Why is object/associative array values not setting when I use $item.data("something").x

Why is it when I do something like

http://jsfiddle.net/sUhn9/

Added relevant HTML

<div id="container" data-physics='{x: 10, y: 5}'>Hello</div>

JavaScript:

$(function() {
    var obj = $("#container").data("physics");
    console.log("b4", obj);
    obj.x = -2;
    obj.y = -6;
    console.log("after", obj);
});

I get

b4 {x: 10, y: 5}
after {x: 10, y: 5}

Where x and y are not being set

Your obj is not an object, it's a string. Changing the x and y properties for the string will not change the string itself.

You should do something like this:

var obj = jQuery.parseJSON($("#container").data("physics"));

Working update here !!!

Well, for data-physics='{x: 10, y: 5}' var obj = $("#container").data("physics"); returns string so obj is not an object so no property and ...

So to get it right, make it data-physics='{"x": 10, "y": 5}', and everything should work fine.