Why does array assignment not work inside Array.forEach?

Question

Consider the following code. Why does the forEach loop not work, but the regular for loop works?

class Nums{
 numbs: number[];
 constructor(nums: number[]){
  this.nums = nums;
 }
}

const [nums1, nums2, nums3] = [new Nums([1]), new Nums([2]), new Nums([3])];
const numSet = [nums1.nums, nums2.nums, nums3.nums];
const magicNums = [5,5,5];

//Gives wrong result.
numSet.forEach((nums) => {
 nums = magicNums;
});

console.log(nums[0]);// [1]

//Gives correct result.
for(let i=0; numSet.length; i++){
 numSet[i] = magicNums;
}

console.log(nums[0]);// [5,5,5,]

@caTS actually here OP tries to reassign nums, not to perform a deep modification. So even if it were an object, it would have been just reassigned, without affecting the array. — ghybs
– ghybs, Commented Oct 21, 2022 at 3:42

ghybs · Accepted Answer · 2022-10-21 03:44:01Z

1

If you want to do the equivalent operation as in the for loop, you also have to access the original array:

numSet.forEach((_nums, index) => {
  numSet[index] = magicNums;
});

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach

Inversely, if you do the same variable re-assignment as in your first forEach attempt within the for loop, you get the undesired result:

for (let i=0; numSet.length; i++){
  let nums = numSet[i];
  nums = magicNums; // Reassigns `nums`, but does not affect the original `numSet` array
}

edited Oct 21, 2022 at 3:44

answered Oct 21, 2022 at 3:31

ghybs

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