Saturday, October 13, 2007

Type-Converting Operator Considered Ridiculous

I have come to question the wisdom of the type-converting == operator in JavaScript. The algorithm (defined in §11.9.3 of the ECMA-262 v3 spec) requires no fewer than 22 steps to execute, and produces such amazing intrasitivities as:

>>> '0' == 0
true

>>> 0 == ''
true

>>> '' == '0'
false

and random quirkiness such as:

>>> ' \t\r\n' == 0
true

but:

>>> null == 0
false

>>> null == false
false

>>> null == ''
false

>>> null == undefined
true

Do these behaviors strike anyone else as, well, a bit off? In the first example (discussed elsewhere) the intransitivity of the equality operator is a result of wanting an equality operator to play well within a weakly-typed language. If you want to be able to compare numbers to their string representations as though they were of the same type, while still preserving string comparison, this is the logical result. The second example is a result of the ToNumber algorithm, described in §9.3ff. The whitespace conversion to number value 0 is strange, but defined in the spec (in fact, it's a result of the same algorithm that dictates Number(' ') => 0). The third example seems bizarre as well; I'm not sure why null would be treated differently than 0 or false, but the same as undefined, in such expressions. Many articles from otherwise reputable sources often describe all of these values in terms of "truthiness" and "falsiness", but these articles sometimes gloss over these distinctions.

I don't condone transitioning JS into a strongly-typed system, but some of these behaviors are quite subtle. Given the fact that the vast majority of JavaScript programmers seem to be unaware of the strict-equality operator === (and its negative counterpart, !==), it's no wonder behaviors such as these can cause confusion. See also this article for even more pitfalls, this time simply with math operators combining string and numeric types.

So as Crockford says, use the strict comparison operator === unless you know what you're doing.

1 comment:

CresceNet said...
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