Use the Rest Operator with Function Parameters_2

Tell us what’s happening:
I am getting a syntax error when using the code below. See ERROR. I am placing the …args operator in the function as explained in the instructions. I assume this is the function they’re talking about and not the return function within the outer SUM function. Also, I don’t know what the purpose of “use strict” is. I’ve seen it it in several challenges, but this is the first time it’s caused problems. Why is it causing problems? Where should I put …args? Do I need the prefix? Thanks.

Your code so far

const sum = (function(...args) {
  "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


SyntaxError: unknown: Non-simple parameter in strict mode (1:22)
> 1 | const sum = (function(...args) {
    |                       ^
  2 |   "use strict";
  3 |   return function sum(x, y, z) {
  4 |     const args = [ x, y, z ];


SyntaxError: Illegal 'use strict' directive in function with non-simple parameter list

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User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.106 Safari/537.36.

Link to the challenge:

First, I think “use strict”; should always be at the top of your code.

Next, x,y,z should not be used because the point of using the spread operator is to not have to worry about the number of arguments.

  1. The outer function is an IIFE, just runs the code inside it immediately. If you log out the variable sum you can see it holds the returned inner function signature.

  2. Right now, the inner function has a fixed number of parameters, your task is to make the function accept a dynamic number of arguments, i.e. it can’t have a fixed number of parameters (must use the rest operator).

Info on the error message you are seeing.

Side note: Why does the last test of the assignment say “The sum function uses the … spread operator on the args parameter.”, shouldn’t it say rest not spread?

For the x,y,z are you referring to the return function sum(x,y,z) or the const args = [x.y.z] or both? These are what came with the challenge.
The “use strict” comes up automatically in the position it’s in. When I delete it I get the same answers (6) for all test cases.

What’s the difference between the rest and spread operators?
(Because I thought they referred to the same thing)

Both. The solution does not need these at all.

I guess you don’t need the “use strict”; in this test but it is probably best practice to use it.

This site talks about both:

Your solution worked. Thanks! We never covered “use strict” or llFE functions.

1 Like

The operator (...) is the same, the usage is not.

This is a function definition using rest for it’s parameter, it can be called with as many arguments as needed.

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


This is a function with a fixed number of parameters, being called using spread on the argument.

function sum(x, y, z) {
  return x + y + z;

const numbers = [1, 2, 3];


It is confusing that the assignment says use spread on the sum function. To me, that implies when the function is called, spreading the arguments into it. As opposed to, using rest on the parameters of the function definition. Correct me if i am wrong, but i think the wording on the assignment’s last test is incorrect. Also, spread is first introduced in the assignment following this assignment.

ES6: Use the Rest Operator with Function Parameters
Modify the function sum so that it uses the rest operator and it works in the same way with any number of parameters.

The sum function uses the … spread operator on the args parameter.

I’m a little confused about how you passed the test. If you use rest correctly on the sum function you don’t need to remove the strict directive.

Please raise this as an issue if you think it needs amending. (I have no strong opinion on this but then I am not a newbie).

I didn’t get rid of “use strict”."