Change the value of an array changes original array JavaScript

Question

The following code causes both elements from id 0 to be set to -, even though I want only one to be set to -1. Am I just creating a reference to the labelArray, or is something else?

labelArray.sort(compare);
valueArray = labelArray;
valueArray[0] = '-1';
labelArray[0] = '-';

All help is appreciated.

UPDATE (2019): It's been several years since I first did this post, and ES6 is used pretty much universally. So, I wanted to come back and add that, instead of using the slice() method recommended in the accepted answer, you can instead use array destructing in the following to make a copy:

valueArray = [...labelArray];

calebds · Accepted Answer · 2012-01-26 20:38:21Z

11

Yes. Both valueArray and labelArray reference the same underlying array. To make a copy, use slice():

valueArray = labelArray.slice(0);

NOTE: Slice() only copies 1 level deep, which works fine for primitive arrays. If the array contains complex objects, use something like jQuery's clone(), credit @Jonathan.

edited Jan 26, 2012 at 20:38

answered Jan 26, 2012 at 20:21

calebds

26.3k9 gold badges51 silver badges75 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Phrogz Over a year ago

@deth4uall Yes, variables, object properties, and array slots in JS all reference values, they do not contain them.

calebds Over a year ago

@deth4uall Not because, per se, but by convention. In many OO languages objects are distinct from their references, unlike primitives, probably because the compiler would have to infer too much to perform a deep-copy.

OpensaurusRex Over a year ago

Thanks guys, if I had known for certain it was an object I would never had needed to ask. Your help has been quite enlightening.

ruakh · Accepted Answer · 2012-01-26 20:19:40Z

2

Am I just creating a reference to the labelArray […] ?

Yes, exactly. valueArray and labelArray still identify the same object, which hasn't been copied.

answered Jan 26, 2012 at 20:19

ruakh

185k29 gold badges292 silver badges324 bronze badges

Comments

Jonathan · Accepted Answer · 2012-01-26 20:21:20Z

1

valueArray is just a reference to labelArray.

What you want to do is clone the array. You can do this using jQuery.clone() or a similar cloning function.

answered Jan 26, 2012 at 20:21

Jonathan

5,5574 gold badges40 silver badges54 bronze badges

1 Comment

OpensaurusRex Over a year ago

The project is not using jQuery so that is not on table right now. I am probably going to end up moving to jQuery anyways.

Collectives™ on Stack Overflow

Change the value of an array changes original array JavaScript

3 Answers 3

3 Comments

Comments

1 Comment

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

Comments

1 Comment

Linked

Related