Question

Is it unsafe to compare a variable against undefined?

if(foo == undefined)

vs

if(foo == 'undefined')

Is the first example sufficient? Or should it be something like

if('undefined' in window){
     //compare against undefined
} else {
    //compare against 'undefined'
}

since undefined exists on the window object? Will it exist in all browsers? Or should I simply compare to == 'undefined'? I’ve found some similar questions on SO, but no answers regarding the existance of the undefined property on the window object.

Answer 1

I think you’re getting mixed up between foo == undefined and typeof foo == "undefined".

Both will yield the same result unless the variable undefined has been set to something else in the current scope. In this case, foo == undefined will compare against that, where-as typeof foo == "undefined" will still resolve correctly.

var undefined = 4;
var reallyUndefined;

reallyUndefined == undefined; // false
typeof reallyUndefined == undefined; // true

Whether it’s a real world scenario that undefined will ever be set to something else is debatable, and I’d question the validity of the library/ code that does that… Because of this however, it’s deemed good practise to always use typeof foo === "undefined".

I’d also be wary about using foo == undefined against foo === undefined (note the triple equals, which does not use type-coercian, compared to == which does).

Using ==, you run the risk of things like null == undefined; // true, where-as null === undefined; // false. This is a good example of why you should always use ===.

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tl;dr: typeof foo === "undefined";