Looking for a resource to explain why when I run the below code, my original array doesn't change.

arr = [1,2,3,4];
for(let val of arr){
  val = val * 2;
  console.log(val);
}
console.log(arr);

I am learning about for...in and for...of loops and not sure why my console.log(arr) doesn't print out [2,4,6,8].

Looking for a resource to explain why when I run the below code, my original array doesn't change.

arr = [1,2,3,4];
for(let val of arr){
  val = val * 2;
  console.log(val);
}
console.log(arr);

I am learning about for...in and for...of loops and not sure why my console.log(arr) doesn't print out [2,4,6,8].

• javascript

• because you are changing val not anything in the array itself – Jaromanda X Commented Jan 30, 2018 at 5:36
• You are not setting up new value back to array. That's why. – Munish Chechi Commented Jan 30, 2018 at 5:38

5 Answers 5

The problem here is that the the identifier val is being overwritten. With another integer and val is just a temp variable invoked each iteration of the loop. If you used an object, and did not reassign the variable, your values would remain intact

// Object values
var x = [{z:1},{z:2}]
for(let y of x){
    // No reassignment
    y.z=3;
}
console.log(x); //[{"z":3},{"z":3}]

If you want to modify an array of simple types in place, you can do something like:

var q = [5,6,7];
for( i in q){
    q[i] = q[i] * 2;
}
console.log(q); //[10, 12, 14]

Use a for loop, which enables you to make the assignments "stick" in the original array.

arr = [1,2,3,4];
for (var i=0; i < arr.length; i++) { 
    arr[i] = 2*arr[i];
    console.log(arr[i]);
}

The issue with what you were originally doing is that val is just a variable with no real connection to the underlying array. Hence, doubling val has no effect on the array itself.

You are modifying and not inserting in back.

Better you use for each this case. So that you'll be able to modify the array. Using of make things plicated.

arr = [1,2,3,4];
arr.forEach(function(part, index, array) {
   array[index] = array[index]*2;
   });
console.log(arr);

You can achieve this using forEach too, which runs a particular function for every value in the array.

arr = [1,2,3,4];
arr.forEach((d, i, arr) => arr[i]*=2);
console.log(arr);

Yes, you can use for of with entries

arr = [1,2,3,4];
for (let [i, val] of arr.entries()){
  arr[i] = val = val * 2;
  console.log(val);
}
console.log(arr);