Truncate not round off decimal numbers in javascript

Question

I am trying to truncate decimal numbers to decimal places  Something like this   5 467   - gt  5 46   985 943 - gt  985 94   toFixed 2  does just  about the right thing but it rounds off the value  I don t need the value rounded off  Hope this is possible in javascript

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upd   So  after all it turned out  rounding bugs will always haunt you  no matter how hard you try to compensate them  Hence the problem should be attacked by representing numbers exactly in decimal notation   Number prototype toFixedDown   function digits        var re   new RegExp     d      d     digits         d             m   this toString   match re       return m   parseFloat m 1     this valueOf            5 467 toFixedDown 2       985 943 toFixedDown 2       17 56 toFixedDown 2        0  toFixedDown 1       1 11 toFixedDown 1    22        5 46  985 94  17 56  0  23 1    Old error-prone solution based on compilation of others    Number prototype toFixedDown   function digits      var n   this - Math pow 10  -digits  2    n    n   Math pow 2  53      added 1360765523  17 56 toFixedDown 2       17 56    return n toFixed digits

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The resulting type remains a number          Return the truncation of n wrt base    var trunc   function n  base        n    n   base    0      return base   n     var t   trunc 5 467  0 01

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Here is simple but working function to truncate number upto 2 decimal places              function truncateNumber num                    var num1                       var num2                       var num1   num split      0                   num2   num split      1                   var decimalNum   num2 substring 0  2                   var strNum   num1       decimalNum                  var finalNum   parseFloat strNum                   return finalNum

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just to point out a simple solution that worked for me  convert it to string and then regex it     var number   123 45678  var number s        number  var number truncated s   number s match   d    d 4    0  var number truncated   parseFloat number truncated s    It can be abbreviated to  var number truncated   parseFloat       123 4568908  match   d    d 4    0

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Dogbert s answer can be improved with Math trunc  which truncates instead of rounding      There is a difference between rounding and truncating  Truncating is   clearly the behaviour this question is seeking  If I call   truncate -3 14  and receive -4 back  I would definitely call that   undesirable       NickKnowlson    var a   5 467  var truncated   Math trunc a   100    100       5 46     var a   -5 467  var truncated   Math trunc a   100    100       -5 46

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I think this function could be a simple solution    x000D   x000D  function trunc decimal n 2   x000D    let x   decimal          string  x000D    return x lastIndexOf      gt  0 parseFloat x substr 0 x lastIndexOf       n 1    decimal     You can use indexOf   instead of lastIndexOf   x000D    x000D   x000D  console log trunc -241 31234 2    x000D  console log trunc 241 312 5    x000D  console log trunc -241 233    x000D  console log trunc 241 2 0      x000D  console log trunc 241    x000D   x000D   x000D

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Here is what I use   var t   1  for  var i   0  i  lt  decimalPrecision  i        t   t   10   var f   parseFloat value   return  Math floor f   t     t

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var a   5 467  var truncated   Math floor a   100    100       5 46

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You can work with strings  It Checks if     exists  and then removes part of string   truncate  7 88  1     --      7 8  truncate  7 889  2    --         7 89  truncate  -7 88  1       --      -7 88  function  truncate number  decimals        const tmp   number           if  tmp indexOf       gt  -1            return  tmp substr 0   tmp indexOf        decimals 1          else           return  number

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Lodash has a few Math utility methods that can round  floor  and ceil a number to a given decimal precision  This leaves off trailing zeroes   They take an interesting approach  using the exponent of a number  Apparently this avoids rounding issues    Note  func is Math round or ceil or floor in the code below      Shift with exponential notation to avoid floating-point issues  var pair    toString number     e   split  e        value   func pair 0     e      pair 1    precision     pair    toString value     e   split  e    return   pair 0     e      pair 1  - precision      Link to the source code

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You can fix the rounding by subtracting 0 5 for toFixed  e g    f - 0 005  toFixed 2

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Truncate using bitwise operators     0 5     0    -0 5      0   14 32794823     14    -439 93      -439

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Number prototype trim   function decimals        var s   this toString        var d   s split           d 1    d 1  substring 0  decimals       return parseFloat d join           console log  5 676  trim 2      logs 5 67

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Consider taking advantage of the double tilde       Take in the number   Multiply by significant digits after the decimal so that you can truncate to zero places with      Divide that multiplier back out   Profit   function truncator numToTruncate  intDecimalPlaces            var numPower   Math pow 10  intDecimalPlaces       numPowerConverter  might be better     return    numToTruncate   numPower  numPower      I m trying to resist wrapping the    call in parens  order of operations should make that work correctly  I believe   alert truncator 5 1231231  1       is 5 1  alert truncator -5 73  1       is -5 7  alert truncator -5 73  0       is -5  JSFiddle link   EDIT  Looking back over  I ve unintentionally also handled cases to round off left of the decimal as well   alert truncator 4343 123  -2       gives 4300   The logic s a little wacky looking for that usage  and may benefit from a quick refactor   But it still works   Better lucky than good

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Here is an ES6 code which does what you want   x000D   x000D  const truncateTo    unRouned  nrOfDecimals   2    gt    x000D        const parts   String unRouned  split       x000D   x000D        if  parts length     2    x000D               without any decimal part x000D          return unRouned  x000D          x000D   x000D        const newDecimals   parts 1  slice 0  nrOfDecimals   x000D          newString      parts 0     newDecimals    x000D   x000D        return Number newString   x000D         x000D   x000D     your examples  x000D   x000D   console log truncateTo 5 467       --- gt  5 46 x000D   x000D   console log truncateTo 985 943       --- gt  985 94 x000D   x000D     other examples  x000D   x000D   console log truncateTo 5       --- gt  5 x000D   x000D   console log truncateTo -5       --- gt  -5 x000D   x000D   console log truncateTo -985 943       --- gt  -985 94 x000D   x000D   x000D

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The answer by  kirilloid seems to be the correct answer  however  the main code needs to be updated  His solution doesn t take care of negative numbers  which someone did mention in the comment section but has not been updated in the main code    Updating that to a complete final tested solution   Number prototype toFixedDown   function digits        var re   new RegExp    -    d      d     digits         d         m   this toString   match re       return m   parseFloat m 1     this valueOf         Sample Usage   var x   3 1415629  Logger log x toFixedDown 2      or use whatever you use to log   Fiddle  JS Number Round down  PS  Not enough repo to comment on that solution

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Number prototype truncate   function places      var shift   Math pow 10  places      return Math trunc this   shift    shift

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Here my take on the subject   convert truncate   function value  decimals      decimals    decimals     undefined   0   decimals     return parseFloat  value- 0 5 Math pow 10  decimals    toFixed decimals  10        It s just a slightly more elaborate version of   f - 0 005  toFixed 2

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I wrote an answer using a shorter method  Here is what I came up with  function truncate value  precision        var step   Math pow 10  precision    0       var temp   Math trunc step   value        return temp   step      The method can be used like so  truncate 132456 25456789  5       Output  132456 25456 truncate 132456 25456789  3       Output  132456 254 truncate 132456 25456789  1       Output  132456 2    truncate 132456 25456789          Output  132456   Or  if you want a shorter syntax  here you go  function truncate v  p        var s   Math pow 10  p    0       return Math trunc s   v    s

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I m a bit confused as to why there are so many different answers to such a fundamentally simple question  there are only two approaches which I saw which seemed to be worth looking at  I did a quick benchmark to see the speed difference using https   jsbench me   This is the solution which is currently  9 26 2020  flagged as the answer   x000D   x000D  function truncate n  digits        var re   new RegExp     d      d     digits         d             m   n toString   match re       return m   parseFloat m 1     n valueOf            truncate 5 467 2       truncate 985 943 2       truncate 17 56 2       truncate 0  1       truncate 1 11  1    22   x000D   x000D   x000D   However  this is doing string and regex stuff  which is usually not very efficient  and there is a Math trunc function which does exactly what the OP wants just with no decimals  Therefore  you can easily use that plus a little extra arithmetic to get the same thing  Here is another solution I found on this thread  which is the one I would use   x000D   x000D  function truncate n  digits        var step   Math pow 10  digits    0       var temp   Math trunc step   n        return temp   step         truncate 5 467 2       truncate 985 943 2       truncate 17 56 2       truncate 0  1       truncate 1 11  1    22        x000D   x000D   x000D   The first method is  quot 99 92  slower quot  than the second  so the second is definitely the one I would recommend using  Okay  back to finding other ways to avoid work

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Nice one-line solution   function truncate  num  places      return Math trunc num   Math pow 10  places     Math pow 10  places       Then call it with   truncate 3 5636232  2      returns 3 56 truncate 5 4332312  3      returns 5 433 truncate 25 463214  4      returns 25 4632

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Dogbert s answer is good  but if your code might have to deal with negative numbers  Math floor by itself may give unexpected results   E g  Math floor 4 3    4  but Math floor -4 3    -5  Use a helper function like this one instead to get consistent results   truncateDecimals   function  number        return Math number  lt  0    ceil     floor   number          Applied to Dogbert s answer  var a   5 467  var truncated   truncateDecimals a   100    100       5 46   Here s a more convenient version of this function   truncateDecimals   function  number  digits        var multiplier   Math pow 10  digits           adjustedNum   number   multiplier          truncatedNum   Math adjustedNum  lt  0    ceil     floor   adjustedNum        return truncatedNum   multiplier         Usage  var a   5 467  var truncated   truncateDecimals a  2        5 46     Negative digits  var b   4235 24  var truncated   truncateDecimals b  -2        4200   If that isn t desired behaviour  insert a call to Math abs on the first line   var multiplier   Math pow 10  Math abs digits      EDIT  shendz correctly points out that using this solution with a   17 56 will incorrectly produce 17 55  For more about why this happens  read What Every Computer Scientist Should Know About Floating-Point Arithmetic  Unfortunately  writing a solution that eliminates all sources of floating-point error is pretty tricky with javascript  In another language you d use integers or maybe a Decimal type  but with javascript     This solution should be 100  accurate  but it will also be slower   function truncateDecimals  num  digits        var numS   num toString            decPos   numS indexOf               substrLength   decPos    -1   numS length   1   decPos   digits          trimmedResult   numS substr 0  substrLength           finalResult   isNaN trimmedResult    0   trimmedResult       return parseFloat finalResult       For those who need speed but also want to avoid floating-point errors  try something like BigDecimal js  You can find other javascript BigDecimal libraries in this SO question   Is there a good Javascript BigDecimal library   and here s a good blog post about math libraries for Javascript

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I thought I d throw in an answer using   since it is simple and works well   truncate   function number  places      var shift   Math pow 10  places      return   number   shift    0    shift

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function toFixed number  digits        var reg ex   new RegExp     d      d     digits         d        var array   number toString   match reg ex       return array   parseFloat array 1     number valueOf      var test   10 123456789 var   fixed   toFixed test  6  console log   fixed       gt  10 123456

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The one that is mark as the solution is the better solution I been found until today  but has a serious problem with 0  for example  0 toFixedDown 2  gives -0 01   So I suggest to use this   Number prototype toFixedDown   function digits      if this    0        return 0        var n   this - Math pow 10  -digits  2    n    n   Math pow 2  53      added 1360765523  17 56 toFixedDown 2       17 56    return n toFixed digits

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I found a problem  considering the next situation  2 1 or 1 2 or -6 4  What if you want always 3 decimals or two or wharever  so  you have to complete the leading zeros to the right     3 decimals numbers 0 5   gt  0 500     6 decimals 0 1   gt  0 10000     4 decimales -2 1   gt  -2 1000     truncate to 3 decimals 3 11568   gt  3 115   This is the fixed function of Nick Knowlson  function truncateDecimals  num  digits         var numS   num toString        var decPos   numS indexOf           var substrLength   decPos    -1   numS length   1   decPos   digits      var trimmedResult   numS substr 0  substrLength       var finalResult   isNaN trimmedResult    0   trimmedResult          adds leading zeros to the right     if  decPos    -1           var s   trimmedResult             decPos   s indexOf               var decLength   s length - decPos               while  decLength  lt   digits                   s   s    0                   decPos   s indexOf                       decLength   s length - decPos                  substrLength   decPos    -1   s length   1   decPos   digits                         finalResult   s            return finalResult       https   jsfiddle net huttn155 7

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const TO FIXED MAX   100   function truncate number  decimalsPrecison         make it a string with precision 1e-100   number   number toFixed TO FIXED MAX         chop off uneccessary digits   const dotIndex   number indexOf         number   number substring 0  dotIndex   decimalsPrecison   1         back to a number data type  app specific    return Number parseFloat number         example truncate 0 00000001999  8   0 00000001   works with    negative numbers very small numbers  Number EPSILON precision

[javascript] Truncate (not round off) decimal numbers in javascript

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