Pattern for enforcing new in javascript

Question

I have been reading JavaScript Patterns book by Stoyan Stefanov and one of the patterns to enforcing the new operator for constructor functions goes like this

function Waffle() {
if (!(this instanceof Waffle)) {
return new Waffle();
}
this.tastes = "yummy";
}
Waffle.prototype.wantAnother = true;

when writing this way you can invoke Waffle either one of these ways

var first = new Waffle(),
second = Waffle(); 

I think this is a helpful feature not sure if it's implemented in future versions of ecma/javascript

I came up with something on my own that I thought could just copy and paste each time when creating a constructor function

something like this

function checkInstance (name) {
    if (name.constructor.name === undefined) {
       return "construct it"
    } else {
       return false;
    }
}

function Waffle() {
    var _self = checkInstance.call(this, this);
    if (_self === "construct it") {
       return new Waffle()
    }
    this.tastes = "yummy"
}

var waffle = Waffle()
waffle

Therefore I can invoke Waffle either way new Waffle or Waffle() and still have it return an object

My problem that I'm having is here

  if (_self === "construct it") {
       return new Waffle()
       }

Is there anyway I can refer to new Waffle() without referring to the actual name of the constructor function meaning so I could copy and paste this each time and not have to change anything. Meaning I could I save Waffle() as a variable and do something like

return new var

I wish I could use this.name but that doesn't work either until it is invoked.

I have a feeling I can't but wanted to at least ask some of the people here on stack overflow if it was a possibility

Again your comments and feedback is appreciated

Answer 1

I have a better solution. This is what you're currently doing:

function Waffle() {
    if (!(this instanceof Waffle))
        return new Waffle;
    this.tastes = "yummy";
}

Waffle.prototype.wantAnother = true;

This pattern isn't really nice because you're mixing the code to construct a new object with the code to check if the new keyword is being used.

I've mentioned before that you shouldn't use the new keyword in JavaScript as it breaks functional features. Instead let's create another function which does the same thing:

Function.prototype.new = (function () {
    return function () {
        functor.prototype = this.prototype;
        return new functor(this, arguments);
    };

    function functor(constructor, args) {
        return constructor.apply(this, args);
    }
}());

This function allows you to create an instance of a function as follows:

var waffle = Waffle.new();

However we don't want to use new at all. So to do away with it we'll create a function which wraps a constructor as follows:

function constructible(constructor) {
    function functor() { return Function.new.apply(constructor, arguments); }
    functor.prototype = constructor.prototype;
    return functor;
}

Now we can define the Waffle function as follows:

var Waffle = constructible(function () {
    this.tastes = "yummy";
});

Waffle.prototype.wantAnother = true;

Now you can create objects with or without using new:

var first = new Waffle;
var second = Waffle();

Note: The constructible function is pretty slow. Use the following version of constructible instead - it's a little faster:

function constructible(constructor) {
    constructor = Function.bind.bind(constructor, null);
    function functor() { return new (constructor.apply(null, arguments)); }
    functor.prototype = constructor.prototype;
    return functor;
}

---

Personally I wouldn't use either of these two methods. I would just remember to write new, or (more likely) I would restructure my code as follows:

var waffle = {
    create: function () {
        var waffle = Object.create(this);
        waffle.tastes = "yummy";
        return waffle;
    },
    wantAnother: true
};

var first = waffle.create();
var second = waffle.create();

If you want to know more about this pattern then read the following answer: https://stackoverflow.com/a/17008403/783743