undefined variable in jquery code

Question

I asked a question about the undefined value in javascript a few days ago. (What is the best way to compare a value against 'undefined'?) I conclude that it is a bad practice to do !== undefined as undefined can be set to 'another' value.

undefined='foo';
var b;
(b !== undefined) // true

I gave a quick look at the jquery code, I realized that in every part, the author use !== undefined and not typeof var !== "undefined"

// Setting one attribute
if ( value !== undefined ) {
// Optionally, function values get executed if exec is true
    exec = !pass && exec && jQuery.isFunction(value);

Is it a possible mistake? Even if I know that we should be crazy to reassign the value of undefined - for the most popular library I think it can cause some mistakes...

Who is in the right way?

*"I conclude that it is a bad practice to do `!== undefined` as `undefined` can be set..."* No, the bad practice is to set `undefined` to another value, and then to write code that accommodates that bad practice. — user113716, Aug 21 '11 at 21:03
@patrick if you have a safer way to check if a variable is not defined, why not using it? — JohnJohnGa, Aug 21 '11 at 21:14
Because I do not believe in writing cryptic code only to accommodate bad coding practices. The appropriate solution is to remove the offending code. Comparing to `undefined` is just fine, though I can see the benefit in creating a local `undefined` so that you don't need to traverse to the global every time you want to access it. To create a local `undefined`, I'd rather use `void`, which always returns `undefined`. `var undef = void 0;` But I'd only do that for performance purposes. — user113716, Aug 21 '11 at 21:22

score 12 · Accepted Answer · answered Aug 21 '11 at 20:55

12

undefined in the jQuery code is actually an undefined parameter of a function wrapping the whole code:

(function(window, undefined) {
    // jQuery code here
    // undefined is the undefined parameter
}(window)); // notice we don't pass a second argument here

That's perfectly safe, as the undefined parameter is local to the function, and nobody except the code in this function can assign to it.

Using a more clear syntax:

var myFunc = function(window, undefined) {
    // jQuery code here

    // The undefined variable is the undefined parameter

    // If the function has been called without a second argument,
    // then the undefined parameter is undefined.
};
myFunc(window); // no second argument

answered Aug 21 '11 at 20:55

Arnaud Le Blanc

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I am curions, if I set `undefined = 'foo'` before including jquery, does that mean `'foo'` gets passed in to the second parameter? – Amir Raminfar Aug 21 '11 at 21:01
no, the function would still be called without a second argument, so the value of this argument is *undefined* – Arnaud Le Blanc Aug 21 '11 at 21:02
oh nice I didn't notice that second parameter not being there. Thanks for clearing up. – Amir Raminfar Aug 21 '11 at 21:07
@Amir: `var a = 5; function test(a){alert("hello");}; test();` See how the parameter wouldn't be 5? – Stefan Kendall Aug 21 '11 at 21:09
@Stefan, I just didn't see the method call earlier. But thanks for providing an example. – Amir Raminfar Aug 21 '11 at 21:11
Thanks, any idea where it can ge useful to set a value to undefined ( = 'foo')? why javascript allow us to do that? – JohnJohnGa Aug 21 '11 at 21:18

jfriend00 · Answer 2 · 2011-08-21T21:21:54.127

1

If you reassign undefined to something else and you expect to use a large library, then you get what you deserve as far as I'm concerned. It's perfectly OK to test if a variable is undefined and jQuery needs to do that in many cases to tell which optional parameters are or aren't passed to various functions.

edited Aug 21 '11 at 21:21

answered Aug 21 '11 at 20:57

jfriend00

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undefined variable in jquery code

2 Answers2

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