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
