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

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?

Solution

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