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The prototype is an object that every function gets automatically, as soon as you create it.

You can add properties and methods to the prototype property of your constructor function. Then all objects created with this constructor will have access to everything you add to the prototype.

var Person = function(name) {
  this.name = name;
};
Person.prototype.say = function() {
  return this.name;
};

if you add properties to the prototype of a "normal" function, meaning function that you don't invoke as a constructor, these will not be used.

Using the prototype

var adam = new Person('Adam');
alert(adam.name); // "Adam"
alert(adam.say()); // "Adam"

As you can see the object has access to the prototype’s members.

The prototype properties are shared among all objects.

Rule of thumb

And a general rule of thumb – the properties and methods that are meant to be reused and shared and inherited, should be added to the prototype.

A constructor is just a function and failing to invoke a constructor with new leads to errors. Not syntax errors but logical errors. Following a naming convention can certainly help, but it doesn't enforce the correct behavior. If you invoke a constructor without new, then this will refer to the global object, called window in browsers.

Here's a pattern that helps you make sure your constructor always behaves as constructor. Instead of adding all members to this, you add them to that.

In this implementation that is the same as this unless this is window which means someone forgot to use new.

function Person() { 
  var that = (this === window) ? {} : this;
 
  that.name = name;
  that.say = function() {
    return "I am " + that.name;
  };
 
  return that; 
}

window is only in browsers

Another way, if you work in non-browser environment is to check if this is an instance of your Person constructor.

Or name-agnostic, general-purpose way is to use arguments.callee instead of hard-coding the constructor name.

this instanceof Person
this instanceof arguments.callee

Constructors are just functions, it all depends on how they are invoked - with or without new.

So a naming pattern is pretty useful to help you visually distinguish between the two. It gives you a clue that the uppercase functions are meant to be called with new.

MyConstructor
myFunction

A simple way to create objects in JavaScript is using object literal notation. An other way, a bit more involved is to use a constructor function.

var adam = new Person("Adam");
adam.say(); // "I am Adam"

The invocation of the constructor looks a bit like a class in other languages, but the constructor is still just a function. (JavaScript doesn’t have classes). So the constructor is the blueprint, the recipe for the object.

And here's how the constructor Person is defined, it adds all members to the this object.

var Person = function(name) {
 
  this.name = name;
  this.say = function() {
    return "I am " + this.name;
  };
 
};

When you define a constructor, something like the following happens behind the scenes. It's as if you defined an object using the literal notation and then returned it.

var Person = function(name) {
  // var this = {};
  this.name = name;
  this.say = function() {
    return “I am ” + this.name;
  };
  // return this;
};

According to Wikipedia:

...the singleton pattern is a design pattern that is used to restrict instantiation of a class to one object.

How does this apply to JavaScript? Well, in JavaScript there are no classes, just objects. So when you create a new object, there's no other object like it and the new object is already a singleton.

var obj = {
  myprop: 'my value'
};

This is an example of creating a simple object in JavaScript using the object literal and it's also an example of a singleton.

Using new

The other question is how about if you want to use the Java-like syntax:

var obj = new Constructor();

This is more of classic-like syntax so the idea of a singleton here is that when you use new to create several objects using the same constructor, you should only get one object.

var obj = new Constructor();
var obj2 = new Constructor();
alert(obj === obj2); // true

obj is created only the first time the constructor is called, the second time (and the third, fourth, etc) the obj created the first time is returned. How to achieve this in JavaScript?

Basically you need your Constructor to cache the object instance this once it's created and then pull this instance the second time it's called. You can cache the object instance:

• in a global variable - not recommended, general principle is that globals are bad, plus anyone can overwrite this global, even by accident;
• in a Constructor property - functions in JavaScript are objects, so they can have properties. You can have something like Constructor.instance and cache the object there. This is a nice clean solution with the only drawback that this property is accessible and code outside of yours might change it, so you lose the instance;
• wrap the instance in a closure

Let's take a look at an example implementation of the last option.

function Constructor() {
  return Constructor.getInstance(this);
}
Constructor.getInstance = function(){
  var instance;
  return function(that) {
    if (typeof instance === 'undefined') {
      instance = that;
    }
    return instance;
  }
}();

And a simpler new

function Single() {
 
  var instance = this; 
 
  // add more to this…
 
  Single = function (){
    return instance;  
  };
}
 
// test
var my1 = new Single();
var my2 = new Single();
alert(my1 === my2) //true