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"

> 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!