javascript - If it’s bad practice to compare an expression against the variable `undefined`, why is this commonplace in jQuery c

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I asked a question about the undefined value in javascript a few days ago. (What is the best way to pare 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 asked a question about the undefined value in javascript a few days ago. (What is the best way to pare 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?

• javascript
• jquery

Share Improve this question Follow edited Dec 20, 2024 at 9:36 dumbass 27.2k44 gold badges3636 silver badges7373 bronze badges asked Aug 21, 2011 at 20:52 JohnJohnGaJohnJohnGa 15.7k2020 gold badges6464 silver badges8787 bronze badges 3

• "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 acmodates that bad practice. – user113716 Commented Aug 21, 2011 at 21:03
• @patrick if you have a safer way to check if a variable is not defined, why not using it? – JohnJohnGa Commented Aug 21, 2011 at 21:14
• Because I do not believe in writing cryptic code only to acmodate 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 Commented Aug 21, 2011 at 21:22

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2 Answers 2

Sorted by: Reset to default 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

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.

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