Changing an array value also changes value in another assigned array

Question

I am learning Javascript and in W3 schools' code editor I try the following code :

    <!DOCTYPE html>
<html>
<body>

<p id="demo"></p>

<script>
var cars = ["Saab", "Volvo", "BMW"];
var newcars = [];
newcars = cars;
newcars[0] = "Benz"
document.getElementById("demo").innerHTML = cars[0];
</script>

</body>
</html>

I expected the value to be : "Saab" since I made no changes to the cars array and the only change was to the newcars array, yet the output was : "Benz".

Could someone please explain what is happening?

Thanks

David Hellsing · Accepted Answer · 2014-06-10 12:20:32Z

4

If you want to clone the cars array, use slice:

var newcars = cars.slice(0);

Otherwise they will point to the same array (by reference) in memory.

answered Jun 10, 2014 at 12:20

David Hellsing

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Community · Accepted Answer · 2017-05-23 12:11:04Z

1

The value of variable "cars" contains a "reference to the array" not "the array value". So, when you go like "var newcars = cars" you are actually copying the reference only, which means that "cars" and "newcars" are pointing to the same array. You need to make another copy of the array which "cars" points to and then let the "newcars" reference to that new copy. You can clone an array in different ways, it could be done using libraries like jQuery (like here), underscore (like here) or without like in here Copying array by value in JavaScript

edited May 23, 2017 at 12:11

CommunityBot

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answered Jun 10, 2014 at 12:53

Qusai Jouda

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1 Comment

Qusai Jouda Over a year ago

yes the behaviour is similar, in JavaScript every thing is passed by value, but one should notice that with Arrays and Objects the value is the reference.

Amir Popovich · Accepted Answer · 2014-06-10 12:51:55Z

1

You are using the same reference.

var cars = ["Saab", "Volvo", "BMW"];
var newcars = []; // new array! good!
newcars = cars; // points to cars. all changes will reflect both.
newcars[0] = "Benz"
document.getElementById("demo").innerHTML = cars[0];

You may want to copy the array and not the reference. If your using Jquery than this is good:

// Shallow copy
var newObject = jQuery.extend([], oldObject);

// Deep copy
var newObject = jQuery.extend(true, [], oldObject);

edited Jun 10, 2014 at 12:51

answered Jun 10, 2014 at 12:18

Amir Popovich

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1 Comment

DJL Over a year ago

for an array should that not be jQuery.extend([], oldObject); ?

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