Differentiating between arrays and “hashes” in Javascript

Question

In order to make the syntax for one of my functions nicer, I need to be able to tell whether a specific parameter is an array or \"hash\" (which I know are just objects).

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Answer 1

You could check the length property as SLaks suggested, but as soon as you pass it a function object you'll be surprised, because it in fact has a length property. Also if the object has a length property defined, you'll get wrong result again.

Your best bet is probably:

function isArray(obj) {
    return Object.prototype.toString.call(obj) === "[object Array]";
}

jQuery uses it, and a "couple of" other people... :)

It is more fail proof than the instanceof way. The method is also suggested by the following article:

'instanceof' considered harmful (or how to write a robust 'isArray') (@kagax)

Another thing to add that this function is almost identical to the Array.isArray function in ES 5 spec:

15.4.3.2 Array.isArray ( arg )

1. If Type(arg) is not Object, return false.
2. If the value of the [[Class]] internal property of arg is "Array", then return true.
3. Return false.