How do I use a predefined function as a callback in array.forEach?

Question

I have 2 arrays that both need to run forEach.

[267, 418, 568].forEach((item, idx) => item * 17 + idx)
[123, 234, 345].forEach((item, idx) => item * 17 + idx)

To reduce repetition, I'd like to replace the callback with an external function.

something like

const otherFunc = (item, idx) => {
    return item * 17 + idx
}

[123, 234, 345].forEach(otherFunc(item, idx))

What's the syntax for doing such?

don't call it just pass it `[123, 234, 345].forEach(otherFunc);` — pilchard, Nov 10 '21 at 22:53
`forEach` doesn't return anything so you can't use the `[].forEach` syntax to change the array elements. Do you want `map` instead to create a new array? — Andy, Nov 10 '21 at 23:09
@Andy No, I do not need `map`. The content of the post was for illustration purposes only. The main point was the syntax of using an external function as a callback. — monsto, Nov 10 '21 at 23:16
But your `forEach` is trying to return a value, and that's not how that method works. Did you just want to log something to the console? — Andy, Nov 10 '21 at 23:18
No, really, it's not. What are you trying to do in your `forEach`? — Andy, Nov 10 '21 at 23:21

score 2 · Answer 1 · answered Nov 10 '21 at 22:57

2

You can use this

function loopItem(item, index) { 
  return item * 17 + index;
}

[267, 418, 568].forEach(loopItem);
[123, 234, 345].forEach(loopItem);

answered Nov 10 '21 at 22:57

Vasim Hayat

You should probably test this. – Andy Nov 10 '21 at 23:02
here is jsfiddle link *https://jsfiddle.net/hy3u1web/1/* – Vasim Hayat Nov 10 '21 at 23:08
But how does that update the original array which is what I assume the OP wants? – Andy Nov 10 '21 at 23:09
@Andy Your assumption is getting into the weeds. I didn't ask about modifying the original array, nor a return or map. I asked about a specific syntax. This response answers that question. – monsto Nov 10 '21 at 23:22
You did ask about a return because _your question involves a return_ which is why I'm questioning whether or not you know how `forEach` works. I'm not trying to be mean. It's just not clear what you want to achieve. – Andy Nov 10 '21 at 23:24
@Andy The code in the post was for an example, quickly typed. What I wanted to achieve was knowing the syntax of calling an external function for a callback. The question, ***as stated at the bottom of the post***, was clear to the 2 people that posted answers to that clear question. Thanks and have a good rest of your day! – monsto Nov 10 '21 at 23:28

score 0 · Answer 2 · answered Nov 10 '21 at 22:58

0

You need to pass your function inside forEach directly(without executing)

[].forEach(someFunc);

answered Nov 10 '21 at 22:58

Eugene

2 Answers2