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Comment by ttty | Hacker News Reader

Which is better `undefinedVar === void 0` or `typeof undefinedVar === 'undefined'`?

I think minifiers use the former because it's shorter, and thus requires less space.

the other benefit is that `void 0` cannot be overwritten in sloppy mode, whereas you can do something crazy like this:

    undefined = 'lol';

No you can't. You can do something like function(undefined) {}, but the global 'undefined' is non-writable and non-configurable.

That's an ES5 change. With ES3, you can overwrite NaN, Infinity, and undefined.

Only in strict mode I though no ?

Generally. Technically, it's a "breaking change".

As you can imagine, this didn't actually affect anyone.

I was curious too, so here's a jsPerf:

On Chrome 35, looks like comparing to void 0 is a bit faster. Would love some more samples though.