Can js forEach take other inputs

Question

function addToArr(num, arr) {
  arr.push(num);
}
theArr = [1, 2, 3, 4];
myArr = [];
theArr.forEach(addToArr(ele, theArr));

console.log(myArr);

I wanted to write a function that could be used in a forEach() that could take a different array as a parameter - that I could push to.

`forEach` expects a function as a parameter. `addToArr(ele, myArr)` isn’t a function, it’s `undefined`. — Sebastian Simon, Feb 27 '17 at 01:59
`arr.forEach(function(ele){ addToArr(ele, myArr); });` should work — haxxxton, Feb 27 '17 at 02:02
also, the arguments to the callback are `item, index, array` — Jaromanda X, Feb 27 '17 at 02:22

4castle · Answer 1 · 2017-02-27T02:12:16.233

3

The parameter you pass to forEach must be a function, meaning your addToArr needs to return a function. This is often called currying, partial application, or a closure.

function addToArr(arr) {
  return function(num) {
    arr.push(num);
  };
}

var arr = [1, 2, 3, 4];
var myArr = [];

arr.forEach(addToArr(myArr));

console.log(myArr);

In ES6 it's much easier to write with arrow functions.

const addToArr = arr => num => arr.push(num);

edited Feb 27 '17 at 02:12

answered Feb 27 '17 at 02:03

4castle

32,613
11
69
106

Or (more commonly), wrap the function call in a function. – Andrew Li Feb 27 '17 at 02:04
Thanks for that @4castle - elegantly written. I will have to study it to fully grasp it all. – Greg Duncan Feb 27 '17 at 02:12

score 1 · Answer 2 · answered Feb 27 '17 at 02:08

Alternatively, the second argument to forEach is used as the this value in the passed callback, so the following should work as well:

function addToArray (x) {
  this.push(x)
}

var array = [1, 2, 3, 4]
var otherArray = []
array.forEach(addToArray, otherArray)

console.log(otherArray)

If your intent was just to make a copy of the original array, though, I would suggest using Array#slice instead:

var array = [1, 2, 3, 4]
var otherArray = array.slice()

console.log(otherArray)

Can js forEach take other inputs

2 Answers2