The Comma Operator in JavaScript

Thu, 12 Jul 2018 14:24:00 +0000

The comma operator takes two expressions, evaluates both and then returns the evaluation of the right expression. For example:

const returnLeft = (x, y) => (y, x);
returnLeft(3, 5); // 3

It’s important to keep in mind that both expressions will be evaluated, meaning that you have to be aware of the possible side-effects of the first expression passed.

For example:

const log = (y) => console.log(`The value of y is ${y}`);
let y = 1;
returnLeft(y, log(y));
// The value of y is 1
// 1

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The Comma Operator in JavaScript