Javascript oddness with array of objects and indexOf

Not quite grasping what's going on here. Given the array (arr):

[
    {
        "first_name": "Dan",
        "last_name": "Woodson",
        "id": 1
    },
    {
        "first_name": "Jen",
        "last_name": "Woodson",
        "id": 2
    },
    {
        "first_name": "Yoshi",
        "last_name": "Woodson",
        "id": 3
    }
]

And the object (obj):

{
    "first_name": "Yoshi",
    "last_name": "Woodson",
    "id": 3
}

Why would arr.indexOf(obj) return -1 (especially since I retrieved the object from the array using it's 'id' parameter earlier in the function)?

Solution 1:

Array.indexOf() will only work on objects if the supplied object is exactly the same object you put in.

An exact copy is insufficient, it has to be the exact same object, i.e. there must be some object in the array such that:

arr[i] === obj

You need to show how you retrieved the object.

Solution 2:

I would like to see the retrieve function, but most likely you are not using the same reference. Because the following is true:

var a = {id: 3};
var b = [a];
b.indexOf(a); // 0
a.id = "not three";
b.indexOf(a); // still 0

However, the following will break:

var a = {id: 3};
var b = [{id: 3}];
b.indexOf(a); // -1 not the same object