It can be useful for a function to accept any number of arguments. For example, Math.max computes the maximum of all the arguments it is given.
To write such a function, you put three dots before the function’s last parameter, like this:
function max(…numbers) {
let result = -Infinity;
for (let number of numbers) {
if (number > result) result = number;
}
return result;
}
console.log(max(4, 1, 9, -2));
// → 9
When such a function is called, the rest parameter is bound to an array containing all further arguments. If there are other parameters before it, their values aren’t part of that array. When, as in max, it is the only parameter, it will hold all arguments.
You can use a similar three-dot notation to call a function with an array of arguments.
let numbers = [5, 1, 7];
console.log(max(…numbers));
// → 7
This “spreads” out the array into the function call, passing its elements as separate arguments. It is possible to include an array like that along with other arguments, as in max(9, …numbers, 2).
Square bracket array notation similarly allows the triple-dot operator to spread another array into the new array.
let words = [“never”, “fully”];
console.log([“will”, …words, “understand”]);
// → [“will”, “never”, “fully”, “understand”]
Source: Haverbeke Marijn (2018), Eloquent JavaScript: A Modern Introduction to Programming,
No Starch Press; 3rd edition.