JavaScript’s parseInt() at the Extremes

3 minute read, published .

@adunkman look what I found today! years = parseInt(this.seconds / 31556926); can you guess what’s wrong? :)

— Scott Smerchek (@smerchek) January 31, 2014

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

@smerchek @adunkman Is this javascript? That small a number is represented in ‘e’ notation. :/

— Make the World Dance (@dustyburwell) January 31, 2014

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:

Just something to look out for!

@smerchek @adunkman The answer is “everything.” Everything is wrong with that.

— David Poeschl (@dpoeschl) January 31, 2014