Why does changing one array alters the other?

Question

Consider this tiny bit of javascript code:

var a = [1, 2, 3],
    b = a;

b[1] = 3;

a; // a === [1, 3, 3] wtf!?

Why does "a" change when I update "b[1]"? I've tested it in Firefox and Chrome. This doesn't happen to a simple number for example. Is this the expected behaviour?

var a = 1,
    b = a;

b = 3;

a; // a === 1 phew!

Answer 1

Because "a" and "b" reference the same array. There aren't two of them; assigning the value of "a" to "b" assigns the reference to the array, not a copy of the array.

When you assign numbers, you're dealing with primitive types. Even on a Number instance there's no method to update the value.

You can see the same "they're pointing to the same object" behavior with Date instances:

var d1 = new Date(), d2 = d1;
d1.setMonth(11); d1.setDate(25);
alert(d2.toString()); // alerts Christmas day

Answer 2

It's the same array (since it's an object, it's the same reference), you need to create a copy to manipulate them separately using .slice() (which creates a new array with the elements at the first level copied over), like this:

var a = [1, 2, 3],
    b = a.slice();

b[1] = 3;

a; // a === [1, 2, 3]

Answer 3

In addition to the other answers, if you want a copy of an array, one approach is to use the slice method:

var b = a.slice(0)

Answer 4

All Javascript objects are passed by reference - you would need to copy the entire object, not assign it.

For arrays this would be simple - just do something like:

var a = [1, 2, 3];
var b = a.slice(0);

where slice(0) returns the array from offset 0 to the end of the array.

This link has more info.

Answer 5

There is a difference between Value types and Reference types.

Basicly Value types are stored on the computers "Stack" directly which means that you actualy have the "Value" and not what is called a Pointer/Reference.

A Reference Type on the other hand, does not hold the actual value of the object/variable but instead it points ( references ) to where you can find the value.

Thus when you say "My Reference Type B points to what Reference Type A points to", you actually have two variables pointing in the same direction and when you interact with either of them and change something, both will be pointing to the changed place, since the pointed to the same place from the begining.

In your other case you say "Hey, copy the value in variable A to variable B" and thus you have the values on seperate places which means that any change to either of them will not affect the other.

Answer 6

Because arrays are references to actual place where they are stored.

Answer 7

the statement x = y is the only special one here, it means "point the variable x at the object y.

Pretty much every other operation is an operation which changes the object referred to by x

b = a;
b[1] = 3;

So, your first statement points the variable b at the array also referred to as a

Your second statement alters the array pointed to by b (and also by a)

Answer 8

Assigning an array does not create a new array, it's just a reference to the same array. Numbers and booleans are copied when assigned to a new variable.