Rest & Spread part 1

ES6 Rest Parameters

Prior to ES6, if we wanted to pass a variablenumber of arguments to a function, we could use the arguments object, an array-like object, to access the parameters passed to the function.
For example, let's write a function that checks if an array contains all the arguments passed:

function containsAll(arr) {
for (let k = 1; k < arguments.length; k++) {
let num = arguments[k];
if (arr.indexOf(num) === -1) {
return false;
}
}
return true;
}
let x = [2, 4, 6, 7];
console.log(containsAll(x, 2, 4, 7));
console.log(containsAll(x, 6, 4, 9));Try It Yourself

We can pass any number of arguments to the function and access it using the arguments object.

While this does the job, ES6 provides a more readable syntax to achieve variable number of parameters by using a rest parameter:

function containsAll(arr, ...nums) {
for (let num of nums) {
if (arr.indexOf(num) === -1) {
return false;
}
}
return true;
}Try It Yourself

The ...nums parameter is called a rest parameter. It takes all the "extra" arguments passed to the function. The three dots (...) are called the Spread operator.
Only the last parameter of a function may be marked as a rest parameter. If there are no extra arguments, the rest parameter will simply be an empty array; the rest parameter will never be undefined.