jQuery - Basics

jQuery is a framework built using JavaScript capabilities. So, you can use all the functions and other capabilities available in JavaScript. This chapter would explain most basic concepts but frequently used in jQuery.

String

A string in JavaScript is an immutable object that contains none, one or many characters. Following are the valid examples of a JavaScript String −

"This is JavaScript String"
'This is JavaScript String'
'This is "really" a JavaScript String'
"This is 'really' a JavaScript String"

Numbers

Numbers in JavaScript are double-precision 64-bit format IEEE 754 values. They are immutable, just as strings. Following are the valid examples of a JavaScript Numbers −

5350
120.27
0.26

Boolean

A boolean in JavaScript can be either true or false. If a number is zero, it defaults to false. If an empty string defaults to false.

Following are the valid examples of a JavaScript Boolean −

true      // true
false     // false
0         // false
1         // true
""        // false
"hello"   // true

Objects

JavaScript supports Object concept very well. You can create an object using the object literal as follows −

var emp = {
   name: "Zara",
   age: 10
};

You can write and read properties of an object using the dot notation as follows −

// Getting object properties
emp.name  // ==> Zara
emp.age   // ==> 10

// Setting object properties
emp.name = "Daisy"  // <== Daisy
emp.age  =  20      // <== 20

Arrays

You can define arrays using the array literal as follows −

var x = [];
var y = [1, 2, 3, 4, 5];

An array has a length property that is useful for iteration −

var x = [1, 2, 3, 4, 5];

for (var i = 0; i < x.length; i++) {
   // Do something with x[i]
}

Functions

A function in JavaScript can be either named or anonymous. A named function can be defined using function keyword as follows −

function named(){
   // do some stuff here
}

An anonymous function can be defined in similar way as a normal function but it would not have any name.

A anonymous function can be assigned to a variable or passed to a method as shown below.

var handler = function (){
   // do some stuff here
}

JQuery makes a use of anonymous functions very frequently as follows −

$(document).ready(function(){
   // do some stuff here
});

Arguments

JavaScript variable arguments is a kind of array which has length property. Following example explains it very well −

function func(x){
   console.log(typeof x, arguments.length);
}

func();                //==> "undefined", 0
func(1);               //==> "number", 1
func("1", "2", "3");   //==> "string", 3

The arguments object also has a callee property, which refers to the function you're inside of. For example −

function func() {
   return arguments.callee; 
}

func();                // ==> func

Context

JavaScript famous keyword this always refers to the current context. Within a function this context can change, depending on how the function is called −

$(document).ready(function() {
   // this refers to window.document
});

$("div").click(function() {
   // this refers to a div DOM element
});

You can specify the context for a function call using the function-built-in methods call() and apply() methods.

The difference between them is how they pass arguments. Call passes all arguments through as arguments to the function, while apply accepts an array as the arguments.

function scope() {
   console.log(this, arguments.length);
}

scope() // window, 0
scope.call("foobar", [1,2]);  //==> "foobar", 1
scope.apply("foobar", [1,2]); //==> "foobar", 2

Scope

The scope of a variable is the region of your program in which it is defined. JavaScript variable will have only two scopes.

• Global Variables − A global variable has global scope which means it is defined everywhere in your JavaScript code.

• Local Variables − A local variable will be visible only within a function where it is defined. Function parameters are always local to that function.

Within the body of a function, a local variable takes precedence over a global variable with the same name −

var myVar = "global";     // ==> Declare a global variable

function ( ) {
   var myVar = "local";   // ==> Declare a local variable
   document.write(myVar); // ==> local
}

Callback

A callback is a plain JavaScript function passed to some method as an argument or option. Some callbacks are just events, called to give the user a chance to react when a certain state is triggered.

jQuery's event system uses such callbacks everywhere for example −

$("body").click(function(event) {
   console.log("clicked: " + event.target);
});

Most callbacks provide arguments and a context. In the event-handler example, the callback is called with one argument, an Event.

Some callbacks are required to return something, others make that return value optional. To prevent a form submission, a submit event handler can return false as follows −

$("#myform").submit(function() {
   return false;
});

Closures

Closures are created whenever a variable that is defined outside the current scope is accessed from within some inner scope.

Following example shows how the variable counter is visible within the create, increment, and print functions, but not outside of them −

function create() {
   var counter = 0;
	
   return {
      increment: function() {
         counter++;
      },
	   print: function() {
         console.log(counter);
      }
   }
}

var c = create();
c.increment();
c.print();     // ==> 1

This pattern allows you to create objects with methods that operate on data that isn't visible to the outside world. It should be noted that data hiding is the very basis of object-oriented programming.

Proxy Pattern

A proxy is an object that can be used to control access to another object. It implements the same interface as this other object and passes on any method invocations to it. This other object is often called the real subject.

A proxy can be instantiated in place of this real subject and allow it to be accessed remotely. We can saves jQuery's setArray method in a closure and overwrites it as follows −

(function() {
   // log all calls to setArray
   var proxied = jQuery.fn.setArray;

   jQuery.fn.setArray = function() {
      console.log(this, arguments);
      return proxied.apply(this, arguments);
   };
	
})();

The above wraps its code in a function to hide the proxied variable. The proxy then logs all calls to the method and delegates the call to the original method. Using apply(this, arguments) guarantees that the caller won't be able to notice the difference between the original and the proxied method.

Built-in Functions

JavaScript comes along with a useful set of built-in functions. These methods can be used to manipulate Strings, Numbers and Dates.

Following are important JavaScript functions −

The Document Object Model

The Document Object Model is a tree structure of various elements of HTML as follows −

<html>
   <head>
      <title>The jQuery Example</title>
   </head>
	
   <body>
      <div>
         <p>This is a paragraph.</p>
         <p>This is second paragraph.</p>
         <p>This is third paragraph.</p>
      </div>
   </body>
</html>

This will produce following result −

Following are the important points about the above tree structure −

• The <html> is the ancestor of all the other elements; in other words, all the other elements are descendants of <html>.

• The <head> and <body> elements are not only descendants, but children of <html>, as well.

• Likewise, in addition to being the ancestor of <head> and <body>, <html> is also their parent.

• The <p> elements are children (and descendants) of <div>, descendants of <body> and <html>, and siblings of each other <p> elements.

While learning jQuery concepts, it will be helpful to have understanding on DOM, if you are not aware of DOM then I would suggest to go through our simple tutorial on DOM Tutorial.