Here are some interesting JavaScript facts that I’ve encountered over the last few years.

Function.length

Call Function.length will return the number of arguments a function is expecting.

The spread operator will not be included in the count.

Array.map(func)

when you call Array.map(func), the mapped function gets called with 3 arguments, not just the value.

So for:

["10", "11", "12"].map(parseInt);

You’d expect to get

[10, 11, 12];

but in reality get

[10, NaN, 1];

This is because .map(parseInt) calls the function with three arguments - (currentValue, index, array). This is normally not an issue, but becomes an issue when the mapped function takes additional arguments that do not correspond to the ones being passed in.

parseInt takes in two arguments - value, [, radix], and thus tries to parse 11 with radix 1, which is NaN

Values are truth-y by default

The only falsey values are:

[0, -0, 0n, "", "", null, undefined, NaN, false];

Everything else is truthy - including [], an empty Set(), and an empty object.

Null comparisons to 0

I ran into a nasty bug once where a value I thought was guaranteed to be a number was actually explicitly set to null. I was doing a comparison with 0 and ran into this weird behavior:

Array.sort sorts by string sequence code

Call .sort() on an array of numbers will not sort them numerically. Which is perplexing

Null is not equal to zero, and is not greater than zero, but is greater than or equal to zero.

String.replace() only replaces the first instance of a match

It’s almost a rite of passage for javascript developers to be bewildered that String.replace only replaces the first instance of a match in a string.

Thankfully in ES2021 we now have String.replaceAll, which behaves as you’d expect.

WTFJS

Someone also pointed out WTFJS.com which is a site that has a lot more javascript oddities and examples.