# Trouble with .reduce and one-line implicit return arrow function

I’m practicing .reduce and attempting to write it as a 1-line implicit return arrow function.

Basically, I’m trying to turn this:

const nums = [25, 5, 200, 1500, 1, 100];
const product = nums.reduce((total, currentVal) => {
});

into this:

const nums = [25, 5, 200, 1500, 1, 100];
const product = nums.reduce(total, currentVal) => total * currentVal;

But I get an error of "Uncaught SyntaxError: Malformed arrow function parameter list
".
What am I doing wrong with my arrow function?

The arrow function will try to take those variables inside the parantheses because that’s what the arrow function does:

(parameter1, parameter2) => {}
//or
parameter1 => {}

So the fix to that is :

const product = nums.reduce((total, currentVal) => total * currentVal);

Thats the great thing about => arrow function. It will always return whatever is after it if there is not brackets {} like the one above.

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Thanks. I guess I don’t understand why you need the outer parentheses surrounding ((total, currentVal) => total * currentVal)?

I thought if you are using a one-line implicit return, then you would not need parentheses?

For example, I don’t use the parentheses here:

const square = num => num * num;

So I guess I’m confused as to why I need them in a one-line implicit return such as:

const product = nums.reduce((total, currentVal) => total * currentVal);

I understand that you need the inner parentheses for the arguments total, currentVal. But I don’t understand why you need the outer parentheses?

Well you are trying to use the function .reduce() which uses a parentheses. So, you want to nest your function inside the .reduce() function. If you would use a normal function it would be the same. You are trying to use a anonymous function which has no name

Array.prototype.reduce(function () {})

Yes, you don’t use parentheses here because you’re not using another function to run the function

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