singleton_1.js

(function(g){
    /** @type {SingletonConstructor} */
    var instance;
    g.Singleton = function() {
        if (instance !== void 0) return instance;
        return instance = this;
    };
    g.Singleton.prototype.foo = 123;
})(window||global);

var obj1 = new Singleton;
var obj2 = new Singleton;

console.log(obj2.foo === 123);
obj1.foo = 456;
console.log(obj2.foo === 456);
console.log(obj1 === obj2 );