JavaScript parenthesis

JavaScript
1

Could you please explain me the purpose of the parenthesis(in bold) on the following code?

Thank you.

const sum =(function() {
“use strict”;
return function sum(x, y, z) {
const args = [ x, y, z ];
return args.reduce((a, b) => a + b, 0);
};
})();
console.log(sum(1, 2, 3)); // 6

1 Like
2

This is what’s called an immediately-invoked function expression (or IIFE). Adding those parentheses right immediately after the function invokes it on the spot. It’s similar to this:

function _iife() {
  'use strict';
  // rest of the function...
}

// notice the parens? They're invoking _iife.
// The same way they invoke the function in the code you posted.
const sum = _iife();
console.log(sum(1,2,3)); // 6

except that an IIFE almost always doesn’t need a name, and that an IIFE is only used on one place.

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3

Thank you for your quick response kevcomedia, I am not sure if I clearly understand this. There is already “()” just after the word “function” and then there is the second “()” just before “)” I understand the outer “()” it is the grouping operator according to MDN link you sent. Is this IIFE the second “()”? Also, when I remove the parenthesis both outer(grouping operator) and IIFE the code doesn’t run but it looks like it should return a value(see below).

const sum = function() {
“use strict”;
return function sum(x, y, z) {
const args = [ x, y, z ];
return args.reduce((a, b) => a + b, 0);
};
};
console.log(sum(1, 2, 3)); // doesn’t return a value

4

The entire function plus the other pair of parens is the IIFE. Without that last pair of parens (regardless whether you keep the grouping parens or not), it’s just a plain function assigned to a variable.

1 Like
5

Thank you very much for the explanation kevcomedia.

1 Like