What's wrong with my forEach method when a simple For Loop works just fine

Question

So, I have a simple task, given the array: let arr = [true, false, true, false, true]; I need to reverse true to false and visa versa. I have managed to do this with a for loop: And it works fine.

Now, I'm trying to do the same with a forEach and I can't figure out why this won't work. So, here's my code:

for (let i = 0; i < arr.length; i++) {
    if (arr[i] === true) arr[i] = false;
    else arr[i] = true;
} // it works

// for some reason, this doesn't
arr2.forEach(el => el === true ? el = false : el = true);
console.log(arr)

//Neither this: 
arr.forEach(el => el === true && el = false || el === false && el = true);
console.log(arr)

The map doesn't work either: Can someone point out my mistake and explain what I am doing wrong? Maybe show other ways to solve it? With filter, reduce or which is more preferrable? A one-liner solution is highly preferred. Thank you for your answers!

Answer 1

What you need is Array.prototype.map, because assigning el is not like assigning arr[i] (it doesn't mutate the array):

arr2 = arr2.map(el => el === true ? false : true);

Which can be simplified to just:

arr2 = arr2.map(el => !el);

Answer 2

You're assigning to el, which is the parameter to your callback. That has no effect whatsoever on the value in the array. The value of the array element is copied to el, after which there is no link between the array element and the el parameter. It's like this:

function example(el) {
  console.log("before", el);
  el = 42;
  console.log("after", el);
}

let a = [1];
console.log("a before", String(a));
example(a[0]);
console.log("a after", String(a));

If you want to assign to the array from within a forEach, you have to do it by index:

arr.forEach((el, index) => el === true ? arr[index] = false : arr[index] = true);

(I strongly recommend not abusing the conditional operator that way, but that would be the nearest to what you're trying to do.)

The more idiomatic thing to do would be to use map, and ! rather than the conditional operator:

const newArray = arr.map(el => !el);