Javascript：简单但奇怪的数组行为？

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?

Answer 1

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.

Answer 2

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.