What is the point of void operator in JavaScript?

What is the point of void operator in JavaScript?

I've seen some people using void operator in their code. I have also seen this in href attributes: javascript:void(0) which doesn't seem any better than javascript:;

So, what is the justification of using the void operator?

回答

The JavaScript, the void operator is used to explicitly return undefined. Its a unary operator, meaning only one operand can be used with it. You can use it like shown below — standalone or with a parenthesis.

void expression;
void(expression);

Lets see some examples

void 0; //returns undefined
void(1); //returns undefined

void 'hello'; //undefined
void {}; //undefined
void []; //undefined

void myFunction(); 
void(myFunction());

If you ask why do you need a special keyword just to return undefined instead of just returning undefined: the reason is that before ES5 you could actually name a global variable undefined, like so: var undefined = "hello" or var undefined = 23, and most browsers would accept it; the identifier undefined was not promised to actually be undefined¹. So to return the actual undefined value, the void operator is/was used. Its not a very popular operator though and is seldom used.

Let's see an example of function with void,

//just a normal function
function test() {
  console.log('hello');
  return 2;
}

//lets call it
console.log(test()); //output is hello followed by 2

//now lets try with void
console.log(void test()); //output is hello followed by undefined

void discards the return value from the function and explicitly returns undefined.

You can read more from my tutorial post: https://josephkhan.me/the-javascript-void-operator/

¹ In ECMAScript 5 and later, the global variable undefined is guaranteed to be undefined (ECMA-262 5th Ed., § 15.1.1.3), though it is still possible to have a variable inside an inner scope be named undefined.