Question

I have just run a very simple JavaScript performance test (don’t ask why). The test declares a variable, but doesn’t assign anything to it:

var x;

It then compares the speed of comparing the value variable to null, and to undefined, in other words:

var y = (x == null); and var y = (x == undefined);.

I was expecting the comparison with undefined to be the fasted. In fact it was nowhere near. The comparison with null was far and away the fastest, around 80% faster.

The results I’ve described above come from running the tests in Chrome (version 13). Running them in Firefox produces results far closer to what I would have expected (the comparison with undefined is faster than with null, albeit very marginally).

So, my question is what could the cause of this be? Why does Chrome seem to favour the comparison with null so greatly?

For quick reference, here’s a screenshot of the results:

Answer 1

null is a reserved keyword which cannot be overriden, so when you are doing a comparison against null, all you have to do is a single comparison.

However, when you are checking against undefined, the engine must do a type lookup and then a comparison, meaning that it is actually slightly more demanding.

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If you need to actually check to see if something is undefined, you should use

if(typeof notSet == "undefined"){ }

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Proof

Try it… and set something to null in your JavaScript console.

null = "will error";
// Errors with --> ReferenceError: invalid assignment left-hand side

However, if you try and do it with undefined, it won’t error. That is not to say that you can override undefined, because you can’t, but that undefined is its own primitive type.

The only real similarity between null and undefined, is that they can both be coerced into a boolean false.