Javascript: Simple yet weird behavior with arrays?

Question

Test 1:

var arr1 = [10, 20];
var arr2 = arr1;

arr2[0]++;

alert(arr1);// [11, 20] --- What?!?!
alert(arr2);// [11, 20] --- correct

Test 2:

var arr1 = [10, 20];
var arr2 = [arr1[0], arr1[1]];

arr2[0]++;

alert(arr1);// [10, 20] --- correct
alert(arr2);// [11, 20] --- correct

In test 1, Why the first array's first element was changed? As far as I know, in other OOP languages like Java & PHP, if we do the Test 1's var arr2 = arr1; this is called "referencing", once we change something with the new variable's value, it creates a new value for that so the first reference "arr1" doesn't get effected. Why it's not the same thing in Javascript!? This totally doesn't make sense! Is it a bug? or it's just that Javascript behaves differently?

Look up "javascript deep copy" vs "javascript shallow copy"

d'alar'cop
– d'alar'cop

2014-06-29 00:25:03 +00:00
Commented Jun 29, 2014 at 0:25 — d'alar'cop
– d'alar'cop, Commented Jun 29, 2014 at 0:25

Pointy · Accepted Answer · 2014-06-29 00:16:46Z

3

JavaScript variables refer to objects. When a variable "is" an object, it contains a reference to the object. Assigning the value of such a variable to another variable just transfers the reference; it does not make a copy of the referenced object.

answered Jun 29, 2014 at 0:16

Pointy

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Comments

Ed Morales · Accepted Answer · 2014-06-29 00:20:34Z

1

In the Test 1 both variables are referencing the same array object, so anything you do on one is going to be reflected on the other one.

On the other hand, on the Test 2 you are copying the values into a new array, so each variable is referencing different array objects.

edit: to remove the "passby" term.

edit 2: on a side note, this happens in java too.

edited Jun 29, 2014 at 0:20

answered Jun 29, 2014 at 0:14

Ed Morales

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5 Comments

Pointy Over a year ago

This is really not correct. There's no "passing" happening here. The terms "pass by value" and "pass by reference" have to do with the semantics of function calls.

Niet the Dark Absol Over a year ago

As a side-note, an easier way to copy the values is arr2 = arr1.slice(0);.

Ed Morales Over a year ago

lol, true passing means as parameters in a function/method, but its the same idea.

Pointy Over a year ago

@EdwardM.B. it's a related concept, but strictly speaking they're really two completely different things.

David Ehrmann Over a year ago

Lots of languages do this. The big exceptions are truly functional languages where data's immutable.

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