2 minute read

JavaScript’s parseInt() at the extremes

Misuse of parseInt in JavaScript can lead to weird behavior if the number being parsed is large or small enough to be represented in E notation.

I received that tweet from an ex-coworker of mine this morning. Well, can you guess what’s wrong?

It has to do with how JavaScript represents extremely small and extremely large numbers: by using E notation.

35 / 31556926 = 0.0000011091067615394477
32 / 31556926 = 0.0000010140404676932093
31 / 31556926 = 9.823517030777965e-7

Because parseInt() is a function for converting strings to integers, it first converts the number to a string, and then an integer.

var number = 31 / 31556926;
> 9.823517030777965e-7

var str = number.toString()
> "9.823517030777965e-7"

parseInt(str);
> 9

So, how do we fix it? Well, stop using parseInt(). We should be using Math.round() here, which doesn’t convert the number to a string first.

Additionally, when using parseInt() a radix should always be supplied as a second argument, becuase it doesn’t always assume you’re using base-10. From MDN:

  • If the input string begins with “0x” or “0X”, radix is 16 (hexadecimal) and the remainder of the string is parsed.

  • If the input string begins with “0”, radix is eight (octal) or 10 (decimal). Exactly which radix is chosen is implementation-dependent. ECMAScript 5 specifies that 10 (decimal) is used, but not all browsers support this yet.

  • If the input string begins with any other value, the radix is 10 (decimal).

Just something to look out for!