I have come across this code:

return someFunctionThatReturnsAnArray()
  .map((action) => void dispatch(action))
  .run();

Can anyone suggest why you would prefix void in the map function so that it returns undefined each time?

I have come across this code:

return someFunctionThatReturnsAnArray()
  .map((action) => void dispatch(action))
  .run();

Can anyone suggest why you would prefix void in the map function so that it returns undefined each time?

• javascript

• 7 looks like they don't know about forEach – Rhumborl Commented Apr 11, 2018 at 15:59
• 6 Maybe run() needs an array of undefined, eh? – sleighty Commented Apr 11, 2018 at 15:59
• 1 Possible duplicate of Why use the void keyword? – 31piy Commented Apr 11, 2018 at 16:02
• 2 forEach does not return an array so its not the same. What is more disturbing in this code it looks like they monkey patched Array.prototype with run function. – Moti Korets Commented Apr 11, 2018 at 16:04
• 2 yeah the answer appears to depend on adding the source of the magical run() function – Rhumborl Commented Apr 11, 2018 at 16:06

3 Answers 3

This is essentially a clever way of defining a function that returns undefined.

From MDN:

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

Your code is essentially the same as:

return someFunctionThatReturnsAnArray()
  .map((action) => { dispatch(action); })
  .run();

I'll be honest: if I came across that code in any project that I work with, I'd flag it for review. It doesn't really make sense and is potentially confusing.

void operator allows evaluating expressions that produce a value into places where an expression that evaluates to undefined is desired.

So it looks like :

• as long as the dispatch method is called, its return value is not important
• the result of the map method is an array with undefined values but its length is equal to the number of dispatched calls
• the run method is not standard for the Array prototype (seems like a custom patch), therefore it is hard to tell what it does.

One of its usage would be info of dispatched calls

Array.prototype.run = function() {
  console.log("Dispatched calls : " + this.length);
};

var getArray = function() {
  return ['1', '2', '3'];
}

var dispatch = function(n) {
  return 10*n;
}

getArray().map((action) => void dispatch(action)).run();

Taken from https://eslint.org/docs/rules/no-void

The void operator takes an operand and returns undefined: void expression will evaluate expression and return undefined. It can be used to ignore any side effects expression may produce:

The common case of using void operator is to get a “pure” undefined value as prior to ES5 the undefined variable was mutable:

// will always return undefined
(function(){
    return void 0;
})();

// will return 1 in ES3 and undefined in ES5+
(function(){
    undefined = 1;
    return undefined;
})();

// will throw TypeError in ES5+
(function(){
    'use strict';
    undefined = 1;
})();

Another common case is to minify code as void 0 is shorter than undefined:

foo = void 0;
foo = undefined; 

When used with IIFE (immediately-invoked function expression), void can be used to force

the function keyword to be treated as an expression instead of a declaration:

var foo = 1;
void function(){ foo = 1; }() // will assign foo a value of 1
+function(){ foo = 1; }() // same as above
function(){ foo = 1; }() // will throw SyntaxError

Some code styles prohibit void operator, marking it as non-obvious and hard to read.

Can't see any good reason to be using it in the snippet that you have provided.