Rest Operator with Function Parameters

Have been working on the lesson with this ((SPOILER)ES6: Use the Rest Operator with Function Parameters) as a guide but I still don’t understand it. I don’t even see a rest operator in the instructions!

The actual lesson is below. You linked to a forum post. Can you explain what part of the instructions are not clear to you?

I’m the same, I don’t understand what I have to do here. I’ve tried using MSDN to understand rest and spread and also reduce since there haven’t been any lessons that I can recall up to this point that help me understand reduce before being asked to work with it.

Thanks in advance for your help! :slight_smile:

I got the answer but I’m still not 100% sure how it works.

// original code to be changed
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

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

So basically all I had to do was remove the args array and add …args into sum() ??
I’m glad I got the answer but I’m still clueless as to how I got it :sweat_smile:

it’s so difficult for me to understand either…
especially don’t know why it need to return args.reduce ((a, b) => a + b, 0);
can someone please explain the second return, how is the function reduce? also how the function know variable (a,b) equal first 2 array and what is the 0 at the end?

i am beginner, very appreciate if anyone could help me this, thanks!

Reduce takes a function and a initial value. So the function is like this:

function sum(a, b) {
  return a + b;

What reduce does is run that function for every value in the array, starting with that initial value (if you don’t give it a initial value, it uses the first value in the array). a is the accumulator, b is the current value in the array. Every time it runs, the result becomes the new value for a.

So say your array is [1,2,3,4] and your start value is 0.

sum(0, 1) is 1
1 is now the first value.
sum(1, 2) is 3
3 is now the first value.
sum(3, 3) is 6
6 is now the first value.
sum(6, 4) is 10
Now at the end of the array, final value is 10.


Thanks you are awesome!
It is so clear and I totally understood now, cheers!

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Thanks @DanCouper for the explanation on this lesson. However, something is still bothering me (an absolute beginner, by the way).

Why can’t the solution simply be:

const sum = function sum(…args) {
return args.reduce((a, b) => a + b, 0);

This does exactly the same in just 3 lines of code. Why bother (in the ‘official’ solution) to create a function that simply returns another function? And I’m puzzled by the empty parentheses () at the end of the ‘official’ solution. Removing them breaks the code, but what are they & what do they do?

That is what is being executed, just those three lines of code. The outer function is just for the benefit of tests — it used to be necessary, but afaik should be being removed when the fix that’s waiting to go in gets put in. The code should pass the tests if you delete the other stuff and just use what you posted

() is how you tell JS to execute a function: add(1,2), sayHello(). The outer code is just a function, and all it does is return the inner function. Parentheses say “execute”, so that outer function will return immediately when that code is evaluated.

The reason it is used is that code inside the function (before the inner function is returned) will stay just within the scope of that. So for example, a “use strict” statement can be added which will apply to everything inside the outer function (this seems to be the main reason for doing things this way)

Wow, I actually get it! :slight_smile: Thanks, @DanCouper!