Use void 0 rather than undefined Back

If you look through the whole source code, you'll never find any undefiend, but only one string "undefined", why?

Firstly, undefined is not a reversed word, and it's only a attribute of all global objects, which can be override in low edition of IE.

var undefined = 10;
console.log(undefined); /**
                         * => undefined (Chrome)
                         * => 10        (IE 8)
                         */

In ES5, undefined has been designed as a read-only attributes, which can't be overridden. However, you will find that you can do it in a local scope:

(function () {
    var undefined = 10;
    console.log(undefined); /**
                             * => 10 (Chrome)
                             */
})();

(function () {
    undefined = 10;
    console.log(undefined); /**
                             * => undefined (Chrome)
                             */
})();

Then, why use the replacement void 0?

What is void in MDN statemet?

The void operator evaluates the given expression and then returns undefined.

That means void will always return undefined, which can completely take the place of undefined. Actually, void 0, void 1 and void "hello world" are all same. The most important thing is that void can't be overridden.

Although undefined can generally be trusted in modern JavaScript environments, there is one trivial advantage of void 0: it's shorter. The difference is not enough to worry about when writing code but it can add up enough over large code bases that most code minifiers replace undefined with void 0 to reduce the number of bytes sent to the browser.

However, the disadvantages of using void 0 is that it has reduced readability of code, and it's recommended to avoid using both void 0 and undefined in our code, despite of the special situation when writing minifier tools, or frameworks.