In JavaScript, object keys are always strings.

object_keys.js

var obj = {};

// null and 'null'
obj[null] = 1;
obj['null'] = 2;
console.log(obj[null]); // 2

// Numbers
obj[3] = 3;
obj['3'] = 4;
console.log(obj[3]); // 4

// Objects
function Foo() {}
function Bar() {}
var f = new Foo();
var b = new Bar();
console.log(f.toString()); // [object Object]
console.log(b.toString()); // [object Object]

obj[f] = 5;
obj[b] = 6;
console.log(obj[f]); // 6