Class constructors without using "new" in JavaScript

When creating a class constructor in JavaScript, you may prefer to allow its instantiation without using the new keyword—just like jQuery does with their $() factory function. Let’s assume the following class as a simple example.

function Person(name, age) {
    this.name = name;
    this.age = age;
}

While var jack = new Person('Jack', 32) would work, var jim = Person('Jim', 34) doesn’t—so far. Actually jim would be simply undefined after the call. But even worse: if someone calls the constructor without using new, the global scope will be poluted by the two variables name and age as calling a function without specifying its context forces the browser to pass it in the global object (which is window). By checking whether the requested object is already an instance of the class, you can return a new instance if it isn’t or execute the actual constructor if it is.

function Person(name, age) {
    if (this instanceof Person) {
        this.name = name;
        this.age = age;
    } else {
        return new Person(name, age);
    }
}

Both var jack = new Person('Jack', 32) and var jim = Person('Jim', 34) would work now. The following way does exactly the same but is a bit shorter and handier.

function Person(name, age) {
    if (!(this instanceof Person)) return new Person(name, age);
    this.name = name;
    this.age = age;
}

JavaScript