iRaylab: Object-Oriented JavaScript questions and answers

1. What is the difference between Classic Inheritance and Prototypical Inheritance?

Keyword:
Classic Inheritance: class and object
Prototypical Inheritance: object, no class

Answer:
In Classic Inheritance, there are two types of abstraction: class and object. To create an object, first define the structure of the object, using a class declaration, then instantiate the class to create a new object. Objects created in this manner have their own copies of all instance attributes, plus a link to the single copy of each of the instance methods. Java and C# use classic inheritance.

In Prototypical Inheritance, there is only one type of abstraction: object. We create an object directly, instead of defining the structure through a class. The object can be reused by new objects. JavaScript use prototypical inheritance.

2. What are the different ways to create an object in JavaScript?

Keyword:
object literal,
constructor function,
Object.create method

Answer:
There are three typical ways to create an object:

a. Using object literal
An object literal is a comma-separated list of colon-separated name:value pairs, enclosed within curly braces.
For example:
var myObject = {
property1 : "www.iraylab.com",
property2 : {
nestedProperty1 : "JavaScript"
}
};

b. Using constructor function
Define the object type by writing a constructor function. Then, create an instance of the object with new keyword.
For example:
function Car(make, model, year) {
this.make = make;
this.model = model;
this.year = year;
}
var myCar = new Car("BMW", "X5", 2014);

c. Using the Object.create method
With ECMAScript 5, we can a static function Object.create() to create an object.
The Object.create() method creates a new object with the specified prototype object and properties: Object.create(prototype [, propertiesObject ])

For example:
var obj = Object.create(null, {
make: "BMW",
model: "X5",
year: 2014
});

3. How to access the properties of an object?

Keyword:
dot notation .
bracket notation []

Answer:
There are two ways to access the properties of an object.

var car = {
make: "BMW",
year: 2014
};

a. Using the dot notation: .
var m = car.make; // get property value
car.make = "GM"; // set property new value

b. Using the bracket notation: []
var m = car["make"];
car["make"] = "GM";

4. How to define public and private properties/methods in an object?

Keyword:
private: var
public: this

Answer:
In a JavaScript object, properties/methods defined with var keyword can only be accessed inside the object;
properties/methods defined with this keyword can be accessed from outside the object.
For example:
function Car (maker, model, year) {
//public properties and methods:
this.maker = maker;
this.model = model;
this.year = year;
this.getInfo = function() {
return "Car info: maker=" + this.maker + ", model="+ this.model +", year=" + this.year;
};
//private properties and methods:
var color;
var getColor = function() {
return "color=" + color;
};

//access a private method inside the object
this.getDetailedInfo = function() {
return this.getInfo() + ", " + getColor();
}
}

5. How to add custom methods and properties to an object?

Keyword:
use prototype

Answer:
To add properties and methods to an object, we can modify its prototype property.

For example:
function Car (maker, model, year) {
this.maker = maker;
this.model = model;
this.year = year
}

//add new property and new method:
Car.prototype.color = "Black";
Car.prototype.getRating = function() {
return 1;
};

6. How to create a namespace in JavaScript?

Answer:
In JavaScript, there is no the concept of namespace, we can create global object to emulate namespace.

For example, the following code create a namespace "iraylab.JSModule" and create two functions inside this namespace:

var iraylab = iraylab || {};
iraylab.JSModule = {
add: function(a, b) {
return a + b;
},
subtract: function(a, b) {
return a -b;
}
};

7. How to implement inheritance in JavaScript?

Keyword:
use prototype and constructor function

Answer:
JavaScript is a class-free, object-oriented language, we can implement prototypal inheritance by using prototype and constructor function.

The following is an example to implement this kind of inheritance.
a. Define super class:
function Vehicle () {}

b. Define sub class:
function Car (maker, model, year) {
this.maker = maker;
this.model = model;
this.year = year
}

c. Set up inheritance:
Car.prototype = new Vehicle;

d. Extend super class using prototype:
Vehicle.prototype.getInfo = function(){
console.log("This is a vehicle.");
}

e. Sub class override super class's method:
Car.prototype.getInfo = function() {
console.log("This is a car.");
};

8. How to implement Singleton Pattern in JavaScript?

Answer:
The singleton pattern is a design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system.

In JavaScript, we can implement a Singleton as follows:

var Singleton = (function () {
var instance;

function init() {

//define private methods and variables here...
function privateMethod(){
console.log( "a private method" );
}
var privateVariable = "a private variable";

return {

// Public methods and variables
publicMethod: function () {
console.log( "a public method" );
},

publicProperty: "a public property"
};
};

return {
// Get the Singleton instance if one exists or create one if it doesn't
getInstance: function () {
if ( !instance ) {
instance = init();
}
return instance;
}
};
})();

// Use the singleton class:
var mySingleton = Singleton.getInstance();
......

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iRaylab

Thursday, September 25, 2014

Object-Oriented JavaScript questions and answers