Anders Tornblad

All about the code

Formatting a truncated float in JavaScript

StackOverflow user nips asked how to format a floating point number in JavaScript, truncating the value instead of rounding it.

According to the question, this was the desired result:

  • The value 12.999 must be displayed as 12.99
  • The value 14 must be displayed as 14.00

The developer.mozilla.org page gives a thorough example of how to perform decimal rounding correctly, avoiding any rounding errors in the process. For this question, however, a simple multiplication-division pair is surely good enough:

// Step by step var multiplied = value * 100; var truncated = Math.floor(multiplied); var divided = truncated * 0.01; var output = divided.toFixed(2); // One-liner version var output = (Math.floor(value * 100) * 0.01).toFixed(2);

The above code performs the following steps, in order:

  • Value is multiplied by 100
    12.999 => 1299.9
    14 => 1400
  • Value is truncated using the Math.floor function
    1299.9 => 1299
    1400 => 1400
  • Value is multiplied by 0.01
    1299 => 12.99
    1400 => 14
  • Value is formatted for printing using two decimal places using the Number.prototype.toFixed function
    12.99 => "12.99"
    14 => "14.00"

Most languages do have functions called round, ceil, floor or similar ones, but almost all of them round to the nearest integer, so the multiply-round-divide chain (or divide-round-multiply for rounding to tens, hundreds, thousands...) is a good pattern to know.

In JavaScript there is no float datatype like in a lot of other languages, but the Number object is used for both integers and floating point numbers. Internally, numbers are just 64 bit floating point numbers (source: ecma262-6.com), conforming to the IEEE 754 standard. So when dealing with numbers in JavaScript, you always need to take floating point precision into consideration!

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