What is "void 0"?

Question

I'm learning Javascript by reading some code, but this function really puzzles me.

    hv:
        function(i) {
            var _this = this;
            return isArr(_this) ? _this.indexOf(i) > -1 : _this[i] !== void 0;
        }

This function is added to Object.prototype.
I don't quite get the void 0 at the end of the ternary expression. Can someone explain it to me?

Thanks.

See the answer here [void 0 stackoverflorw](http://stackoverflow.com/questions/1291942/what-does-javascriptvoid0-mean) — alexmac, Mar 22 '14 at 03:24
Already answered: http://stackoverflow.com/questions/1291942/what-does-javascriptvoid0-mean — André Kugland, Mar 22 '14 at 03:24
@kojiro https://github.com/foray1010/Popup-my-Bookmarks/blob/master/js/common.js — octref, Mar 22 '14 at 03:25

suhailvs · Accepted Answer · 2014-03-22T03:41:25.293

6

The void operator is often used merely to obtain the undefined primitive value, usually using “void(0)” (which is equivalent to “void 0”). In these cases, the global variable undefined can be used instead (assuming it has not been assigned to a non-default value).

in this cases, the global variable undefined can be used instead: ie:

_this[i] !== undefined;

Jsfiddle Demo

edited Mar 22 '14 at 03:41

answered Mar 22 '14 at 03:24

suhailvs

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its been answered already by a link – alexmac Mar 22 '14 at 03:25
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/void – suhailvs Mar 22 '14 at 03:26
Its a duplicate question – alexmac Mar 22 '14 at 03:27
Thanks, actually I found void(0) while googling but I thought it'd be different from void 0. – octref Mar 22 '14 at 03:27
@alexmac i actually googled it. so `undefined` can also be used?? – suhailvs Mar 22 '14 at 03:29
No idea, I saw that it is a duplicate question is all. never used it, why use code like that, when it confuses the programmers :) – alexmac Mar 22 '14 at 03:34
@alexmac for example: `var x; if (x === undefined) alert('novalue'); else alert(x);` [jsfiddle demo](http://jsfiddle.net/2XkR4/1) – suhailvs Mar 22 '14 at 03:36
@suhail MDN says `undefined` can be used instead. – octref Mar 22 '14 at 03:37

score 2 · Answer 2 · answered Mar 22 '14 at 03:25

2

void 0 is a way of getting undefined without fail. Some browsers allow overriding the undefined variable, but you can't override void

answered Mar 22 '14 at 03:25

Zoey Mertes

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What is "void 0"?

2 Answers2