Adding item to a json object

I saw many questions in Stackoverflow but i didn't find what exactly responds to my query.

I need to add an element to my json object:

var JSONObject = {
    "shape0": {
        "id": "id0",
        "x1": 0,
        "x2": 0,
        "y1": 0,
        "y2": 0
    },
    "shape1": {
        "id": "id1",
        "x1": 2,
        "x2": 2,
        "y1": 2,
        "y2": 2
    }
};

I used this syntax but in vain:

var newShape = "shape2";
JSONObject.newShape.id = "id2";

Any help will be appreciated