Can't we reassign array values inside forEach?

Question

The problem statement is, i should replace the any digit below 5 with 0 and any digit 5 and above with 1.

I am trying to reassign values, but it is not affecting, Why?

function fakeBinary(n) {
    let numbersArr = n.split('');
    numbersArr.forEach(num => {
        if(Number(num) < 5) {
            num = '0';
        } else if(Number(num) >= 5) {
            num = '1';
        }
    });
    return numbersArr.join('');
}

console.log(fakeBinary('3457'));

I except the output of 0011, but the actual output is 3457.

The output should be `1110`, correct? That's what your code does... — Jack Bashford, Aug 06 '19 at 14:07
You *can* reassign the parameter, but that will have no effect on the source array. The `.forEach()` callback is passed a *copy* of the array element. That's why `.map()` is a better solution: you can return each value and it builds the resulting array for you. — Pointy, Aug 06 '19 at 14:10
@Ponty Actually, for objects (or, really, anything which is not a primitive), isn't the element passed in foreach (so, not a copy of it?) — briosheje, Aug 06 '19 at 14:20

briosheje · Answer 1 · 2019-08-06T14:19:25.373

forEach doesn't bring the element's reference for primitive values but rather brings a copy of the value in your case. You can easily access that manually through the index, though:

function fakeBinary(n) {
    let numbersArr = n.split('');
    numbersArr.forEach((num, i) => {
//                          ^--- note that `i` is brought and used below to access the element at index [i].
        if(Number(num) < 5) {
            numbersArr[i] = '0';
        } else if(Number(num) >= 5) {
            numbersArr[i] = '1';
        }
    });
    return numbersArr.join('');
}


console.log(fakeBinary('3457'));

Please note that you may also use other prototypes, I just tried to stay as close as possible to your solution, you may also want to use map or, even (not appropriate, though) reduce or even a regular for loop.

Ahhhhg this is so weird lol, if you're iterating JSON objs instead of num with forEach. You can update properties on ea elem and the changes will be reflected in the root array afterwards, however you can't re-assign the entire obj. Probably more accurate to say it brings a **shallow clone** no? If it was a true copy you wouldn't be able to have property changes affect the original array? — Llama D'Attore, Apr 17 '23 at 13:57

score 5 · Answer 2 · answered Aug 06 '19 at 14:08

forEach used like that won't do anything - use map instead.

let numbersArr = n.split("").map(num => {
  if (Number(num) > 5) {
    num = "0";
  } else if (Number(num) <= 5) {
    num = "1";
  }
  return num;
});

return numbersArr.join("");

Note that to produce your desired output, you need to change your conditions slightly:

if (Number(num) >= 5) {
  num = "1";
} else {
  num = "0";
}

score 1 · Answer 3 · answered Aug 06 '19 at 14:21

1

To do it with forEach, you would need to use the additional arguments to reference the array and the index.

function fakeBinary(n) {
    let numbersArr = n.split('');
    numbersArr.forEach((num, index, arr) => {
        if(Number(num) < 5) {
            arr[index] = '0';
        } else if(Number(num) >= 5) {
            arr[index] = '1';
        }
    });
    return numbersArr.join('');
}

console.log(fakeBinary('3457'));

But forEach is not the correct way to return a new array. You want to use map()

function fakeBinary(n) {
    return n
      .split('')
      .map(num => (Number(num) < 5) ? '0' : '1')
      .join('');
}

console.log(fakeBinary('3457'));

answered Aug 06 '19 at 14:21

epascarello

204,599
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any reason to bring the third argument in the `forEach`? it shoulnd't be revelant, right? – briosheje Aug 06 '19 at 14:23
You can use the array directly, but if you were to do the split and forEach on one line it would not be possible to do it. – epascarello Aug 06 '19 at 14:26
Oh, didn't notice that, just copied the raw code from the OP and didn't notice that first. Indeed, that makes sense. – briosheje Aug 06 '19 at 14:27

score 0 · Answer 4 · answered Aug 06 '19 at 14:50

0

A shorter version using Array.from()

const fakeBinary = (str) => Array.from(str, n => +(+n >= 5)).join('');

console.log(fakeBinary('3457'));

answered Aug 06 '19 at 14:50

User863

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41

Can't we reassign array values inside forEach?

4 Answers4

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