Node.js Tutorial - Node.js Class Creation

Functions that return objects are a great way to create similar objects.

Example

The following code shows how to create an object using this pattern.


function Message() {//from   w  ww. ja  va2  s  .c om
   var message = 'hello';
   function setMessage(newMessage) {
      if (!newMessage) 
         throw new Error('cannot set empty message');
      message = newMessage;
   }
   function getMessage() {
      return message;
   }
   function printMessage() { 
      console.log(message); 
   }
   return {
      setMessage: setMessage,
      getMessage: getMessage,
      printMessage: printMessage
   }; 
} 
// Pattern in use 
var hi1 = Message(); 
hi1.printMessage(); // hello 

var hi2 = Message(); 
hi2.setMessage('hi'); 
hi2.printMessage(); // hi 

hi1.printMessage(); // hello

Understanding this

Javascript this object behaves differently depending upon how we call it.

this object refers to the calling context.

The calling context is the prefix used to call a function.


var myData = { 
   myValue: 123, 
   myFunction: function () {
      console.log('inside this.myValue is:', this.myValue); 
   } 
} 
console.log('myData.myValue is: ', myData.myValue); // myData.myValue is: 123 
myData.myFunction(); // inside this.myValue is: 123

The default calling context is the Node.js global variable.


function myData() {
    console.log('is this called from globals? : ', this === global); // true 
} 
myData();

We can attach a function to any object and change the calling context.


var myData = { 
    myValue: 123 /*w  w w.j a va 2s.  com*/
}; 

function myFunction() { 
    if (this === global) 
        console.log('called from global'); 
    if (this === myData) 
        console.log('called from myData'); 
} 

// global context 
myFunction(); // called from global 

// from myData 
myData.myFunction = myFunction; 
myData.myFunction(); // called from myData

If you call a function with the new JavaScript operator, it creates a new JavaScript object and this within the function refers to this newly created object.


function myData() {//  w  ww  . j a  v  a2  s  . c  om
   this.myData = 123;
   console.log('Is this global?: ', this == global); 
} 

// without the new operator 
myData(); // Is this global?: true 
console.log(global.myData); // 123 

// with the new operator 
var newFoo = new myData(); // Is this global?: false 
console.log(newFoo.myData); // 123

In the code above we modified this.myData inside the function and newFoo.myData was set to that value.

Understanding Prototype

Every object in JavaScript has an internal link to another object called the prototype.

When you read a property on an object, myData.myValue reads the property myValue from myData, JavaScript checks that this property exists on myData.

If not, JavaScript checks if the property exists on myData.__proto__ and so on till __proto__ itself is not present.

If a value is found at any level, it is returned.

Otherwise, JavaScript returns undefined.


var shape = {}; 
shape.__proto__.myValue= 123; 
console.log(shape.myValue); // 123

__ prefix in JavaScript is should not be used by user code.

When creating an object using the new operator on a function, the __proto__ is set to the function's .prototype member.


function shape() { }; 
shape.prototype.myValue = 123; 

var bas = new shape(); 

console.log(bas.__proto__ === shape.prototype); // true 
console.log(bas.myValue); // 123

Note

prototypes are shared between all the objects created from the same function.


function shape() { }; //www. j  av  a 2 s  .  co m
shape.prototype.myValue = 123; 

var bas = new shape(); 
var myItem = new shape(); 

console.log(bas.myValue); // 123 
console.log(myItem.myValue); // 123 

shape.prototype.myValue = 456; 

console.log(bas.myValue); // 456 
console.log(myItem.myValue); // 456

The code above generates the following result.

Suppose we have 1,000 instances created of a certain object. All the properties and functions on prototype is shared.

Therefore prototype saves memory.

Note 2

A prototype property is shadowed by a property on an object.


function shape() { }; //from   w  w w . j  a  v  a 2 s. c  o m
shape.prototype.myValue = 123; 

var bas = new shape(); 
var myItem = new shape(); 

bas.myValue = 456; 
console.log(bas.myValue);

console.log(myItem.myValue); // 123

this object is a perfect candidate for read/write properties(data) and you should use it for all properties(data).

But functions are generally not altered after creation. So functions are great candidates to put on .prototype.

functionality (functions/methods) is shared between all instances, and properties belong on individual objects.

Example 2

The following code shows a pattern to write a class in JavaScript.


function someClass() { /*from   w ww  .  j  av a2 s  .c om*/
   // Properties go here 
   this.someProperty = 'some initial value'; 
} 
// Member functions go here: 
someClass.prototype.someMemberFunction = function () {
   this.someProperty = 'modified value'; 
   console.log("called from prototype");
} 
// Creation 
var instance = new someClass(); 
// Usage 
console.log(instance.someProperty); // some initial value 
instance.someMemberFunction(); 
console.log(instance.someProperty); // modified value

The code above generates the following result.

Within the member function, we can get access to the current instance using this even though the same function body is shared between all instances.

Most classes inside core Node.js are written using this pattern.