What does Object(this) do?

Question

I noticed in several MDN Array method shims, such as this one on line 7, that they use the following syntax:

var t = Object(this);

Where this is an array. Not only does it not hint in my validator, I’m also clueless as to what it does.

Can anyone shed a light?

Answer 1

As far as I can tell, the only use of it in there is to cover the case when you pass a string literal to Array.prototype.indexOf. If you remove it and pass a string in you get an error:

TypeError: Cannot use 'in' operator to search for '0' in string

However, by casting the string literal to an instance of String, you end up with an object with a numerical property for each character of the string, and since it's a normal object, you can use the in operator on it and the method will return the index of the character in the string:

Array.prototype.indexOf.call("hello", "e"); // Returns 1

Here's an example with the cast to Object, and here's an example without it.

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In more general cases, passing anything to the Object constructor will attempt to convert that thing to an object. So you can pass it a string literal and get an instance of String back, or pass it a numeric literal and get an instance of Number.