[Registry] a simple registry without moving elements

Registry.js

#!/usr/local/bin/node

var Registry = function() {
    this.r = [null];
};

Registry.prototype.register = function(object) {
    var id = this.r[0];
    if (id === null) {
        this.r.push(object);
        return this.r.length - 1;
    } else {
        this.r[0] = this.r[id];
        this.r[id] = object;
        return id;
    }
};

Registry.prototype.unregister = function(id) {
    this.r[id] = this.r[0];
    this.r[0] = id;
};

var reg = new Registry();
reg.register('module1');
var id = reg.register('module2');
reg.register('module3');

reg.unregister(id);
reg.register('new_module2');
console.log(reg);