Round to at most 2 decimal places only if necessary

Question

I d like to round at most 2 decimal places  but only if necessary   Input   10 1 7777777 9 1   Output   10 1 78 9 1   How can I do this in JavaScript

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This is the simplest  more elegant solution  and I am the best of the world     function roundToX num  X            return   Math round num    e   X      e-  X       roundToX 66 66666666 2    gt  66 67   roundToX 10 2    gt  10   roundToX 10 904 2    gt  10 9

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Here s my solution to this problem   function roundNumber number  precision   0    var num   number toString   replace           var integer  decimal  significantDigit   if  num indexOf       gt  0  amp  amp  num substring num indexOf        1  length  gt  precision  amp  amp  precision  gt  0        integer   parseInt num  toString        decimal   num substring num indexOf        1       significantDigit   Number decimal substr precision  1         if  significantDigit  gt   5            decimal    Number decimal substr 0  precision     1  toString            return integer         decimal        else           decimal    Number decimal substr 0  precision     1  toString            return integer         decimal          else if  num indexOf       gt  0        integer   parseInt num  toString        decimal   num substring num indexOf        1       significantDigit   num substring num length - 1  1        if  significantDigit  gt   5            decimal    Number decimal    1  toString            return integer         decimal        else                       return integer         decimal            return number

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To not deal with many 0s  use this variant   Math round num   1e2    1e2

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Try this light weight solution   function round x  digits     return parseFloat x toFixed digits       round 1 222   2        1 22  round 1 222  10        1 222

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If you are using lodash library  you can use the round method of lodash like following     round number  precision    Eg      round 1 7777777  2    1 78

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I wrote the following set of functions for myself  Maybe it will help you too   function float exponent number        exponent   1      while  number  lt  1 0            exponent    1         number    10           return exponent    function format float number  extra precision        precision   float exponent number     extra precision    0      return number toFixed precision  split     0     0      Usage   format float 1 01      1 format float 1 06      1 1 format float 0 126      0 13 format float 0 000189      0 00019   For you case   format float 10  1      10 format float 9 1  1      9 1 format float 1 77777  1      1 78

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Quick helper function where rounging is You default rounding  let rounding 4   let round  number   gt   let multiply Math pow 10 rounding    return Math round number multiply  multiply    console log round 0 040579431         0 0406

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A simpler ES6 way is  const round    x  n    gt     Number parseFloat Math round x   Math pow 10  n     Math pow 10  n   toFixed n      This pattern also returns the precision asked for   ex   round 44 7826456  4      yields 44 7826 round 78 12  4           yields 78 12

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I ll add one more approach to this   number   16 6666666  console log parseFloat number toFixed 2      16 67   number   16 6  console log parseFloat number toFixed 2      16 6   number   16  console log parseFloat number toFixed 2      16     toFixed 2  returns a string with exactily 2 decimal points  that may or may not be trailing zeros  Doing a parseFloat   will eliminate those trailing zeros

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I know there are many answers  but most of them have side effect in some specific cases   Easiest and shortest solution without any side effects is following   Number  2 3456789  toFixed 2      2 35   It rounds properly and returns number instead of string  console log Number  2 345  toFixed 2        2 35 console log Number  2 344  toFixed 2        2 34 console log Number  2  toFixed 2            2 console log Number  -2  toFixed 2           -2 console log Number  -2 345  toFixed 2       -2 35  console log Number  2 345678  toFixed 3       2 346

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You should use   Math round  num   100   Number EPSILON     100   No one seems to be aware of Number EPSILON   Also it s worth noting that this is not a JavaScript weirdness like some people stated   That is simply the way floating point numbers works in a computer  Like 99  of programming languages  JavaScript doesn t have home made floating point numbers  it relies on the CPU FPU for that  A computer uses binary  and in binary  there isn t any numbers like 0 1  but a mere binary approximation for that  Why  For the same reason than 1 3 cannot be written in decimal  its value is 0 33333333    with an infinity of threes   Here come Number EPSILON  That number is the difference between 1 and the next number existing in the double precision floating point numbers  That s it  There is no number between 1 and 1   Number EPSILON   EDIT   As asked in the comments  let s clarify one thing  adding Number EPSILON is relevant only when the value to round is the result of an arithmetic operation  as it can swallow some floating point error delta   It s not useful when the value comes from a direct source  e g   literal  user input or sensor    EDIT  2019    Like  maganap and some peoples have pointed out  it s best to add Number EPSILON before multiplying   Math round    num   Number EPSILON     100     100   EDIT  december 2019    Lately  I use a function similar to this one for comparing numbers epsilon-aware   const ESPILON RATE   1   Number EPSILON   const ESPILON ZERO   Number MIN VALUE    function epsilonEquals  a   b       if   Number isNaN  a      Number isNaN  b           return false         if   a     0    b     0         return a  lt   b   EPSILON ZERO  amp  amp  b  lt   a   EPSILON ZERO         return a  lt   b   EPSILON RATE  amp  amp  b  lt   a   EPSILON RATE       My use-case is an assertion   data validation lib I m developing for many years   In fact  in the code I m using ESPILON RATE   1   4   Number EPSILON and EPSILON ZERO   4   Number MIN VALUE  four times the epsilon   because I want an equality checker loose enough for cumulating floating point error   So far  it looks perfect for me  I hope it will help

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This answer is more about speed    var precalculatedPrecisions    1e0  1e1  1e2  1e3  1e4  1e5  1e6  1e7  1e8  1e9  1e10    function round num   prec         precision   precalculatedPrecisions  prec      return Math round num    precision   1e-14     precision       jsPerf about this

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I ve read all the answers  the answers of similar questions and the complexity of the most  quot good quot  solutions didn t satisfy me  I don t want to put a huge round function set  or a small one but fails on scientific notation  So  I came up with this function  It may help someone in my situation  function round num  dec       const  sv  ev    num toString   split  e       return Number Number Math round parseFloat sv    e    dec      e-    dec     e     ev    0       I didn t run any performance test because I will call this just to update the UI of my application  The function gives the following results for a quick test     1 3563143   2 806510993243886e-7 round 1 3563143  2      returns  2 81e-7   round 1 31645  4        returns 1 3165  round -17 3954  2       returns -17 4  This is enough for me

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It may work for you   Math round num   100  100    to know the difference between toFixed and round  You can have a look at Math round num  vs num toFixed 0  and browser inconsistencies

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I have found this works for all my use cases   const round    value  decimalPlaces   0    gt        const multiplier   Math pow 10  decimalPlaces       return Math round value   multiplier   Number EPSILON    multiplier       Keep in mind that is ES6  An ES5 equiv  would be very easy to code though so I m not gonna add it

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If you happen to already be using the d3 library  they have a powerful number formatting library  https   github com mbostock d3 wiki Formatting  Rounding specifically is here  https   github com mbostock d3 wiki Formatting d3 round  In your case  the answer is    gt  d3 round 1 777777  2  1 78  gt  d3 round 1 7  2  1 7  gt  d3 round 1  2  1

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10  toFixed 2        10   10 12345  toFixed 2        10 12   10  toFixed 2        10 00  10 12345  toFixed 2        10 12

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This did the trick for me  TypeScript    round decimal  number  decimalPoints  number   number      let roundedValue   Math round decimal   Math pow 10  decimalPoints     Math pow 10  decimalPoints        console log  Rounded   decimal  to   roundedValue         return roundedValue        Sample output  Rounded 18 339840000000436 to 18 34 Rounded 52 48283999999984 to 52 48 Rounded 57 24612000000036 to 57 25 Rounded 23 068320000000142 to 23 07 Rounded 7 792980000000398 to 7 79 Rounded 31 54157999999981 to 31 54 Rounded 36 79686000000004 to 36 8 Rounded 34 723080000000124 to 34 72 Rounded 8 4375 to 8 44 Rounded 15 666960000000074 to 15 67 Rounded 29 531279999999924 to 29 53 Rounded 8 277420000000006 to 8 28

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Keep type as integer for later sorting or other math operations   Math round 1 7777777   100  100      1 78      Round up  Math ceil 1 7777777   100  100       1 78      Round down  Math floor 1 7777777   100  100      1 77   Or convert to string    1 7777777  toFixed 2        1 77

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I created this function  for rounding a number  The value can be a string  ex   1 005   or a number 1 005 that will be 1 by default and if you specify the decimal to be 2  the result will be 1 01   round value  string   number  decimals  number   string    0    number   null       return    Math round Number value    e   decimals      e-    decimals       Usage  round 1 005  2     1 01 or  Usage  round  1 005   2    1 01

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Mathematic floor and round definitions    lead us to   x000D   x000D  let round  x  gt    x 0 005 -  x 0 005  0 01       replace                1    x000D   x000D     for case like 1 384 we need to use regexp to get only 2 digits after dot x000D     and cut off machine-error  epsilon  x000D   x000D  console log round 10    x000D  console log round 1 7777777    x000D  console log round 1 7747777    x000D  console log round 1 384    x000D   x000D   x000D

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For me Math round   was not giving correct answer  I found toFixed 2  works better   Below are examples of both    x000D   x000D  console log Math round 43000   80000    100      wrong answer x000D   x000D  console log   43000   80000    100  toFixed 2       correct answer x000D   x000D   x000D

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Here is the shortest and complete answer   function round num  decimals            var n   Math pow 10  decimals           return Math round   n   num  toFixed decimals       n       This also takes care of the example case 1 005 which will return 1 01

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The rounding problem can be avoided by using numbers represented in exponential notation   public roundFinancial amount  number  decimals  number        return Number Math round Number    amount e  decimals        e-  decimals

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var roundUpto   function number  upto       return Number number toFixed upto      roundUpto 0 1464676  2     toFixed 2  here 2 is number of digits upto which we want to round this num

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number  parseInt  number  0 005  100   100         add 0 005 if you want to normal round  2 decimals        8 123  0 005  gt  8 128 100  gt 812 100  gt 8 12     8 126  0 005  gt  8 131 100  gt 813 100  gt 8 13

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From the existing answers I found another solution which seems to work great  which also works with sending in a string and eliminates trailing zeros   function roundToDecimal string  decimals        return parseFloat parseFloat string  toFixed decimals        It doesn t take in to account if you send in some bull   like  apa  though  Or it will probably throw an error which I think is the proper way anyway  it s never good to hide errors that should be fixed  by the calling function

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Here is a simple way to do it   Math round value   100    100   You might want to go ahead and make a separate function to do it for you though   function roundToTwo value        return Math round value   100    100       Then you would simply pass in the value   You could enhance it to round to any arbitrary number of decimals by adding a second parameter   function myRound value  places        var multiplier   Math pow 10  places        return  Math round value   multiplier    multiplier

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Try to use the jQuery  number plug-in   var number   19 8000000007  var res   1     number number  2

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Consider  toFixed   and  toPrecision     http   www javascriptkit com javatutors formatnumber shtml

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To round at decimal positions pos  including no decimals  do Math round num   Math pow 10 pos     Math pow 10 pos    x000D   x000D  var console     x000D   log  function s    x000D    document getElementById  console   innerHTML    s     lt br  gt   x000D     x000D    x000D  var roundDecimals function num pos    x000D   return  Math round num   Math pow 10 pos     Math pow 10 pos     x000D    x000D    https   en wikipedia org wiki Pi x000D  var pi 3 14159265358979323846264338327950288419716939937510  x000D  for var i 2 i lt 15 i    console log  pi   roundDecimals pi i    x000D  for var i 15 i gt  0 --i  console log  pi   roundDecimals pi i    x000D   lt div id  console    gt  x000D   x000D   x000D

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None of the answers found here is correct   stinkycheeseman asked to round up  you all rounded the number   To round up  use this   Math ceil num   100  100

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A Generic Answer for all browsers and precisions   function round num  places          if  places          return Math round num                  var val   Math pow 10  places         return Math round num   val    val     round num  2

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A simple general rounding function could be following   Steps are    Multiply the number by  10 to the power of number of decimal place  using Math pow 10 places   Round the result to whole integer using Math Round  Divide the result back by  10 to the power of number of decimal place  Math pow 10 places     Example   number is  1 2375 to be rounded to 3 decimal places   1 2375    10 3       1 2375   1000   1237 5 Round to integer     1238 Divide 1238 by  10 3      1238   1000   1 238    note  10 3 means Math pow 10 3      x000D   x000D   function numberRoundDecimal v n    x000D   return Math round  v Number EPSILON  Math pow 10 n   Math pow 10 n   x000D   x000D   x000D     ------- tests -------- x000D  console log numberRoundDecimal -0 024641163062896567 3       -0 025 x000D  console log numberRoundDecimal 0 9993360575508052 3          0 999 x000D  console log numberRoundDecimal 1 0020739645577939 3          1 002 x000D  console log numberRoundDecimal 0 975 0                       1 x000D  console log numberRoundDecimal 0 975 1                       1 x000D  console log numberRoundDecimal 0 975 2                       0 98 x000D  console log numberRoundDecimal 1 005 2                       1 01 x000D   x000D   x000D

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Use this function Number x  toFixed 2

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This may help you   var result   Math round input 100  100    for more information  you can have a look at this link  Math round num  vs num toFixed 0  and browser inconsistencies

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MarkG s answer is the correct one   Here s a generic extension for any number of decimal places   Number prototype round   function places      return   Math round this    e     places      e-    places       Usage   var n   1 7777      n round 2      1 78   Unit test   it only  should round floats to 2 places   function        var cases           n  10       e  10     p 2          n  1 7777   e  1 78   p 2          n  1 005    e  1 01   p 2          n  1 005    e  1      p 0          n  1 77777  e  1 8    p 1          cases forEach function testCase        var r   testCase n round testCase p       assert equal r  testCase e   didn  t get right number

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Using Brian Ustas s solution   function roundDecimal value  precision        var multiplier   Math pow 10  precision       return Math round value   multiplier    multiplier

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There is a solution working for all numbers  give it a try  expression is given below   Math round  num   0 00001    100    100  Try Math round  1 005   0 00001    100    100 and Math round  1 0049   0 00001    100    100   I recently tested every possible solution and finally arrived at the output after trying almost 10 times  Here is a screenshot of issue arised during caculations     head over to the amount field  It s returning almost infinite  I gave a try to toFixed   method but it s not working for some cases i e try with PI  and finally derived s solution given above

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Since ES6 there is a  proper  way  without overriding statics and creating workarounds  to do this by using toPrecision   x000D   x000D  var x   1 49999999999  x000D  console log x toPrecision 4    x000D  console log x toPrecision 3    x000D  console log x toPrecision 2    x000D   x000D  var y   Math PI  x000D  console log y toPrecision 6    x000D  console log y toPrecision 5    x000D  console log y toPrecision 4    x000D   x000D  var z   222 987654 x000D  console log z toPrecision 6    x000D  console log z toPrecision 5    x000D  console log z toPrecision 4    x000D   x000D   x000D    then you can just parseFloat and zeroes will  go away      x000D   x000D  console log parseFloat  1 4999  toPrecision 3     x000D  console log parseFloat  1 005  toPrecision 3     x000D  console log parseFloat  1 0051  toPrecision 3     x000D   x000D   x000D    It doesn t solve the  1 005 rounding problem  though - since it is intrinsic to how float fractions are being processed    x000D   x000D  console log 1 005 - 0 005   x000D   x000D   x000D    If you are open to libraries you can use bignumber js   x000D   x000D  console log 1 005 - 0 005   x000D  console log new BigNumber 1 005  minus 0 005    x000D   x000D  console log new BigNumber 1 005  round 4    x000D  console log new BigNumber 1 005  round 3    x000D  console log new BigNumber 1 005  round 2    x000D  console log new BigNumber 1 005  round 1    x000D   lt script src  https   cdnjs cloudflare com ajax libs bignumber js 2 3 0 bignumber min js  gt  lt  script gt  x000D   x000D   x000D

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Starting from the example proposed over the precisionRound that I found on MDN   that event for 1 005 returs 1 and not 1 01   I write a custom precisionRound that manage a random precision number and for 1 005 returns 1 01   This is the function   function precisionRound number  precision       if precision  lt  0            var factor   Math pow 10  precision         return Math round number   factor    factor          else       return   Math round number    e   precision       e-  precision      console log precisionRound 1234 5678  1        output  1234 6 console log precisionRound 1234 5678  -1       output  1230 console log precisionRound 1 005  2            output  1 01 console log precisionRound 1 0005  2           output  1 console log precisionRound 1 0005  3           output  1 001 console log precisionRound 1 0005  4           output  1 0005   For TypeScript   public static precisionRound number  number  precision  number      if  precision  lt  0          let factor   Math pow 10  precision       return Math round number   factor    factor        else     return   Math round Number number    e     precision            e-    precision

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I just wanted to share my approach  based on previously mentioned answers   Let s create a function that rounds any given numeric value to a given amount of decimal places   function roundWDecimals n  decimals        if   isNaN parseFloat n    amp  amp  isFinite n             if  typeof decimals     typeof undefined                 decimals   0                    var decimalPower   Math pow 10  decimals           return Math round parseFloat n    decimalPower    decimalPower            return NaN      And introduce a new  round  method for numbers prototype   Object defineProperty Number prototype   round         enumerable  false      value  function decimals            return roundWDecimals this  decimals               And you can test it    x000D   x000D  function roundWDecimals n  decimals    x000D      if   isNaN parseFloat n    amp  amp  isFinite n     x000D          if  typeof decimals     typeof undefined     x000D              decimals   0  x000D            x000D          var decimalPower   Math pow 10  decimals   x000D          return Math round parseFloat n    decimalPower    decimalPower  x000D        x000D      return NaN  x000D    x000D  Object defineProperty Number prototype   round     x000D      enumerable  false  x000D      value  function decimals    x000D          return roundWDecimals this  decimals   x000D        x000D      x000D   x000D  var roundables     x000D       num  10  decimals  2   x000D       num  1 7777777  decimals  2   x000D       num  9 1  decimals  2   x000D       num  55 55  decimals  1   x000D       num  55 549  decimals  1   x000D       num  55  decimals  0   x000D       num  54 9  decimals  0   x000D       num  -55 55  decimals  1   x000D       num  -55 551  decimals  1   x000D       num  -55  decimals  0   x000D       num  1 005  decimals  2   x000D       num  1 005  decimals  2   x000D       num  19 8000000007  decimals  2   x000D       x000D    table     lt table border  1  gt  lt tr gt  lt th gt Num lt  th gt  lt th gt Decimals lt  th gt  lt th gt Result lt  th gt  lt  tr gt    x000D    each roundables  function     x000D    table    x000D        lt tr gt    x000D          lt td gt   this num   lt  td gt    x000D          lt td gt   this decimals   lt  td gt    x000D          lt td gt   this num round this decimals    lt  td gt    x000D        lt  tr gt   x000D      x000D      x000D  table      lt  table gt    x000D      results   append table   x000D   lt script src  https   ajax googleapis com ajax libs jquery 1 11 1 jquery min js  gt  lt  script gt  x000D   lt div class  results  gt  lt  div gt  x000D   x000D   x000D

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There are a couple of ways to do that  For people like me  the Lodash s variant  function round number  precision        var pair    number    e   split  e       var value   Math round pair 0     e      pair 1    precision       pair    value    e   split  e       return   pair 0     e      pair 1  - precision       Usage   round 0 015  2     0 02 round 1 005  2     1 01     If your project uses jQuery or lodash  you can also find proper round method in the libraries   Update 1  I removed the variant n toFixed 2   because it is not correct  Thank you  avalanche1

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This function works for me  You just pass in the number and the places you want to round and it does what it needs to do easily   round source n     let places   Math pow 10 n     return Math round source   places    places

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I reviewed every answer of this post  Here is my take on the matter   x000D   x000D  const nbRounds   7  const round    x  n 2    gt        const precision   Math pow 10  n      return Math round  x Number EPSILON    precision     precision     new Array nbRounds  fill 1  forEach    i   gt        console log  round 1 00083899    i 1     gt     round 1 00083899  i 1       console log  round 1 83999305    i 1     gt     round 1 83999305  i 1      x000D   x000D   x000D

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Please use the below function if you don t want to round off   function ConvertToDecimal num      num   num toString       If it s not already a String   num   num slice 0   num indexOf         3      With 3 exposing the hundredths place     alert  M       Number num       If you need it back as a Number

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Use something like this   parseFloat parseFloat value  toFixed 2      parseFloat parseFloat  1 7777777   toFixed 2  -- gt 1 78  parseFloat parseFloat  10   toFixed 2  -- gt 10  parseFloat parseFloat  9 1   toFixed 2  -- gt 9 1

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I still don t think anyone gave him the answer to how to only do the rounding if needed  The easiest way I see to do it is to check if there is even a decimal in the number  like so   var num   3 21  if    num     indexOf       gt   0       at least assert to string first           whatever code you decide to use to round

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In the node environment I just use the roundTo module   const roundTo   require  round-to        roundTo 123 4567  2       123 46

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I tried my very own code  try this  function AmountDispalyFormat value        value   value toFixed 3       var amount   value toString   split           var result   0      if  amount length  gt  1            var secondValue   parseInt amount 1  toString   slice 0  2            if  amount 1  toString   length  gt  2                if  parseInt amount 1  toString   slice 2  3    gt  4                    secondValue                    if  secondValue    100                        amount 0    parseInt amount 0     1                      secondValue   0                                                     if  secondValue toString   length    1                secondValue    0    secondValue                    result   parseFloat amount 0          secondValue         else           result   parseFloat amount             return result

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The easiest approach would be to use toFixed and then strip trailing zeros using the Number function   const number   15 5  Number number toFixed 2       15 5   const number   1 7777777  Number number toFixed 2       1 78

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As per the answer already given in comment with the link to http   jsfiddle net AsRqx  Following one worked for me perfectly   function C num       return   Math round num    e 2       e-2         function N num  places       return   Math round num    e     places      e-    places        C 1 005    N 1 005 0   N 1 005 1     up to 1 decimal places N 1 005 2     up to 2 decimal places N 1 005 3     up to 3 decimal places

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Simple solution would be use lodash s ceil function if you want to round up     https   lodash com docs 4 17 10 ceil    round 6 001 2    gives 6    ceil 6 001   2     gives 6 01    ceil 37 4929 2     gives 37 5    round 37 4929 2     gives 37 49

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Easiest way    num toFixed 2   It converts it to a string  and then back into an integer   float

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I was building a simple tipCalculator and there was a lot of answers here that seem to overcomplicate the issue  So I found summarizing the issue to be the best way to truly answer this question  if you want to create a rounded decimal number  first you call toFixed   of decimal places you want to keep  and then wrap that in a Number     so end result   let amountDue   286 44  tip   Number  amountDue   0 2  toFixed 2    console log tip      57 29 instead of 57 288

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You could also override the Math round function to do the rounding correct and add a parameter for decimals and use it like  Math round Number  Decimals   Keep in mind that this overrides the built in component Math round and giving it another property then it original is   var round   Math round  Math round   function  value  decimals      decimals   decimals    0    return Number round value    e    decimals     e-    decimals       Then you can simply use it like this   Math round 1 005  2     https   jsfiddle net k5tpq3pd 3

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This worked pretty well for me when wanting to always round up to a certain decimal   The key here is that we will always be rounding up with the Math ceil function   You could conditionally select ceil or floor if needed    x000D   x000D           x000D         Possibility to lose precision at large numbers x000D          param number x000D          returns Number number x000D          x000D      var roundUpToNearestHundredth   function number    x000D   x000D             Ensure that we use high precision Number x000D          number   Number number   x000D   x000D             Save the original number so when we extract the Hundredth decimal place we don t bit switch or lose precision x000D          var numberSave   Number number toFixed 0    x000D   x000D             Remove the  integer  values off the top of the number x000D          number   number - numberSave  x000D   x000D             Get the Hundredth decimal places x000D          number    100  x000D   x000D             Ceil the decimals   Therefore  15000001 will equal  151  etc  x000D          number   Math ceil number   x000D   x000D             Put the decimals back into their correct spot x000D          number    100  x000D   x000D             Add the  integer  back onto the number x000D          return number   numberSave  x000D   x000D         x000D   x000D  console log roundUpToNearestHundredth 6132423 1200000000001   x000D   x000D   x000D

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the proposed answers while generally correct doesn t consider the precision of the passed in number  which is not expressed as requirement in the original question  but may be a requirement in case of scientific application where 3 is different from 3 00  for example  as the number of decimal digits represents the precision of the instrument that have acquired the value or the accuracy of a calculation  In fact the proposed answers rounds 3 001 to 3 while by keeping the information about the precision of the number should be 3 00  below a function that takes in account that   x000D   x000D  function roundTo value  decimal    x000D   x000D      let absValue   Math abs value   x000D      let int   Math floor absValue  toString   length  x000D      let dec   absValue toString   length - int  x000D      dec -   Number isInteger absValue    0   1   x000D      return value toPrecision int   Math min dec  decimal    x000D     x000D    x000D   x000D   x000D

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Based on the choosen answer and the upvoted comment on the same question   Math round  num   0 00001    100    100   This works for both these examples   Math round  1 005   0 00001    100    100  Math round  1 0049   0 00001    100    100

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You can use  function roundToTwo num            return   Math round num    e 2       e-2        I found this over on MDN  Their way avoids the problem with 1 005 that was mentioned   roundToTwo 1 005  1 01 roundToTwo 10  10 roundToTwo 1 7777777  1 78 roundToTwo 9 1  9 1 roundToTwo 1234 5678  1234 57

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Slight modification of this answer that seems to work well   Function  function roundToStep value  stepParam       var step   stepParam    1 0     var inv   1 0   step     return Math round value   inv    inv      Usage  roundToStep 2 55    3 roundToStep 2 55  0 1    2 6 roundToStep 2 55  0 01    2 55

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Use Math round     Math round num   100    100  Or to be more specific and to ensure things like 1 005 round correctly  use Number EPSILON    Math round  num   Number EPSILON    100    100

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MarkG and Lavamantis offered a much better solution than the one that has been accepted  It s a shame they don t get more upvotes   Here is the function I use to solve the floating point decimals issues also based on MDN  It is even more generic  but less concise  than Lavamantis s solution   function round value  exp      if  typeof exp      undefined      exp     0      return Math round value      value    value    exp     exp     if  isNaN value       typeof exp      number   amp  amp  exp   1     0       return NaN        Shift   value   value toString   split  e      value   Math round   value 0     e     value 1      value 1    exp    exp           Shift back   value   value toString   split  e      return   value 0     e     value 1      value 1  - exp    -exp        Use it with   round 10 8034  2           Returns 10 8 round 1 275  2             Returns 1 28 round 1 27499  2           Returns 1 27 round 1 2345678e 2  2      Returns 123 46   Compared to Lavamantis s solution  we can do     round 1234 5678  -2      Returns 1200 round  123 45            Returns 123

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Just for the record  the scaling method could theoretically return Infinity if the number and the digits you want to round to are big enough  In JavaScript that shouldn t be a problem since the maximum number is 1 7976931348623157e 308  but if you re working with really big numbers or a lot of decimal places you could try this function instead    x000D   x000D  Number prototype roundTo   function digits  x000D    x000D      var str   this toString    x000D      var split   this toString   split  e    x000D      var scientific   split length  gt  1  x000D      var index  x000D      if  scientific  x000D        x000D          str   split 0   x000D          var decimal   str split       x000D          if  decimal length  lt  2  x000D              return this  x000D          index   decimal 0  length   1   digits  x000D        x000D      else x000D          index   Math floor this  toString   length   1   digits  x000D      if  str length  lt   index  x000D          return this  x000D      var digit   str index   1   x000D      var num   Number parseFloat str substring 0  index    x000D      if  digit  gt   5  x000D        x000D          var extra   Math pow 10  -digits   x000D          return this  lt  0   num - extra   num   extra  x000D        x000D      if  scientific  x000D          num     e    split 1   x000D      return num  x000D    x000D   x000D   x000D

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In general  decimal rounding is done by scaling  round num   p    p Naive implementation Using the following function with halfway numbers  you will get either the upper rounded value as expected  or the lower rounded value sometimes depending on the input  This inconsistency in rounding may introduce hard to detect bugs in the client code   x000D   x000D  function naiveRound num  decimalPlaces   0        var p   Math pow 10  decimalPlaces       return Math round num   p    p     console log  naiveRound 1 245  2         1 25 correct  rounded as expected  console log  naiveRound 1 255  2         1 25 incorrect  should be 1 26      testing edge cases console log  naiveRound 1 005  2         1    incorrect  should be 1 01  console log  naiveRound 2 175  2         2 17 incorrect  should be 2 18  console log  naiveRound 5 015  2         5 01 incorrect  should be 5 02  x000D   x000D   x000D   Better implementations Exponential notation By converting the number to a string in the exponential notation  positive numbers are rounded as expected  But  be aware that negative numbers round differently than positive numbers  In fact  it performs what is basically equivalent to  quot round half up quot  as the rule  you will see that round -1 005  2  evaluates to -1 even though round 1 005  2  evaluates to 1 01  The lodash   round method uses this technique   x000D   x000D         Round half up   round half towards positive infinity      Uses exponential notation to avoid floating-point issues     Negative numbers round differently than positive numbers      function round num  decimalPlaces   0        num   Math round num    e    decimalPlaces       return Number num    e    -decimalPlaces         test rounding of half console log  round 0 5  0         1 console log  round -0 5  0        0     testing edge cases console log  round 1 005  2          1 01 console log  round 2 175  2          2 18 console log  round 5 015  2          5 02  console log  round -1 005  2         -1 console log  round -2 175  2         -2 17 console log  round -5 015  2         -5 01 x000D   x000D   x000D   If you want the usual behavior when rounding negative numbers  you would need to convert negative numbers to positive before calling Math round    and then convert them back to negative numbers before returning     Round half away from zero function round num  decimalPlaces   0        num   Math round Math abs num     quot e quot    decimalPlaces    Math sign num       return Number num    quot e quot    -decimalPlaces      Number EPSILON There is a different purely mathematical technique to perform round-to-nearest  using  quot round half away from zero quot    in which epsilon correction is applied before calling the rounding function  Simply  we add the smallest possible float value    1 0 ulp  unit in the last place  to the product before rounding  This moves to the next representable float value  away from zero   x000D   x000D         Round half away from zero   commercial  rounding     Uses correction to offset floating-point inaccuracies     Works symmetrically for positive and negative numbers      function round num  decimalPlaces   0        var p   Math pow 10  decimalPlaces       var m    num   p     1   Number EPSILON       return Math round m    p        test rounding of half console log  round 0 5  0         1 console log  round -0 5  0        -1     testing edge cases console log  round 1 005  2         1 01 console log  round 2 175  2         2 18 console log  round 5 015  2         5 02  console log  round -1 005  2        -1 01 console log  round -2 175  2        -2 18 console log  round -5 015  2        -5 02 x000D   x000D   x000D   This is needed to offset the implicit round-off error that may occur during encoding of decimal numbers  particularly those  having  quot 5 quot  in the last decimal position  like 1 005  2 675 and 16 235  Actually  1 005 in decimal system is encoded to 1 0049999999999999 in 64-bit binary float  while  1234567 005 in decimal system is encoded to 1234567 0049999998882413 in 64-bit binary float  It is worth noting that the maximum binary round-off error is dependent upon  1  the magnitude of the number and  2  the relative machine epsilon  2 -52   Double rounding Here  we use the toPrecision   method to strip the floating-point round-off errors in the intermediate calculations  Simply  we round to 15 significant figures to strip the round-off error at the 16th significant digit  This technique to preround the result to significant digits is also used by PHP 7 round function   x000D   x000D     Round half away from zero function round num  decimalPlaces   0        var p   Math pow 10  decimalPlaces       var m   Number  Math abs num    p  toPrecision 15        return Math round m    p   Math sign num         test rounding of half console log  round 0 5  0         1 console log  round -0 5  0        -1     testing edge cases console log  round 1 005  2         1 01 console log  round 2 175  2         2 18 console log  round 5 015  2         5 02  console log  round -1 005  2        -1 01 console log  round -2 175  2        -2 18 console log  round -5 015  2        -5 02 x000D   x000D   x000D   Arbitrary-precision JavaScript library - decimal js  x000D   x000D     Round half away from zero function round num  decimalPlaces   0        return new Decimal num  toDecimalPlaces decimalPlaces  toNumber          test rounding of half console log  round 0 5  0         1 console log  round -0 5  0        -1     testing edge cases console log  round 1 005  2         1 01 console log  round 2 175  2         2 18 console log  round 5 015  2         5 02  console log  round -1 005  2        -1 01 console log  round -2 175  2        -2 18 console log  round -5 015  2        -5 02 x000D   lt script src  https   cdnjs cloudflare com ajax libs decimal js 10 2 1 decimal js  integrity  sha512-GKse2KVGCCMVBn4riigHjXE8j5hCxYLPXDw8AvcjUtrt a9TbZFtIKGdArXwYOlZvdmkhQLWQ46ZE3Q1RIa7uQ    crossorigin  anonymous  gt  lt  script gt  x000D   x000D   x000D   Solution 1  string in exponential notation Inspired by the solution provided by KFish here  https   stackoverflow com a 55521592 4208440 A simple drop in solution that provides accurate decimal rounding  flooring  and ceiling to a specific number of decimal places without adding a whole library  It treats floats more like decimals by fixing the binary rounding issues to avoid unexpected results  for example  floor  0 1 0 7  10  will return the expected result 8  Numbers are rounded to a specific number of fractional digits  Specifying a negative precision will round to any number of places to the left of the decimal point   x000D   x000D     Solution 1 var DecimalPrecision    function         if  Math sign     undefined            Math sign   function x                return   x  gt  0  -  x  lt  0       x                       if  Math trunc     undefined            Math trunc   function v                return v  lt  0   Math ceil v    Math floor v                        var decimalAdjust   function type  num  decimalPlaces            var shift   function value  exponent                value    value    e   split  e                return   value 0     e      value 1     exponent    0                        var n   type      round    Math abs num    num          var m   shift n   decimalPlaces           var r   shift Math type  m   -decimalPlaces           return type      round    Math sign num    r   r             return              Decimal round  half away from zero          round  function num  decimalPlaces                return decimalAdjust  round   num  decimalPlaces                         Decimal ceil         ceil  function num  decimalPlaces                return decimalAdjust  ceil   num  decimalPlaces                         Decimal floor         floor  function num  decimalPlaces                return decimalAdjust  floor   num  decimalPlaces                         Decimal trunc         trunc  function num  decimalPlaces                return decimalAdjust  trunc   num  decimalPlaces                         Format using fixed-point notation         toFixed  function num  decimalPlaces                return decimalAdjust  round   num  decimalPlaces  toFixed decimalPlaces                              test rounding of half console log DecimalPrecision round 0 5        1 console log DecimalPrecision round -0 5       -1     testing very small numbers console log DecimalPrecision ceil 1e-8  2      0 01              0 01 console log DecimalPrecision floor 1e-8  2      0                   0     testing simple cases console log DecimalPrecision round 5 12  1      5 1               5 1 console log DecimalPrecision round -5 12  1      -5 1            -5 1 console log DecimalPrecision ceil 5 12  1      5 2                5 2 console log DecimalPrecision ceil -5 12  1      -5 1             -5 1 console log DecimalPrecision floor 5 12  1      5 1               5 1 console log DecimalPrecision floor -5 12  1      -5 2            -5 2 console log DecimalPrecision trunc 5 12  1      5 1               5 1 console log DecimalPrecision trunc -5 12  1      -5 1            -5 1     testing edge cases for round console log DecimalPrecision round 1 005  2      1 01            1 01 console log DecimalPrecision round 39 425  2      39 43         39 43 console log DecimalPrecision round -1 005  2      -1 01         -1 01 console log DecimalPrecision round -39 425  2      -39 43      -39 43     testing edge cases for ceil console log DecimalPrecision ceil 9 130  2      9 13             9 13 console log DecimalPrecision ceil 65 180  2      65 18          65 18 console log DecimalPrecision ceil -2 260  2      -2 26          -2 26 console log DecimalPrecision ceil -18 150  2      -18 15       -18 15     testing edge cases for floor console log DecimalPrecision floor 2 260  2      2 26            2 26 console log DecimalPrecision floor 18 150  2      18 15         18 15 console log DecimalPrecision floor -9 130  2      -9 13         -9 13 console log DecimalPrecision floor -65 180  2      -65 18      -65 18     testing edge cases for trunc console log DecimalPrecision trunc 2 260  2      2 26            2 26 console log DecimalPrecision trunc 18 150  2      18 15         18 15 console log DecimalPrecision trunc -2 260  2      -2 26         -2 26 console log DecimalPrecision trunc -18 150  2      -18 15      -18 15     testing round to tens and hundreds console log DecimalPrecision round 1262 48  -1      1260         1260 console log DecimalPrecision round 1262 48  -2      1300         1300     testing toFixed   console log DecimalPrecision toFixed 1 005  2       1 01          1 01  x000D   x000D   x000D   Solution 2  purely mathematical  Number EPSILON  This solution avoids any string conversion   manipulation of any kind for performance reasons  Solution 1  25 838 ops sec Solution 2  655 087 ops sec http   jsbench github io  31ec3a8b3d22bd840f8e6822e681a3ac  x000D   x000D     Solution 2 var DecimalPrecision2    function         if  Number EPSILON     undefined            Number EPSILON   Math pow 2  -52             if  Math trunc     undefined            Math trunc   function v                return v  lt  0   Math ceil v    Math floor v                        var isRound   function num  decimalPlaces              return decimalPlaces  gt   0  amp  amp                 num toFixed decimalPlaces      num          var p   Math pow 10  decimalPlaces           return Math round num   p    p     num             var decimalAdjust   function type  num  decimalPlaces            if  isRound num  decimalPlaces    0               return num          var p   Math pow 10  decimalPlaces    0           var m    num   p     1   Number EPSILON           return Math type  m    p             return              Decimal round  half away from zero          round  function num  decimalPlaces                return decimalAdjust  round   num  decimalPlaces                         Decimal ceil         ceil  function num  decimalPlaces                return decimalAdjust  ceil   num  decimalPlaces                         Decimal floor         floor  function num  decimalPlaces                return decimalAdjust  floor   num  decimalPlaces                         Decimal trunc         trunc  function num  decimalPlaces                return decimalAdjust  trunc   num  decimalPlaces                         Format using fixed-point notation         toFixed  function num  decimalPlaces                return decimalAdjust  round   num  decimalPlaces  toFixed decimalPlaces                              test rounding of half console log DecimalPrecision2 round 0 5        1 console log DecimalPrecision2 round -0 5       -1     testing very small numbers console log DecimalPrecision2 ceil 1e-8  2      0 01              0 01 console log DecimalPrecision2 floor 1e-8  2      0                   0     testing simple cases console log DecimalPrecision2 round 5 12  1      5 1               5 1 console log DecimalPrecision2 round -5 12  1      -5 1            -5 1 console log DecimalPrecision2 ceil 5 12  1      5 2                5 2 console log DecimalPrecision2 ceil -5 12  1      -5 1             -5 1 console log DecimalPrecision2 floor 5 12  1      5 1               5 1 console log DecimalPrecision2 floor -5 12  1      -5 2            -5 2 console log DecimalPrecision2 trunc 5 12  1      5 1               5 1 console log DecimalPrecision2 trunc -5 12  1      -5 1            -5 1     testing edge cases for round console log DecimalPrecision2 round 1 005  2      1 01            1 01 console log DecimalPrecision2 round 39 425  2      39 43         39 43 console log DecimalPrecision2 round -1 005  2      -1 01         -1 01 console log DecimalPrecision2 round -39 425  2      -39 43      -39 43     testing edge cases for ceil console log DecimalPrecision2 ceil 9 130  2      9 13             9 13 console log DecimalPrecision2 ceil 65 180  2      65 18          65 18 console log DecimalPrecision2 ceil -2 260  2      -2 26          -2 26 console log DecimalPrecision2 ceil -18 150  2      -18 15       -18 15     testing edge cases for floor console log DecimalPrecision2 floor 2 260  2      2 26            2 26 console log DecimalPrecision2 floor 18 150  2      18 15         18 15 console log DecimalPrecision2 floor -9 130  2      -9 13         -9 13 console log DecimalPrecision2 floor -65 180  2      -65 18      -65 18     testing edge cases for trunc console log DecimalPrecision2 trunc 2 260  2      2 26            2 26 console log DecimalPrecision2 trunc 18 150  2      18 15         18 15 console log DecimalPrecision2 trunc -2 260  2      -2 26         -2 26 console log DecimalPrecision2 trunc -18 150  2      -18 15      -18 15     testing round to tens and hundreds console log DecimalPrecision2 round 1262 48  -1      1260         1260 console log DecimalPrecision2 round 1262 48  -2      1300         1300     testing toFixed   console log DecimalPrecision2 toFixed 1 005  2       1 01          1 01  x000D   x000D   x000D   Solution 3  double rounding This solution uses the toPrecision   method to strip the floating-point round-off errors   x000D   x000D     Solution 3 var DecimalPrecision3    function         if  Math sign     undefined            Math sign   function x                return   x  gt  0  -  x  lt  0       x                       if  Math trunc     undefined            Math trunc   function v                return v  lt  0   Math ceil v    Math floor v                           Eliminate binary floating-point inaccuracies      var stripError   function num            if  Number isInteger num               return num          return parseFloat num toPrecision 15               var decimalAdjust   function type  num  decimalPlaces            var n   type      round    Math abs num    num          var p   Math pow 10  decimalPlaces    0           var m   stripError n   p           var r   Math type  m    p          return type      round    Math sign num    r   r             return              Decimal round  half away from zero          round  function num  decimalPlaces                return decimalAdjust  round   num  decimalPlaces                         Decimal ceil         ceil  function num  decimalPlaces                return decimalAdjust  ceil   num  decimalPlaces                         Decimal floor         floor  function num  decimalPlaces                return decimalAdjust  floor   num  decimalPlaces                         Decimal trunc         trunc  function num  decimalPlaces                return decimalAdjust  trunc   num  decimalPlaces                         Format using fixed-point notation         toFixed  function num  decimalPlaces                return decimalAdjust  round   num  decimalPlaces  toFixed decimalPlaces                              test rounding of half console log DecimalPrecision3 round 0 5        1 console log DecimalPrecision3 round -0 5       -1     testing very small numbers console log DecimalPrecision3 ceil 1e-8  2      0 01              0 01 console log DecimalPrecision3 floor 1e-8  2      0                   0     testing simple cases console log DecimalPrecision3 round 5 12  1      5 1               5 1 console log DecimalPrecision3 round -5 12  1      -5 1            -5 1 console log DecimalPrecision3 ceil 5 12  1      5 2                5 2 console log DecimalPrecision3 ceil -5 12  1      -5 1             -5 1 console log DecimalPrecision3 floor 5 12  1      5 1               5 1 console log DecimalPrecision3 floor -5 12  1      -5 2            -5 2 console log DecimalPrecision3 trunc 5 12  1      5 1               5 1 console log DecimalPrecision3 trunc -5 12  1      -5 1            -5 1     testing edge cases for round console log DecimalPrecision3 round 1 005  2      1 01            1 01 console log DecimalPrecision3 round 39 425  2      39 43         39 43 console log DecimalPrecision3 round -1 005  2      -1 01         -1 01 console log DecimalPrecision3 round -39 425  2      -39 43      -39 43     testing edge cases for ceil console log DecimalPrecision3 ceil 9 130  2      9 13             9 13 console log DecimalPrecision3 ceil 65 180  2      65 18          65 18 console log DecimalPrecision3 ceil -2 260  2      -2 26          -2 26 console log DecimalPrecision3 ceil -18 150  2      -18 15       -18 15     testing edge cases for floor console log DecimalPrecision3 floor 2 260  2      2 26            2 26 console log DecimalPrecision3 floor 18 150  2      18 15         18 15 console log DecimalPrecision3 floor -9 130  2      -9 13         -9 13 console log DecimalPrecision3 floor -65 180  2      -65 18      -65 18     testing edge cases for trunc console log DecimalPrecision3 trunc 2 260  2      2 26            2 26 console log DecimalPrecision3 trunc 18 150  2      18 15         18 15 console log DecimalPrecision3 trunc -2 260  2      -2 26         -2 26 console log DecimalPrecision3 trunc -18 150  2      -18 15      -18 15     testing round to tens and hundreds console log DecimalPrecision3 round 1262 48  -1      1260         1260 console log DecimalPrecision3 round 1262 48  -2      1300         1300     testing toFixed   console log DecimalPrecision3 toFixed 1 005  2       1 01          1 01  x000D   x000D   x000D   Solution 4  double rounding v2 This solution is just like Solution 3  however it uses a custom toPrecision   function   x000D   x000D     Solution 4 var DecimalPrecision4    function         if  Math sign     undefined            Math sign   function x                return   x  gt  0  -  x  lt  0       x                       if  Math trunc     undefined            Math trunc   function v                return v  lt  0   Math ceil v    Math floor v                        var toPrecision   function num  significantDigits               Return early for   0  NaN and Infinity          if   num     Number isFinite num               return num             Compute shift of the decimal point          var shift   significantDigits - 1 - Math floor Math log10 Math abs num                Return if rounding to the same or higher precision          var decimalPlaces   0          for  var p   1   Number isInteger num   p   p    10  decimalPlaces            if  shift  gt   decimalPlaces              return num             Round to sf-1 fractional digits of normalized mantissa x dddd         var scale   Math pow 10  Math abs shift            return shift  gt  0               Math round num   scale    scale               Math round num   scale    scale                Eliminate binary floating-point inaccuracies      var stripError   function num            if  Number isInteger num               return num          return toPrecision num  15              var decimalAdjust   function type  num  decimalPlaces            var n   type      round    Math abs num    num          var p   Math pow 10  decimalPlaces    0           var m   stripError n   p           var r   Math type  m    p          return type      round    Math sign num    r   r             return              Decimal round  half away from zero          round  function num  decimalPlaces                return decimalAdjust  round   num  decimalPlaces                         Decimal ceil         ceil  function num  decimalPlaces                return decimalAdjust  ceil   num  decimalPlaces                         Decimal floor         floor  function num  decimalPlaces                return decimalAdjust  floor   num  decimalPlaces                         Decimal trunc         trunc  function num  decimalPlaces                return decimalAdjust  trunc   num  decimalPlaces                         Format using fixed-point notation         toFixed  function num  decimalPlaces                return decimalAdjust  round   num  decimalPlaces  toFixed decimalPlaces                              test rounding of half console log DecimalPrecision4 round 0 5        1 console log DecimalPrecision4 round -0 5       -1     testing very small numbers console log DecimalPrecision4 ceil 1e-8  2      0 01              0 01 console log DecimalPrecision4 floor 1e-8  2      0                   0     testing simple cases console log DecimalPrecision4 round 5 12  1      5 1               5 1 console log DecimalPrecision4 round -5 12  1      -5 1            -5 1 console log DecimalPrecision4 ceil 5 12  1      5 2                5 2 console log DecimalPrecision4 ceil -5 12  1      -5 1             -5 1 console log DecimalPrecision4 floor 5 12  1      5 1               5 1 console log DecimalPrecision4 floor -5 12  1      -5 2            -5 2 console log DecimalPrecision4 trunc 5 12  1      5 1               5 1 console log DecimalPrecision4 trunc -5 12  1      -5 1            -5 1     testing edge cases for round console log DecimalPrecision4 round 1 005  2      1 01            1 01 console log DecimalPrecision4 round 39 425  2      39 43         39 43 console log DecimalPrecision4 round -1 005  2      -1 01         -1 01 console log DecimalPrecision4 round -39 425  2      -39 43      -39 43     testing edge cases for ceil console log DecimalPrecision4 ceil 9 130  2      9 13             9 13 console log DecimalPrecision4 ceil 65 180  2      65 18          65 18 console log DecimalPrecision4 ceil -2 260  2      -2 26          -2 26 console log DecimalPrecision4 ceil -18 150  2      -18 15       -18 15     testing edge cases for floor console log DecimalPrecision4 floor 2 260  2      2 26            2 26 console log DecimalPrecision4 floor 18 150  2      18 15         18 15 console log DecimalPrecision4 floor -9 130  2      -9 13         -9 13 console log DecimalPrecision4 floor -65 180  2      -65 18      -65 18     testing edge cases for trunc console log DecimalPrecision4 trunc 2 260  2      2 26            2 26 console log DecimalPrecision4 trunc 18 150  2      18 15         18 15 console log DecimalPrecision4 trunc -2 260  2      -2 26         -2 26 console log DecimalPrecision4 trunc -18 150  2      -18 15      -18 15     testing round to tens and hundreds console log DecimalPrecision4 round 1262 48  -1      1260         1260 console log DecimalPrecision4 round 1262 48  -2      1300         1300     testing toFixed   console log DecimalPrecision4 toFixed 1 005  2       1 01          1 01  x000D   x000D   x000D   Benchmarks http   jsbench github io  31ec3a8b3d22bd840f8e6822e681a3ac Here is a benchmark comparing the operations per second in the solutions above on Chrome 85 0 4183 83  Obviously all browsers differ  so your mileage may vary    Note  More is better  Thanks  Mike for adding a screenshot of the benchmark

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This question is complicated   Suppose we have a function  roundTo2DP num   that takes a float as an argument and returns a value rounded to 2 decimal places  What should each of these expressions evaluate to    roundTo2DP 0 014999999999999999  roundTo2DP 0 0150000000000000001  roundTo2DP 0 015    The  obvious  answer is that the first example should round to 0 01  because it s closer to 0 01 than to 0 02  while the other two should round to 0 02  because 0 0150000000000000001 is closer to 0 02 than to 0 01  and because 0 015 is exactly halfway between them and there is a mathematical convention that such numbers get rounded up    The catch  which you may have guessed  is that roundTo2DP cannot possibly be implemented to give those obvious answers  because all three numbers passed to it are the same number  IEEE 754 binary floating point numbers  the kind used by JavaScript  can t exactly represent most non-integer numbers  and so all three numeric literals above get rounded to a nearby valid floating point number  This number  as it happens  is exactly  0 01499999999999999944488848768742172978818416595458984375  which is closer to 0 01 than to 0 02   You can see that all three numbers are the same at your browser console  Node shell  or other JavaScript interpreter  Just compare them     0 014999999999999999     0 0150000000000000001 true  So when I write m   0 0150000000000000001  the exact value of m that I end up with is closer to 0 01 than it is to 0 02  And yet  if I convert m to a String       var m   0 0150000000000000001    console log String m    0 015   var m   0 014999999999999999    console log String m    0 015      I get 0 015  which should round to 0 02  and which is noticeably not the 56-decimal-place number I earlier said that all of these numbers were exactly equal to  So what dark magic is this   The answer can be found in the ECMAScript specification  in section 7 1 12 1  ToString applied to the Number type  Here the rules for converting some Number m to a String are laid down  The key part is point 5  in which an integer s is generated whose digits will be used in the String representation of m      let n  k  and s be integers such that k   1  10k-1   s  lt  10k  the Number value for s    10n-k is m  and k is as small as possible  Note that k is the number of digits in the decimal representation of s  that s is not divisible by 10  and that the least significant digit of s is not necessarily uniquely determined by these criteria    The key part here is the requirement that  k is as small as possible   What that requirement amounts to is a requirement that  given a Number m  the value of String m  must have the least possible number of digits while still satisfying the requirement that Number String m       m  Since we already know that 0 015     0 0150000000000000001  it s now clear why String 0 0150000000000000001       0 015  must be true   Of course  none of this discussion has directly answered what roundTo2DP m  should return  If m s exact value is 0 01499999999999999944488848768742172978818416595458984375  but its String representation is  0 015   then what is the correct answer - mathematically  practically  philosophically  or whatever - when we round it to two decimal places   There is no single correct answer to this  It depends upon your use case  You probably want to respect the String representation and round upwards when    The value being represented is inherently discrete  e g  an amount of currency in a 3-decimal-place currency like dinars  In this case  the true value of a Number like 0 015 is 0 015  and the 0 0149999999    representation that it gets in binary floating point is a rounding error   Of course  many will argue  reasonably  that you should use a decimal library for handling such values and never represent them as binary floating point Numbers in the first place   The value was typed by a user  In this case  again  the exact decimal number entered is more  true  than the nearest binary floating point representation    On the other hand  you probably want to respect the binary floating point value and round downwards when your value is from an inherently continuous scale - for instance  if it s a reading from a sensor   These two approaches require different code  To respect the String representation of the Number  we can  with quite a bit of reasonably subtle code  implement our own rounding that acts directly on the String representation  digit by digit  using the same algorithm you would ve used in school when you were taught how to round numbers  Below is an example which respects the OP s requirement of representing the number to 2 decimal places  only when necessary  by stripping trailing zeroes after the decimal point  you may  of course  need to tweak it to your precise needs          Converts num to a decimal string  if it isn t one already  and then rounds it    to at most dp decimal places        For explanation of why you d want to perform rounding operations on a String    rather than a Number  see http   stackoverflow com a 38676273 1709587        param   number string   num     param  number  dp     return  string      function roundStringNumberWithoutTrailingZeroes  num  dp        if  arguments length    2  throw new Error  2 arguments required         num   String num       if  num indexOf  e       -1               Can t round numbers this large because their string representation            contains an exponent  like 9 99e 37         throw new Error  num too large              if  num indexOf         -1               Nothing to do         return num             var parts   num split               beforePoint   parts 0           afterPoint   parts 1           shouldRoundUp   afterPoint dp   gt   5          finalNumber       afterPoint   afterPoint slice 0  dp       if   shouldRoundUp            finalNumber   beforePoint         afterPoint        else if    9    test afterPoint                If we need to round up a number like 1 9999  increment the integer            before the decimal point and discard the fractional part          finalNumber   Number beforePoint  1        else              Starting from the last digit  increment digits until we find one            that is not 9  then stop         var i   dp-1          while  true                if  afterPoint i      9                     afterPoint   afterPoint substr 0  i                                  0                                 afterPoint substr i 1                   i--                else                   afterPoint   afterPoint substr 0  i                                  Number afterPoint i     1                                 afterPoint substr i 1                   break                                   finalNumber   beforePoint         afterPoint                Remove trailing zeroes from fractional part before returning     return finalNumber replace  0             Example usage     roundStringNumberWithoutTrailingZeroes 1 6  2   1 6    roundStringNumberWithoutTrailingZeroes 10000  2   10000    roundStringNumberWithoutTrailingZeroes 0 015  2   0 02    roundStringNumberWithoutTrailingZeroes  0 015000   2   0 02    roundStringNumberWithoutTrailingZeroes 1  1   1    roundStringNumberWithoutTrailingZeroes  0 015   2   0 02    roundStringNumberWithoutTrailingZeroes 0 01499999999999999944488848768742172978818416595458984375  2   0 02    roundStringNumberWithoutTrailingZeroes  0 01499999999999999944488848768742172978818416595458984375   2   0 01   The function above is probably what you want to use to avoid users ever witnessing numbers that they have entered being rounded wrongly    As an alternative  you could also try the round10 library which provides a similarly-behaving function with a wildly different implementation    But what if you have the second kind of Number - a value taken from a continuous scale  where there s no reason to think that approximate decimal representations with fewer decimal places are more accurate than those with more  In that case  we don t want to respect the String representation  because that representation  as explained in the spec  is already sort-of-rounded  we don t want to make the mistake of saying  0 014999999   375 rounds up to 0 015  which rounds up to 0 02  so 0 014999999   375 rounds up to 0 02    Here we can simply use the built-in toFixed method  Note that by calling Number   on the String returned by toFixed  we get a Number whose String representation has no trailing zeroes  thanks to the way JavaScript computes the String representation of a Number  discussed earlier in this answer           Takes a float and rounds it to at most dp decimal places  For example           roundFloatNumberWithoutTrailingZeroes 1 2345  3        returns 1 234       Note that since this treats the value passed to it as a floating point    number  it will have counterintuitive results in some cases  For instance             roundFloatNumberWithoutTrailingZeroes 0 015  2        gives 0 01 where 0 02 might be expected  For an explanation of why  see    http   stackoverflow com a 38676273 1709587  You may want to consider using the    roundStringNumberWithoutTrailingZeroes function there instead         param  number  num     param  number  dp     return  number      function roundFloatNumberWithoutTrailingZeroes  num  dp        var numToFixedDp   Number num  toFixed dp       return Number numToFixedDp

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One can use  toFixed NumberOfDecimalPlaces    var str   10 234 toFixed 2        gt   10 23  var number   Number str        gt  10 23

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One way to achieve such a rounding only if necessary is to use Number prototype toLocaleString     myNumber toLocaleString  en    maximumFractionDigits 2  useGrouping false     This will provide exactly the output you expect  but as strings  You can still convert those back to numbers if that s not the data type you expect

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the question is to rounds of 2 decimal   let s not make this complicated modifying prototype chain etc    here is one line solution    x000D   x000D  let round2dec   num   gt  Math round num   100    100   x000D   x000D  console log round2dec 1 77    x000D  console log round2dec 1 774    x000D  console log round2dec 1 777    x000D  console log round2dec 10    x000D   x000D   x000D

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Node js  This did the trick for me on Node js in a matter of seconds   npm install math   Source  http   mathjs org examples basic usage js html

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A precise rounding method  Source  Mozilla   function                    Decimal adjustment of a number                 param    String     type    The type of adjustment          param    Number     value   The number          param    Integer    exp     The exponent  the 10 logarithm of the adjustment base           returns  Number             The adjusted value              function decimalAdjust type  value  exp               If the exp is undefined or zero            if  typeof exp      undefined      exp     0                return Math type  value                     value    value          exp    exp             If the value is not a number or the exp is not an integer            if  isNaN value       typeof exp      number   amp  amp  exp   1     0                 return NaN                       Shift         value   value toString   split  e            value   Math type    value 0     e     value 1      value 1  - exp    -exp                Shift back         value   value toString   split  e            return   value 0     e     value 1      value 1    exp    exp                  Decimal round     if   Math round10            Math round10   function value  exp                return decimalAdjust  round   value  exp                           Decimal floor     if   Math floor10            Math floor10   function value  exp                return decimalAdjust  floor   value  exp                           Decimal ceil     if   Math ceil10            Math ceil10   function value  exp                return decimalAdjust  ceil   value  exp                            Examples      Round Math round10 55 55  -1      55 6 Math round10 55 549  -1      55 5 Math round10 55  1      60 Math round10 54 9  1      50 Math round10 -55 55  -1      -55 5 Math round10 -55 551  -1      -55 6 Math round10 -55  1      -50 Math round10 -55 1  1      -60 Math round10 1 005  -2      1 01 -- compare this with Math round 1 005 100  100 above    Floor Math floor10 55 59  -1      55 5 Math floor10 59  1      50 Math floor10 -55 51  -1      -55 6 Math floor10 -51  1      -60    Ceil Math ceil10 55 51  -1      55 6 Math ceil10 51  1      60 Math ceil10 -55 59  -1      -55 5 Math ceil10 -59  1      -50

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If the value is a text type  parseFloat  quot 123 456 quot   toFixed 2    If the value is a number  var numb   123 23454  numb   numb toFixed 2    There is a downside that values like 1 5 will give  quot 1 50 quot  as the output  A fix suggested by  minitech  var numb   1 5  numb    numb toFixed 2      Note the plus sign that drops any  quot extra quot  zeroes at the end     It changes the result  which is a string  into a number again  think  quot 0   foo quot       which means that it uses only as many digits as necessary   It seems like Math round is a better solution  But it is not  In some cases it will NOT round correctly  Math round 1 005   1000  1000    Returns 1 instead of expected 1 01   toFixed   will also NOT round correctly in some cases  tested in Chrome v 55 0 2883 87   Examples  parseFloat  quot 1 555 quot   toFixed 2      Returns 1 55 instead of 1 56  parseFloat  quot 1 5550 quot   toFixed 2      Returns 1 55 instead of 1 56     However  it will return correct result if you round 1 5551  parseFloat  quot 1 5551 quot   toFixed 2      Returns 1 56 as expected   1 3555 toFixed 3     Returns 1 355 instead of expected 1 356     However  it will return correct result if you round 1 35551  1 35551 toFixed 2      Returns 1 36 as expected   I guess  this is because 1 555 is actually something like float 1 55499994 behind the scenes  Solution 1 is to use a script with required rounding algorithm  for example  function roundNumber num  scale      if    quot  quot    num  includes  quot e quot          return   Math round num    quot e  quot    scale      quot e- quot    scale       else       var arr     quot  quot    num  split  quot e quot        var sig    quot  quot      if  arr 1    scale  gt  0          sig    quot   quot             return   Math round  arr 0     quot e quot    sig     arr 1    scale      quot e- quot    scale          https   plnkr co edit uau8BlS1cqbvWPCHJeOy p preview NOTE  This is not a universal solution for everyone  There are several different rounding algorithms  your implementation can be different  depends on your requirements  https   en wikipedia org wiki Rounding Solution 2 is to avoid front end calculations and pull rounded values from the backend server  Edit  Another possible solution  which is not a bullet proof also  Math round  num   Number EPSILON    100    100  In some cases  when you round number like 1 3549999999999998 it will return incorrect result  Should be 1 35 but result is 1 36

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A slight variation on this is if you need to format a currency amount as either being a whole amount of currency or an amount with fractional currency parts   For example   1 should output  1  1 1 should output  1 10  1 01 should output  1 01  Assuming amount is a number   const formatAmount    amount    gt  amount   1     0   amount   amount toFixed 2    If amount is not a number then use parseFloat amount  to convert it to a number

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A different approach is to use a library  Why not lodash   const     require  lodash   const roundedNumber     round originalNumber  2

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The big challenge on this seemingly simple task is that we want it to yield psychologically expected results even if the input contains minimal rounding errors to start with  not mentioning the errors which will happen within our calculation   If we know that the real result is exactly 1 005  we expect that rounding to two digits yields 1 01  even if the 1 005 is the result of a large computation with loads of rounding errors on the way   The problem becomes even more obvious when dealing with floor   instead of round    For example  when cutting everything away after the last two digits behind the dot of 33 3  we would certainly not expect to get 33 29 as a result  but that is what happens    x000D   x000D  console log Math floor 33 3   100    100  x000D   x000D   x000D    In simple cases  the solution is to perform calculation on strings instead of floating point numbers  and thus avoid rounding errors completely  However  this option fails at the first non-trivial mathematical operation  including most divsions   and it is slow   When operating on floating point numbers  the solution is to introduce a parameter which names the amount by which we are willing to deviate from the actual computation result  in order to output the psychologically expected result    x000D   x000D  var round   function num  digits   2  compensateErrors   2    x000D    if  num  lt  0    x000D      return -this round -num  digits  compensateErrors   x000D      x000D    const pow   Math pow 10  digits   x000D    return  Math round num   pow    1   compensateErrors   Number EPSILON     pow   x000D    x000D   x000D     --- testing ---    x000D   x000D  console log  Edge cases mentioned in this thread    x000D  var values     0 015  1 005  5 555  156893 145  362 42499999999995  1 275  1 27499  1 2345678e 2  2 175  5 015  58 9   0 15    x000D  values forEach  n    gt    x000D    console log n     - gt      round n    x000D    console log -n     - gt      round -n    x000D      x000D   x000D  console log   nFor numbers which are so large that rounding cannot be performed anyway within computation precision  only string-based computation can help    x000D  console log  Standard      round 1e 19    x000D  console log  Compensation   1      round 1e 19  2  1    x000D  console log  Effectively no compensation      round 1e 19  2  0 4    x000D   x000D   x000D    Note  Internet Explorer does not know Number EPSILON  If you are in the unhappy position of still having to support it  you can use a shim  or just define the constant yourself for that specific browser family

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2017 Just use native code  toFixed    number   1 2345  number toFixed 2      1 23    If you need to be strict and add digits just if needed it can use replace  number   1      1  number toFixed 5  replace     0   g

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Here is a prototype method   Number prototype round   function places       places   Math pow 10  places        return Math round this   places  places     var yournum   10 55555  yournum   yournum round 2

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FINAL UPDATE  Leaving this answer here for posterity  but I would recommend using  AmrAli s adaptation of the DecimalPrecision function  as it is equipped to handle exponential notation as well  I had originally sought to avoid any string conversion manipulation of any kind for performance reasons  but there is virtually no difference in performance with his implementation  EDIT 8 22 2020  I think it should be clarified here that the goal of these efforts is not to completely eliminate the inherent rounding errors caused by the floating point data type  as that will never be possible without switching to a data type that is actually storing the value as a base10  decimal   The goal really should be to push the inaccuracies as far out to the edge as possible  so that you can perform mathematical operations on a given value without producing the bug  The moment your value hits the absolute edge  where simply invoking the value would cause JS to produce the bug  either before or after you ve manipulated it  there s nothing more you could do to alleviate that  For example  if you instantiate the value 0 014999999999999999  JS will immediately round it to 0 015  Therefore if you passed that value to any of these functions  you are actually passing 0 015  At that point you couldn t even convert to string first and then manipulate it  the value would have to be instantiated as a string from the start for that to work  The goal  and only reasonable expectation  of any functions created to alleviate this bug is simply to allow for mathematical operations to be performed on floating point values while pushing the bug all the way out to the edge where either the starting value  or resulting value would produce the bug simply by being invoked anyway  The only other alternative solutions would be to store the whole numbers and decimal values independently  both as integers  so that they are only ever invoked as such  or to always store the value as a string and use a combination of string manipulation and integer based math to perform operations on it  After running through various iterations of all the possible ways to achieve true accurate decimal rounding precision  it is clear that the most accurate and efficient solution is to use Number EPSILON  This provides a true mathematical solution to the problem of floating point math precision  It can be easily polyfilled as shown here  https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Number EPSILON to support all of the last remaining IE users  then again maybe we should stop doing that   Adapted from the solution provided here  https   stackoverflow com a 48850944 6910392 A simple drop in solution that provides accurate decimal rounding  flooring  and ceiling  with an optional precision variable without adding a whole library  05-19-2020 UPDATE  As Sergey noted in the comments  there is a limitation to this  or any  method that s worth pointing out  In the case of numbers like 0 014999999999999999  you will still experience inaccuracies which are the result of hitting the absolute edge of accuracy limitations for floating point value storage  There is no math or other solution that can be applied to account for that  as the value itself is immediately evaluated as 0 015  You can confirm this by simply invoking that value by itself in the console  Due to this limitation  it would not even be possible to use string manipulation to reduce this value  as its string representation is simply  quot 0 015 quot   Any solution to account for this would need to be applied logically at the source of the data before ever accepting the value into a script  eg restricting the character length of a field etc  That would be a consideration that would need to be taken into account on a case by case basis to determine the best approach  08-19-2020 UPDATE  Per Amr s comment  the ceil and floor functions will produce undesired results when the input value is an integer  This is due to the addition applied to the input with Number EPSILON in order to offset the expected float inaccuracy  The function has been updated to check if the input value is an integer and return the value unchanged  as that is the correct result of either function when applied to a whole number   Note  This issue also reveals that while the ceil and floor functions still require the application of the Number EPSILON adjustment  they do produce undesirable results when applied to a value where the number of decimals in the input number is lower than the number of decimals requested for the output  p   For example  ceil 17 1  5  should return 17 1 in relation to expected  quot ceil quot  function behavior when applied to whole numbers in mathematics  where all decimal places after  quot 1 quot  are assumed to be 0  To correct for this  I ve added an additional function check to identify if the number of decimals in the input number is lower than the requested output decimals  and return the number unchanged  just as with whole numbers   x000D   x000D  var DecimalPrecision    function            if  Number EPSILON     undefined                Number EPSILON   Math pow 2  -52                     if Number isInteger     undefined               Number isInteger   function value                    return typeof value      number   amp  amp                   isFinite value   amp  amp                   Math floor value      value                                   this isRound   function n p               let l   n toString   split      1  length              return  p  gt   l                     this round   function n  p 2               if Number isInteger n     this isRound n p                   return n              let r   0 5   Number EPSILON   n              let o   1  while p--  gt  0  o    10              if n lt 0                  o    -1              return Math round  n   r    o    o                    this ceil   function n  p 2               if Number isInteger n     this isRound n p                   return n              let r   0 5   Number EPSILON   n              let o   1  while p--  gt  0  o    10                           return Math ceil  n   r    o    o                    this floor   function n  p 2               if Number isInteger n     this isRound n p                   return n              let r   0 5   Number EPSILON   n              let o   1  while p--  gt  0  o    10                           return Math floor  n   r    o    o                    return this                console log DecimalPrecision round 1 005        console log DecimalPrecision ceil 1 005        console log DecimalPrecision floor 1 005        console log DecimalPrecision round 1 0049999        console log DecimalPrecision ceil 1 0049999        console log DecimalPrecision floor 1 0049999        console log DecimalPrecision round 2 175495134384 7        console log DecimalPrecision round 2 1753543549 8        console log DecimalPrecision round 2 1755465135353 4        console log DecimalPrecision ceil 17 4        console log DecimalPrecision ceil 17 1 4        console log DecimalPrecision ceil 17 1 15    x000D   x000D   x000D

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parseFloat  1 555   toFixed 2      Returns 1 55 instead of 1 56    1 55 is the absolute correct result  because there exists no exact representation of 1 555 in the computer  If reading 1 555 it is rounded to the nearest possible value   1 55499999999999994  64 bit float   And rounding this number by toFixed 2  results in 1 55   All other functions provided here give fault result  if the input is 1 55499999999999   Solution  Append the digit  5  before scanning to rounding up  more exact  rounding away from 0  the number  Do this only  if the number is really a float  has a decimal point    parseFloat  1 555   5   toFixed 2      Returns 1 56

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Here is a function I came up with to do  round up   I used double Math round to compensate for JavaScript s inaccurate multiplying  so 1 005 will be correctly rounded as 1 01   function myRound number  decimalplaces       if decimalplaces  gt  0           var multiply1   Math pow 10  decimalplaces   4            var divide1   Math pow 10  decimalplaces           return Math round Math round number   multiply1  10000   divide1            if decimalplaces  lt  0           var divide2   Math pow 10  Math abs decimalplaces            var multiply2   Math pow 10  Math abs decimalplaces            return Math round Math round number   divide2    multiply2             return Math round number

[javascript] Round to at most 2 decimal places (only if necessary)

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