The undefined variable in JavaScript is actually a property of the global object. As such, in ECMAScript 3 it was possible to overwrite the value of undefined. While ECMAScript 5 disallows overwriting undefined, it's still possible to shadow undefined, such as:

function doSomething(data) {
    var undefined = "hi";

    // doesn't do what you think it does
    if (data === undefined) {
        // ...
    }
}

Because undefined can be overwritten or shadowed, reading undefined can give an unexpected value. (This is not the case for null, which is a keyword that always produces the same value.) To guard against this, you can avoid all uses of undefined.

This ensures that undefined will always hold its original, expected value.

Bad Practice

var foo = undefined;

var undefined = "foo";

if (foo === undefined) {
    // ...
}

function foo(undefined) {
    // ...
}

Recommended

var foo = void 0;

var Undefined = "foo";

if (typeof foo === "undefined") {
    // ...
}

global.undefined = "foo";