How do I declare a namespace in JavaScript?

Question

How do I create a namespace in JavaScript so that my objects and functions aren't overwritten by other same-named objects and functions? I've used the following:

if (Foo == null || typeof(Foo) != "object") { var Foo = new Object();}

Is there a more elegant or succinct way of doing this?

Answer 1

My favorite pattern has become lately this:

var namespace = (function() {
  
  // expose to public
  return {
    a: internalA,
    c: internalC
  }

  // all private
  
  /**
   * Full JSDoc
   */
  function internalA() {
    // ...
  }
  
  /**
   * Full JSDoc
   */
  function internalB() {
    // ...
  }
  
  /**
   * Full JSDoc
   */
  function internalC() {
    // ...
  }
  
  /**
   * Full JSDoc
   */
  function internalD() {
    // ...
  }
  
})();

Of course, return can be at the end, but if only function declarations follow it, it's much easier to see what's the namespace all about, and what API is exposed.

The pattern of using function expressions in such cases results in not being able to know what methods are exposed without going over the entire code.

Answer 2

JavaScript doesn’t support namespace by default. So if you create any element(function, method, object, variable) then it becomes global and pollute the global namespace. Let's take an example of defining two functions without any namespace,

function func1() {
    console.log("This is a first definition");

}
function func1() {
    console.log("This is a second definition");
}
func1(); // This is a second definition

It always calls the second function definition. In this case, namespace will solve the name collision problem.

Answer 3

I normally build it in a closure:

var MYNS = MYNS || {};

MYNS.subns = (function() {

    function privateMethod() {
        // Do private stuff, or build internal.
        return "Message";
    }

    return {
        someProperty: 'prop value',
        publicMethod: function() {
            return privateMethod() + " stuff";
        }
    };
})();

My style over the years has had a subtle change since writing this, and I now find myself writing the closure like this:

var MYNS = MYNS || {};

MYNS.subns = (function() {
    var internalState = "Message";

    var privateMethod = function() {
        // Do private stuff, or build internal.
        return internalState;
    };
    var publicMethod = function() {
        return privateMethod() + " stuff";
    };

    return {
        someProperty: 'prop value',
        publicMethod: publicMethod
    };
})();

In this way I find the public API and implementation easier to understand. Think of the return statement as being a public interface to the implementation.

Answer 4

I created namespace which is inspired by Erlang's modules. It is a very functional approach, but that is how I write my JavaScript code these days.

It gives a closure a global namespace and exposes a defined set functions within that closure.

(function(){

  namespace("images", previous, next);
  // ^^ This creates or finds a root object, images, and binds the two functions to it.
  // It works even though those functions are not yet defined.

  function previous(){ ... }

  function next(){ ... }

  function find(){ ... } // A private function

})();

Answer 5

I use the approach found on the Enterprise jQuery site:

Here is their example showing how to declare private & public properties and functions. Everything is done as a self-executing anonymous function.

(function( skillet, $, undefined ) {
    //Private Property
    var isHot = true;

    //Public Property
    skillet.ingredient = "Bacon Strips";

    //Public Method
    skillet.fry = function() {
        var oliveOil;

        addItem( "\t\n Butter \n\t" );
        addItem( oliveOil );
        console.log( "Frying " + skillet.ingredient );
    };

    //Private Method
    function addItem( item ) {
        if ( item !== undefined ) {
            console.log( "Adding " + $.trim(item) );
        }
    }
}( window.skillet = window.skillet || {}, jQuery ));

So if you want to access one of the public members you would just go skillet.fry() or skillet.ingredients.

What's really cool is that you can now extend the namespace using the exact same syntax.

//Adding new Functionality to the skillet
(function( skillet, $, undefined ) {
    //Private Property
    var amountOfGrease = "1 Cup";

    //Public Method
    skillet.toString = function() {
        console.log( skillet.quantity + " " +
                     skillet.ingredient + " & " +
                     amountOfGrease + " of Grease" );
        console.log( isHot ? "Hot" : "Cold" );
    };
}( window.skillet = window.skillet || {}, jQuery ));

The third undefined argument

The third, undefined argument is the source of the variable of value undefined. I'm not sure if it's still relevant today, but while working with older browsers / JavaScript standards (ecmascript 5, javascript < 1.8.5 ~ firefox 4), the global-scope variable undefined is writable, so anyone could rewrite its value. The third argument (when not passed a value) creates a variable named undefined which is scoped to the namespace/function. Because no value was passed when you created the name space, it defaults to the value undefined.

Answer 6

Here's how Stoyan Stefanov does it in his JavaScript Patterns book which I found to be very good (it also shows how he does comments that allows for auto-generated API documentation, and how to add a method to a custom object's prototype):

/**
* My JavaScript application
*
* @module myapp
*/

/** @namespace Namespace for MYAPP classes and functions. */
var MYAPP = MYAPP || {};

/**
* A maths utility
* @namespace MYAPP
* @class math_stuff
*/
MYAPP.math_stuff = {

    /**
    * Sums two numbers
    *
    * @method sum
    * @param {Number} a First number
    * @param {Number} b Second number
    * @return {Number} Sum of the inputs
    */
    sum: function (a, b) {
        return a + b;
    },

    /**
    * Multiplies two numbers
    *
    * @method multi
    * @param {Number} a First number
    * @param {Number} b Second number
    * @return {Number} The inputs multiplied
    */
    multi: function (a, b) {
        return a * b;
    }
};

/**
* Constructs Person objects
* @class Person
* @constructor
* @namespace MYAPP
* @param {String} First name
* @param {String} Last name
*/
MYAPP.Person = function (first, last) {

    /**
    * First name of the Person
    * @property first_name
    * @type String
    */
    this.first_name = first;

    /**
    * Last name of the Person
    * @property last_name
    * @type String
    */
    this.last_name = last;
};

/**
* Return Person's full name
*
* @method getName
* @return {String} First name + last name
*/
MYAPP.Person.prototype.getName = function () {
    return this.first_name + ' ' + this.last_name;
};