Question

Why is underscore.js’s isUndefined defined this way?

_.isUndefined = function(obj) {
return obj === void 0;
};

Why can’t this work?

typeof obj === 'undefined'

Answer 1

Ok, for a start typeof obj === 'undefined' is slower as you can easily verify.

The question then is why make the comparison

obj === void 0 

vs

obj === undefined

Let’s see:

void 0; returns the result of the unary operator void which will always return undefined (i.e. void 1 would make no difference)

undefined points to the global variable undefined.

Under normal circumstances the two are the same. I presume though void 0 is preferred because it is possible to shadow undefined with a local variable undefined 🙂 This is idiotic but it happens.