JavaScript Decimal Math[Answered] RSS

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  joegreen2005

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  JavaScript Decimal Math

  Mar 04, 2021 05:10 PM|joegreen2005|LINK

  Hello,

  When I do 34.09 * 11, I get 374.99 but if I add 340.91 and 34.09 in JavaScript, I get 375.00 instead of 374.99.  Here is my code:

  <p id="demo"></p>

  var num = (parseFloat(340.90) + parseFloat(34.09));
  num = parseFloat(num).toFixed(2);
  document.getElementById("demo").innerHTML = n;
  

  Why JavaScript is rounding 374.99 to 375. What do I need to do to get 374.99?

  Joe

• • Reply
  mgebhard

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  Re: JavaScript Decimal Math

  Mar 04, 2021 06:42 PM|mgebhard|LINK

  joegreen2005

  Hello,

  When I do 34.09 * 11, I get 374.99 but if I add 340.91 and 34.09 in JavaScript, I get 375.00 instead of 374.99.  Here is my code:

  <p id="demo"></p>

  var num = (parseFloat(340.91) + parseFloat(34.09));
  num = parseFloat(num).toFixed(2);
  document.getElementById("demo").innerHTML = n;

  Why JavaScript is rounding 374.99 to 375. What do I need to do to get 374.99?

  Joe

  Your math is wrong.  

  340.91 + 34.09  = 375

  Maybe you mean

  340.91 + 34.08 = 374.99

  Plus your code has a bug.

• • Reply
  joegreen2005

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  Re: JavaScript Decimal Math

  Mar 04, 2021 07:27 PM|joegreen2005|LINK

  The error is happening when I add 34.09 + 34.09 + 34.09 + 34.09 + 34.09,  I get 170.455 and when I do 170.455.toFixed(2) I get 170.46 which should be 170.45 and not 170.46.

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  bruce (sqlwo...

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  Re: JavaScript Decimal Math

  Mar 04, 2021 09:22 PM|bruce (sqlwork.com)|LINK

  javascript does not do decimal arithmetic. all numbers are 64 bit floats. floats (because they are binary rather than decimal) can not be used for money, as you will get precision errors. this is usually covered in comp sci 101, so I assume you don't have a formal edition in computer science.

  just like 1/3 is .33333.. in decimal, several decimal number are repeating binaries.   see:

    https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

  the typical solution to calculating money with floats is to use a fixed precision library (like C# money) or do all the calculations in pennies (or mills if more precision required) and divide by 100 (or 1000 if mills) 

    > (3409 + 3409 + 3409 + 3409 + 3409) / 100 
    < 170.45

  you can do a simple javascript math package:

  const pennyMath = { 
      toPenny: (d) => Math.round(d * 100), 
      add: (d1,d2) => (pennyMath.toPenny(d1) + pennyMath.toPenny(d2)) / 100, 
      sub: (d1,d2) => (pennyMath.toPenny(d1) - pennyMath.toPenny(d2)) / 100, 
      mul: (d1,d2) => Math.round(pennyMath.toPenny(d1) * pennyMath.toPenny(d2)) / 100, 
      div: (d1,d2) => Math.round(pennyMath.toPenny(d1) / pennyMath.toPenny(d2)) / 100, 
      sum: (...d) => d.reduce((a,c) => pennyMath.toPenny(c) + a,0) / 100
  };

  then:

    pennyMath.sum(34.09 , 34.09 , 34.09 , 34.09 , 34.09)

  gives 170.45

  
  
        

  bruce (sqlwork.com)
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  PaulTheSmith

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  Re: JavaScript Decimal Math

  Mar 04, 2021 10:28 PM|PaulTheSmith|LINK

  toFixed rounds to the number of decimal places requested. 170.455 when rounded to two decimal places id 170.46.

  One way to truncate to two decimal places would be to multiply by 100, take the .floor (which finds the largest integer smaller than or equal to the argument) and then divide by 100. (If you want three decimal places then use 1,000 instead of 100, etc)

  const truncated = (Math.floor(myNum * 100) / 100).toString();

  (What do you want to do with negative numbers?  The above will display -4.567 as -4.57)