Index

• What is JavaScript
• Applications of Javascript Programming
• Client-Side JavaScript
• Advantages of JavaScript
• Limitations of JavaScript
• Your First JavaScript Code
• Case Sensitivity, Comments in JavaScript
• Enable JavaScript in different Browsers
• JavaScript Location in HTML document
• JavaScript in External File

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What is JavaScript�

JavaScript is a dynamic computer programming language. It is lightweight and most commonly used as a part of web pages, whose implementations allow client-side script to interact with the user and make dynamic pages. It is an interpreted programming language with object-oriented capabilities.

JavaScript is a lightweight, interpreted programming language. It is designed for creating network-centric applications. It is complimentary to and integrated with Java. JavaScript is very easy to implement because it is integrated with HTML. It is open and cross-platform.

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There are many useful Java script frameworks and libraries available:

• Angular
• React
• jQuery
• Mithril
• Node.js
• Polymer
• Aurelia
• Backbone.js

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It is really impossible to give a complete list of all the available Java script frameworks and libraries. The Java script world is just too large and too much new is happening.

JavaScript was first known as Live Script, but Netscape changed its name to JavaScript, possibly because of the excitement being generated by Java. JavaScript made its first appearance in Netscape 2.0 in 1995 with the name Live Script.

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The general-purpose core of the language has been embedded in Netscape, Internet Explorer, and other web browsers.

• JavaScript is a lightweight, interpreted programming language.
• Designed for creating network-centric applications.
• Complementary to and integrated with Java.
• Complementary to and integrated with HTML.
• Open and cross-platform

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�Applications of Javascript Programming�

As mentioned before, Javascript is one of the most widely used programming languages (Front-end as well as Back-end). It has it's presence in almost every area of software development. I'm going to list few of them here:

• Client side validation - This is really important to verify any user input before submitting it to the server and Javascript plays an important role in validting those inputs at front-end itself.
• Manipulating HTML Pages - Javascript helps in manipulating HTML page on the fly. This helps in adding and deleting any HTML tag very easily using javascript and modify your HTML to change its look and feel based on different devices and requirements.
• User Notifications - You can use Javascript to raise dynamic pop-ups on the webpages to give different types of notifications to your website visitors.

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• Back-end Data Loading - Javascript provides Ajax library which helps in loading back-end data while you are doing some other processing. This really gives an amazing experience to your website visitors.
• Presentations - JavaScript also provides the facility of creating presentations which gives website look and feel. JavaScript provides RevealJS and BespokeJS libraries to build a web-based slide presentations.
• Server Applications - Node JS is built on Chrome's Javascript runtime for building fast and scalable network applications. This is an event based library which helps in developing very sophisticated server applications including Web Servers.

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Client-Side JavaScript�

Client-side JavaScript is the most common form of the language. The script should be included in or referenced by an HTML document for the code to be interpreted by the browser.

It means that a web page need not be a static HTML, but can include programs that interact with the user, control the browser, and dynamically create HTML content.

The JavaScript client-side mechanism provides many advantages over traditional CGI server-side scripts. For example, you might use JavaScript to check if the user has entered a valid e-mail address in a form field.

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The JavaScript code is executed when the user submits the form, and only if all the entries are valid, they would be submitted to the Web Server.

JavaScript can be used to trap user-initiated events such as button clicks, link navigation, and other actions that the user initiates explicitly or implicitly.

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Advantages of JavaScript�

The merits of using JavaScript are −

• Less server interaction − You can validate user input before sending the page off to the server. This saves server traffic, which means less load on your server.
• Immediate feedback to the visitors − They don't have to wait for a page reload to see if they have forgotten to enter something.
• Increased interactivity − You can create interfaces that react when the user hovers over them with a mouse or activates them via the keyboard.
• Richer interfaces − You can use JavaScript to include such items as drag-and-drop components and sliders to give a Rich Interface to your site visitors.

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Limitations of JavaScript�

We cannot treat JavaScript as a full-fledged programming language. It lacks the following important features −

• Client-side JavaScript does not allow the reading or writing of files. This has been kept for security reason.
• JavaScript cannot be used for networking applications because there is no such support available.
• JavaScript doesn't have any multi-threading or multiprocessor capabilities.

Once again, JavaScript is a lightweight, interpreted programming language that allows you to build interactivity into otherwise static HTML pages.

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JavaScript Code�

JavaScript can be implemented using JavaScript statements that are placed within the <script>... </script> HTML tags in a web page.

You can place the <script> tags, containing your JavaScript, anywhere within your web page, but it is normally recommended that you should keep it within the <head> tags.

The <script> tag alerts the browser program to start interpreting all the text between these tags as a script. A simple syntax of your JavaScript will appear as follows.

<script ...>

JavaScript code

</script>

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The script tag takes two important attributes −

• Language − This attribute specifies what scripting language you are using. Typically, its value will be javascript. Although recent versions of HTML (and XHTML, its successor) have phased out the use of this attribute.
• Type − This attribute is what is now recommended to indicate the scripting language in use and its value should be set to "text/javascript".

So your JavaScript segment will look like −

<script language = "javascript" type = "text/javascript">

JavaScript code

</script>

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Let us take a sample example to print out "Hello World". We added an optional HTML comment that surrounds our JavaScript code. This is to save our code from a browser that does not support JavaScript. The comment ends with a "//-->". Here "//" signifies a comment in JavaScript, so we add that to prevent a browser from reading the end of the HTML comment as a piece of JavaScript code. Next, we call a function document.write which writes a string into our HTML document.

This function can be used to write text, HTML, or both. Take a look at the following code.

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Semicolons are Optional

Simple statements in JavaScript are generally followed by a semicolon character, just as they are in C, C++, and Java. JavaScript, however, allows you to omit this semicolon if each of your statements are placed on a separate line. For example, the following code could be written without semicolons.

<script language = "javascript" type = "text/javascript">

<!--

var1 = 10

var2 = 20

//-->

</script>

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But when formatted in a single line as follows, you must use semicolons −

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<script language = "javascript" type = "text/javascript">

<!--

var1 = 10; var2 = 20;

//-->

</script>

Note − It is a good programming practice to use semicolons.

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<html>

<body>

<script language = "javascript" type = "text/javascript">

<!--

document.write("Hello World!")

//-->

</script>

</body>

</html>

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Case Sensitivity

JavaScript is a case-sensitive language. This means that the language keywords, variables, function names, and any other identifiers must always be typed with a consistent capitalization of letters.

So the identifiers Time and TIME will convey different meanings in JavaScript.

NOTE − Care should be taken while writing variable and function names in JavaScript.

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Comments in JavaScript

JavaScript supports both C-style and C++-style comments, Thus −

• Any text between a // and the end of a line is treated as a comment and is ignored by JavaScript.
• Any text between the characters /* and */ is treated as a comment. This may span multiple lines.
• JavaScript also recognizes the HTML comment opening sequence <!--. JavaScript treats this as a single-line comment, just as it does the // comment.
• The HTML comment closing sequence --> is not recognized by JavaScript so it should be written as //-->.

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The following example shows how to use comments in JavaScript.

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<script language = "javascript" type = "text/javascript">

<!--

// This is a comment. It is similar to comments in C++

/* * This is a multi-line comment in JavaScript

* It is very similar to comments in C Programming

*/

//-->

</script>

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All the modern browsers come with built-in support for JavaScript. Frequently, you may need to enable or disable this support manually. This chapter explains the procedure of enabling and disabling JavaScript support in your browsers: Internet Explorer, Firefox, chrome, and Opera.

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Enable JavaScript in Internet Explorer�

Here are simple steps to turn on or turn off JavaScript in your Internet Explorer −

• Follow Tools → Internet Options from the menu.
• Select Security tab from the dialog box.
• Click the Custom Level button.
• Scroll down till you find Scripting option.
• Select Enable radio button under Active scripting.
• Finally click OK and come out

To disable JavaScript support in your Internet Explorer, you need to select Disable radio button under Active scripting.

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Enable JavaScript in Firefox�

Here are the steps to turn on or turn off JavaScript in Firefox −

• Open a new tab → type about: config in the address bar.
• Then you will find the warning dialog. Select I’ll be careful, I promise!
• Then you will find the list of configure options in the browser.
• In the search bar, type javascript.enabled.
• There you will find the option to enable or disable javascript by right-clicking on the value of that option → select toggle.

If javascript.enabled is true; it converts to false upon clicking toogle. If javascript is disabled; it gets enabled upon clicking toggle.

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Enable JavaScript in Chrome�

Here are the steps to turn on or turn off JavaScript in Chrome −

• Click the Chrome menu at the top right hand corner of your browser.
• Select Settings.
• Click Show advanced settings at the end of the page.
• Under the Privacy section, click the Content settings button.

In the "Javascript" section, select "Do not allow any site to run JavaScript" or "Allow all sites to run JavaScript (recommended)".

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Enable JavaScript in Opera�

Here are the steps to turn on or turn off JavaScript in Opera −

• Follow Tools → Preferences from the menu.
• Select Advanced option from the dialog box.
• Select Content from the listed items.
• Select Enable JavaScript checkbox.
• Finally click OK and come out.

To disable JavaScript support in your Opera, you should not select the Enable JavaScript checkbox.

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Warning for Non-JavaScript Browsers�

If you have to do something important using JavaScript, then you can display a warning message to the user using <noscript> tags.

You can add a noscript block immediately after the script block as follows:

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<html>

<body>

<script language = "javascript" type = "text/javascript">

<!--

document.write("Hello World!")

//-->

</script>

<noscript>

Sorry...JavaScript is needed to go ahead.

</noscript>

</body>

</html>

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Now, if the user's browser does not support JavaScript or JavaScript is not enabled, then the message from </noscript> will be displayed on the screen.

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JavaScript Location in HTML document�

There is a flexibility given to include JavaScript code anywhere in an HTML document. However the most preferred ways to include JavaScript in an HTML file are as follows −

• Script in <head>...</head> section.
• Script in <body>...</body> section.
• Script in <body>...</body> and <head>...</head> sections.
• Script in an external file and then include in <head>...</head> section.

In the following section, we will see how we can place JavaScript in an HTML file in different ways.

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JavaScript in <head>...</head> section�

If you want to have a script run on some event, such as when a user clicks somewhere, then you will place that script in the head as follows −

<html>

<head>

<script type = "text/javascript">

<!--

function sayHello() {

alert("Hello World")

}

//-->

</script>

</head>

<body>

<input type = "button" onclick = "sayHello()" value = "Say Hello" />

</body>

</html>

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JavaScript in <body>...</body> section�

If you need a script to run as the page loads so that the script generates content in the page, then the script goes in the <body> portion of the document. In this case, you would not have any function defined using JavaScript. Take a look at the following code.

<html>

<head>

</head>

<body>

<script type = "text/javascript">

<!--

document.write("Hello World")

//-->

</script>

<p>This is web page body </p>

</body>

</html>

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JavaScript in <body> and <head> Sections�

You can put your JavaScript code in <head> and <body> section altogether as follows −

<html>

<head>

<script type = "text/javascript">

<!--

function sayHello() {

alert("Hello World")

}

//-->

</script>

</head>

<body>

<script type = "text/javascript">

<!--

document.write("Hello World")

//-->

</script>

<input type = "button" onclick = "sayHello()" value = "Say Hello" />

</body>

</html>

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JavaScript in External File�

As you begin to work more extensively with JavaScript, you will be likely to find that there are cases where you are reusing identical JavaScript code on multiple pages of a site.

You are not restricted to be maintaining identical code in multiple HTML files. The script tag provides a mechanism to allow you to store JavaScript in an external file and then include it into your HTML files.

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Here is an example to show how you can include an external JavaScript file in your HTML code using script tag and its src attribute.

<html>

<head>

<script type = "text/javascript" src = "filename.js" > </script>

</head>

<body>

.......

</body>

</html>

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