Explain the difference between void 0 and undefined

I'm learning js Library or framework , I found that it was used in many places void 0, I studied it carefully .

void Operator

Evaluate a given expression , Then return undefined.
void Operators are usually used only to get undefined Original value , In general use void(0)（ Equate to void 0）. In the above case , You can also use global variables undefined Instead of .— Excerpt from MDN

let me put it another way void 0 and undefined It is equivalent. . as follows ：



volid The following cannot be followed by undeclared variables , Otherwise, an error will be reported .
void+xxx All are undefined, So why are some tool Libraries 0 Well , Maybe for convenience .

void 0 And undefined

undefined Is an attribute of a global object . in other words , It is a variable in the global scope .undefined The initial value of is the original data type undefined;
In modern browsers （JavaScript 1.8.5/Firefox 4+）, since ECMAscript5 Since the standard undefined One cannot be configured （non-configurable）, Can't be rewritten （non-writable） Properties of . Even if this is not the case , Also avoid rewriting it . But in the local scope , It can be rewritten .

//  Don't do this ！

//  Print  'foo string' PS： explain undefined The value and type of have changed 
(function() {
var undefined = 'foo';
console.log(undefined, typeof undefined)
})()

//  Print  'foo string' PS： explain undefined The value and type of have changed 
(function(undefined) {
console.log(undefined, typeof undefined)
})('foo')

undefined It could be rewritten ,void 0 Will not be rewritten .
and void 0 No matter when it comes back undefined, Like to see , Use void 0 Instead of undefined It's safer , No mistakes .