CODEWITHSUNDEEP
javascript
exebook
exebook

Reputation: 33900

JavaScript: how to protect from "undefined" is defined confusion?

In JavaScript undefined is not a keyword, it is a variable. Sort of special variable. This is known to cause trouble because undefined could be reassigned. I do not really know why would anyone want to do that, but this is possible, and maybe you can experience this by using one of those genius frameworks out there, see the code:

function f() {
    var z, undefined = 42
    console.log(z, undefined, z == undefined)
} f()

Outputs:

undefined 42 false

Now how does one protect himself from such a confusion? Should one just cross his fingers that undefined is undefined?

Upvotes: 0

Views: 570

Answers (3)

aljordan82
aljordan82

Reputation: 1720

You can pass undefined as a function parameter, this will ensure that undefined is undefined in the scope of the function.

Many JavaScript libraries use this technique, for example look at jQuery source code

//jQuery
(function( window, undefined ) {
...
})( window );

Because the function expects two formal parameters, but we only give it one, the other gets the (true) value undefined, and then we can rely on that within the function.

Upvotes: 3

sailingthoms
sailingthoms

Reputation: 938

one possibility is to check the type of the variable instead of checking equality to undefied:

if (typeof(VARIABLE) != "undefined")

or read on here: How to check for “undefined” in JavaScript?.

Upvotes: 0

Esailija
Esailija

Reputation: 140220

Just use void 0, it's simply bulletproof. And it's a keyword too.

Upvotes: 4

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