typeof !== "undefined" vs. != null

Question

I often see JavaScript code which checks for undefined parameters etc. this way:

if (typeof input !== "undefined") {     // do stuff } 

This seems kind of wasteful, since it involves both a type lookup and a string comparison, not to mention its verbosity. It's needed because undefined could be renamed, though.

My question is:
How is that code any better than this approach:

if (null != input) {     // do stuff } 

As far as I know, you can't redefine null, so it's not going to break unexpectedly. And, because of the type-coercion of the != operator, this checks for both undefined and null... which is often exactly what you want (e.g. for optional function parameters).

Yet this form does not seem widespread, and it even causes JSLint to yell at you for using the evil != operator.

Why is this considered bad style?

Answer 1

typeof is safer as it allows the identifier to never have been declared before:

if(typeof neverDeclared === "undefined") // no errors  if(neverDeclared === null) // throws ReferenceError: neverDeclared is not defined 

Answer 2

If the variable is declared (either with the var keyword, as a function argument, or as a global variable), I think the best way to do it is:

if (my_variable === undefined) 

jQuery does it, so it's good enough for me :-)

Otherwise, you'll have to use typeof to avoid a ReferenceError.

If you expect undefined to be redefined, you could wrap your code like this:

(function(undefined){     // undefined is now what it's supposed to be })(); 

Or obtain it via the void operator:

const undefined = void 0; // also safe