Friday, February 13, 2015

Getting started with jQuery

Getting started with jQuery

Introduction

jQuery is a lightweight, "write less, do more", JavaScript library.

The purpose of jQuery is to make it much easier to use JavaScript on your website.

jQuery takes a lot of common tasks that require many lines of JavaScript code to accomplish, and wraps them into methods that you can call with a single line of code.
jQuery also simplifies a lot of the complicated things from JavaScript, like AJAX calls and DOM manipulation.

The jQuery library contains the following features:
  •          HTML/DOM manipulation
  •          CSS manipulation
  •          HTML event methods
  •          Effects and animations
  •         AJAX
  •         Utilities


Downloading jQuery


There are two versions of jQuery available for downloading:

  • Production version - this is for your live website because it has been minified and compressed
  • Development version - this is for testing and development (uncompressed and readable code)


Both versions can be downloaded from jQuery.com.

The jQuery library is a single JavaScript file, and you reference it with the HTML <script> tag (notice that the <script> tag should be inside the <head> section):
<head>
<script src="jquery-1.11.2.min.js"></script>
</head>


jQuery CDN


If you don't want to download and host jQuery yourself, you can include it from a CDN (Content Delivery Network).

Both Google and Microsoft host jQuery.

To use jQuery from Google or Microsoft, use one of the following:


Google CDN:

<head>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>
</head>


Microsoft CDN:

<head>
<script src="http://ajax.aspnetcdn.com/ajax/jQuery/jquery-1.11.2.min.js"></script>
</head>

One big advantage of using the hosted jQuery from Google or Microsoft:


Many users already have downloaded jQuery from Google or Microsoft when visiting another site. As a result, it will be loaded from cache when they visit your site, which leads to faster loading time. Also, most CDN's will make sure that once a user requests a file from it, it will be served from the server closest to them, which also leads to faster loading time.


jQuery Syntax

The jQuery syntax is tailor made for selecting HTML elements and performing some action on the element(s).


Basic syntax is: $(selector).action()

  •          A $ sign to define/access jQuery
  •          A (selector) to "query (or find)" HTML elements
  •          A jQuery action() to be performed on the element(s


The Document Ready Event

It is good practice to wait for the document to be fully loaded and ready before working with it. This also allows you to have your JavaScript code before the body of your document, in the head section.


$(document).ready(function(){
   // jQuery methods go here...
});


This is to prevent any jQuery code from running before the document is finished loading.


The jQuery team has also created an even shorter method for the document ready event:


$(function(){
   // jQuery methods go here...
});


jQuery Selectors

jQuery selectors allow you to select and manipulate HTML element(s).

jQuery selectors are used to "find" (or select) HTML elements based on their id, classes, types, attributes, values of attributes and much more. It's based on the existing CSS Selectors, and in addition, it has some own custom selectors.

All selectors in jQuery start with the dollar sign and parentheses: $().


The #id Selector

The jQuery #id selector uses the id attribute of an HTML tag to find the specific element.

An id should be unique within a page, so you should use the #id selector when you want to find a single, unique element.

To find an element with a specific id, write a hash character, followed by the id of the HTML element:

$("#test")


The .class Selector

The jQuery class selector finds elements with a specific class.
To find elements with a specific class, write a period character, followed by the name of the class:


$(".test")

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