defineLazyValue(obj, prop, fn, writable)

Use Object.defineProperty to define properties whose values will be created lazily when they are first read.

var _id = 1;
function getId() {
    return _id++;
}

function MyObj(name) {
    this.name = name;
}

defineLazyValue(MyObj.prototype, 'id', getId);

var obj1 = new MyObj('obj1');
var obj2 = new MyObj('obj2');

console.log( obj2 ); // -> { name: "obj2" }
console.log( obj1 ); // -> { name: "obj1" }

// the first `id` read will get id `1`
console( obj2.id ); // -> 1
console( obj1.id ); // -> 2

console.log( obj2 ); // -> { name: "obj2", id: 1 }
console.log( obj1 ); // -> { name: "obj1", id: 2 }