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Ben Nadel at the jQuery Conference 2009 (Cambridge, MA) with: Scott Gonzalez
Ben Nadel at the jQuery Conference 2009 (Cambridge, MA) with: Scott Gonzalez ( @scott_gonzalez )

Javascript Objects Cannot Have Complex Index Values

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This was just a shot in the dark, but I wanted to see if Javascript Objects could be indexed by other objects rather than just strings (yes I know Strings are technically objects too). I wanted to see if something along these lines would work:

var objIndex = {
	DOMElement: { ... },
	DOMElement: { ... },
	DOMElement: { ... },
	... etc ...
	};

Basically, I wanted to see if Javascript objects could act like Java HashTables that can take any type of object as an index value.

After some quick testing, I realized that this did not work. At first, though, I thought it might because no errors were thrown. Javascript actually lets you do this; but, not in the I wanted - what it actually does (from what I can gather) is that it creates an index value in the following manner:

var objIndex = {
	DOMElement: { ... }
	};

... becomes:

var objIndex = {
	DOMElement.toString(): { ... }
	};

I believe that Javascript objects requires string-based indexing and therefore takes any object that you pass to it (to be used as an index value) and converts it to a string. Part of the problem is that this string version of the object is rather generic. The P and SPAN tags convert to the following regardless of markup (in FireFox):

[object HTMLParagraphElement]
[object HTMLSpanElement]

Therefore, all P and SPAN tags would be indexed using the same values (respectively) no matter how many there were in the page.

Anyway, it was just a thought.

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Reader Comments

2 Comments

If you override an object's toString() method, then you can control how JavaScript sets the key.

domElement.toString = function() {
return this.id;
};

Sadly, IE doesn't let you override for DOM objects. But this works for custom objects in all browsers.

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Ben Nadel