Question

I’m trying to cleanup some code (fix jshint warnings).

What’s the best replacement for

if (x == undefined) {...}

?

I’m using underscore.js, is the following equivalent:

if (x === null || _.isUndefined(x)) {...}

Or am I missing some edge cases?
What is equal (==) to undefined? (assuming nobody did var undefined = 'asdf' or anything evil like that).

Answer 1

With x == undefined you actually check for two values of x – undefined itself and null. Take note, though, that absence of declared x variable will trigger an error here; to prevent this, use…

if (typeof x === 'undefined' || x === null) { ... }

But I actually think the true meaning of this warning is that you shouldn’t check for both null and undefined in one branch of code, as these are different values.

Yet if you feel safe about this, you can turn off these particular checks in JSHint, with eqnull option. Then…

if (x == null) { ... }

… will check both for null and undefined, yet won’t trigger the warning. Note, though, that == undefined comparisons are not ‘covered’ by this option.

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As a sidenote, underscore’s isUndefined method is an interesting piece of code:

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

It’s essentially the same as simple obj === undefined, yet covers one special case: when someone decided that it’s a good idea to overwrite the value of undefined in their script. As void construct always returns “the real undefined“, this function will work correctly even in this situation.