Javascript array is syncing with another assigned array

Question

I've been racking my brain trying to figure out why these arrays are syncing after I assign one to the other. The output should be "1, 2, 3, 4" but instead it's "5, 6, 7, 8". Do I need to copy the arrays differently?

var firstArray = [1, 2, 3, 4];
var secondArray = [5, 6, 7, 8];

for (i = 0; i < firstArray.length; i++) {
  var myTempArray = firstArray; 
  myTempArray[i] = secondArray[i];
}
console.log("Result: " + firstArray);

Expected output:

Result: 1,2,3,4

Actual output:

Result: 5,6,7,8

How do I alter the second array without changing the first array?

your code is clearly altering the first array through myTempArray, and the code doesn't even attempt to alter the second array ever — Jaromanda X
– Jaromanda X, Commented Dec 23, 2016 at 2:20
myTempArray and firstArray both reference the same array. In JavaScript, objects (including arrays) are reference values. — Felix Kling
– Felix Kling, Commented Dec 23, 2016 at 2:21
var myTempArray = firstArray doesn't COPY the array ... it makes myTempArray reference the same data as is in firstArray - to make a (simple) copy of an array use array.slice ... i.e. var myTempArray = firstArray.slice() - this should be done BEFORE the loop — Jaromanda X
– Jaromanda X, Commented Dec 23, 2016 at 2:22
If you want to get fancy.. var myTempArray = [...firstArray]; Using a Spread will create a copy, not a reference. — Sterling Archer
– Sterling Archer, Commented Dec 23, 2016 at 2:29

Community · Accepted Answer · 2017-02-08 15:13:03Z

3

Arrays are mutable objects. So they just contain references. You need to "copy" an array in order to make a copy, not just assign like primitive objects. To copy an array, there are various methods. One best method is:

myTempArray = firstArray.slice();

What you are doing is a shallow copy:

Also, another big issue is that, you have the array assignment inside the loop, which keeps the myTempArray changing. You need to take it out of the loop. Your final code should look like:

var firstArray = [1, 2, 3, 4];
var secondArray = [5, 6, 7, 8];

var myTempArray = firstArray.slice();
for (i = 0; i < firstArray.length; i++) {
  myTempArray[i] = secondArray[i];
}
console.log("Result: " + firstArray);

edited Feb 8, 2017 at 15:13

CommunityBot

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answered Dec 23, 2016 at 2:30

Praveen Kumar Purushothaman

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1 Comment

Victor Stoddard Over a year ago

I moved the array declaration out of the loop and it a made no difference. The slice() ended the reference. Thanks

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