CODEWITHSUNDEEP
javascript
Jacob
Jacob

Reputation: 87

Using For...Of loop to change an array

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].

Upvotes: 5

Views: 5989

Answers (5)

Bruno Marotta
Bruno Marotta

Reputation: 488

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);

Upvotes: 0

Varun Sharma
Varun Sharma

Reputation: 1752

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);

Upvotes: 1

Suresh Atta
Suresh Atta

Reputation: 121998

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 complicated.

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

Upvotes: 2

Chirag Ravindra
Chirag Ravindra

Reputation: 4830

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]

Upvotes: 2

Tim Biegeleisen
Tim Biegeleisen

Reputation: 520968

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.

Upvotes: 2

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