Question

I have a complex javascript object with an array. When I try to set the value for one attribute of an index, it’s applied to all items of the array.

Here is a basic example:

var obj = new Object();    
obj.arr = [];
obj.arr[0] = {pos:[0,0]};
obj.arr[1] = {pos:[0,0]};

Now if i set a value for an attribute of the object, via a specific index,

obj.arr[0].pos = [10,10];
obj.arr[1].pos = [5,5];

Here it seems to be setting the value [5,5] for both items of the array. The resulting values are:

console.log(obj.arr[0].pos) returns [5,5]
and

console.log(obj.arr[1].pos) also returns [5,5]

My actual object is far more complex, but this is the basic idea of what’s happening…

Any ideas?

Answer 1

They share the same link, i.e. several variables/properties of an object are referring to the same value.

Exact answer (where’s the error) depends on how your object is composed.

var nested = {a:1};
var obj = {arr:[]};
obj.arr[0] = {pos:0, n:nested};
obj.arr[1] = {pos:0, n:nested};

obj.arr[0].pos = 1;
obj.arr[1].pos == 1; // false
obj.arr[0].nested.a = 2;
obj.arr[1].nested.a == 2; // true

Assignment of the same array/object literals is not the same.

var a = [0];
var b = [0];
b[0] = 1;
a[0] == 1; // false
b = a;
a[0] = 2;
b[0] == 2; // true

a = b = [0];
a[0] = 1;
b[0] == 1; // true