How can I round down a number in Javascript

Question

How can I round down a number in JavaScript   math round   doesn t work because it rounds it to the nearest decimal    I m not sure if there is a better way of doing it other than breaking it apart at the decimal point at keeping the first bit  There must be

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Round towards negative infinity - Math floor    3 5   gt   3 0 -3 5   gt  -4 0  Round towards zero can be done using Math trunc    Older browsers do not support this function  If you need to support these  you can use Math ceil   for negative numbers and Math floor   for positive numbers   3 5   gt   3 0 using Math floor   -3 5   gt  -3 0 using Math ceil

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Rounding a number towards 0 can be done by subtracting its signed fractional part number   1   rounded   number - number   1    Like Math floor  rounds towards -Infinity  this method is perfectly accurate   There are differences in the handling of -0   Infinity and -Infinity though   Math floor -0    gt  -0 -0 - -0   1      gt   0  Math floor Infinity       gt  Infinity Infinity - Infinity   1   gt  NaN  Math floor -Infinity        gt  -Infinity -Infinity - -Infinity   1   gt  NaN

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To round down towards negative infinity  use   rounded Math floor number     To round down towards zero  if the number can round to a 32-bit integer between -2147483648 and 2147483647   use    rounded number 0    To round down towards zero  for any number   use   if number gt 0 rounded Math floor number  else rounded Math ceil number

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Math floor 1 7 8

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Here is math floor being used in a simple example  This might help a new developer to get an idea how to use it in a function and what it does  Hope it helps    lt script gt   var marks   0   function getRandomNumbers           generate a random number between 1  amp  10     var number   Math floor  Math random     10    1       return number     function getNew               This function can create a new problem by generating two random numbers  When the page is loading as the first time  this function is executed with the onload event and the onclick event of  new  button     document getElementById  ans   focus    var num1   getRandomNumbers    var num2   getRandomNumbers    document getElementById  num1   value   num1  document getElementById  num2   value   num2   document getElementById  ans   value      document getElementById  resultBox   style backgroundColor    maroon  document getElementById  resultBox   innerHTML             function checkAns           After entering the answer  the entered answer will be compared with the correct answer           If the answer is correct  the text of the result box should be  Correct  with a green background and 10 marks should be added to the total marks          If the answer is incorrect  the text of the result box should be  Incorrect  with a red background and 3 marks should be deducted from the total          The updated total marks should be always displayed at the total marks box      var num1   eval document getElementById  num1   value   var num2   eval document getElementById  num2   value   var answer   eval document getElementById  ans   value    if answer   num1 num2        marks   marks   10      document getElementById  resultBox   innerHTML    Correct       document getElementById  resultBox   style backgroundColor    green       document getElementById  totalMarks   innerHTML   Total marks       marks      else      marks   marks - 3      document getElementById  resultBox   innerHTML    Wrong       document getElementById  resultBox   style backgroundColor    red       document getElementById  totalMarks   innerHTML    Total Marks      marks             lt  script gt   lt  head gt    lt body onLoad  getNew    gt       lt div class  container  gt           lt h1 gt Let s add numbers lt  h1 gt           lt div class  sum  gt               lt input id  num1  type  text  readonly gt     lt input id  num2  type  text  readonly gt           lt  div gt           lt h2 gt Enter the answer below and click  Check  lt  h2 gt           lt div class  answer  gt               lt input id  ans  type  text  value    gt           lt  div gt           lt input id  btnchk  onClick  checkAns    type  button  value  Check   gt           lt div id  resultBox  gt     lt  div gt           lt input id  btnnew  onClick  getNew    type  button  value  New  gt           lt div id  totalMarks  gt Total marks   0 lt  div gt         lt  div gt   lt  body gt   lt  html gt

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This was the best solution I found that works reliably  function round value  decimals        return Number Math floor parseFloat value    e    decimals      e-    decimals       Credit to  Jack L Moore s blog

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Math floor   will work  but it s very slow compared to using a bitwise OR operation   var rounded   34 923   0  alert  rounded      alerts  34    EDIT Math floor   is not slower than using the   operator  Thanks to Jason S for checking my work   Here s the code I used to test   var a       var time   new Date   getTime    for  i   0  i  lt  100000  i             a push  Math random     100000    0        a push  Math floor  Math random     100000        var elapsed   new Date   getTime   - time  alert   elapsed time      elapsed

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You need to put -1 to round half down and after that multiply by -1 like the example down bellow    lt script type  text javascript  gt     function roundNumber number  precision  isDown        var factor   Math pow 10  precision       var tempNumber   number   factor      var roundedTempNumber   0      if  isDown          tempNumber   -tempNumber        roundedTempNumber   Math round tempNumber    -1        else         roundedTempNumber   Math round tempNumber             return roundedTempNumber   factor       lt  script gt    lt div class  col-sm-12  gt     lt p gt Round number 1 25 down   lt script gt document write roundNumber 1 25  1  true    lt  script gt     lt  p gt     lt p gt Round number 1 25 up   lt script gt document write roundNumber 1 25  1  false    lt  script gt  lt  p gt   lt  div gt

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You can try to use this function if you need to round down to a specific number of decimal places  function roundDown number  decimals        decimals   decimals    0      return   Math floor  number   Math pow 10  decimals      Math pow 10  decimals         examples  alert roundDown 999 999999       999 alert roundDown 999 999999  3       999 999 alert roundDown 999 999999  -1       990

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Using Math floor   is one way of doing this   More information  https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Math floor

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Was fiddling round with someone elses code today and found the following which seems rounds down as well   var dec   12 3453465  int   dec  gt  gt  0     returns 12   For more info on the Sign-propagating right shift      see MDN Bitwise Operators  It took me a while to work out what this was doing  D  But as highlighted above  Math floor   works and looks more readable in my opinion

[javascript] How can I round down a number in Javascript?

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