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JavaScript

JavaScript is THE scripting language of the Web.

JavaScript is used in millions of Web pages to add functionality, validate forms, detect browsers, and much more.


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JavaScript is the most popular scripting language on the internet, and works in all major browsers, such as Internet Explorer, Firefox, Chrome, Opera, and Safari.

What is JavaScript?

  • JavaScript was designed to add interactivity to HTML pages
  • JavaScript is a scripting language
  • A scripting language is a lightweight programming language
  • JavaScript is usually embedded directly into HTML pages
  • JavaScript is an interpreted language (means that scripts execute without preliminary compilation)
<html>
<body>

<script type="text/javascript">
document.write("This is my first JavaScript!");
</script>

</body>
</html>

The example below shows how to add HTML tags to the JavaScript:
<html>
<body>
<script type="text/javascript">
document.write("<h1>Hello World!</h1>");
</script>
</body>
</html>

JavaScript is Case Sensitive

Unlike HTML, JavaScript is case sensitive - therefore watch your capitalization closely when you write JavaScript statements, create or call variables, objects and functions.

JavaScript Statements

A JavaScript statement is a command to a browser. The purpose of the command is to tell the browser what to do.

JavaScript Code

JavaScript code (or just JavaScript) is a sequence of JavaScript statements.

Each statement is executed by the browser in the sequence they are written.

What can a JavaScript do?

  • JavaScript gives HTML designers a programming tool - HTML authors are normally not programmers, but JavaScript is a scripting language with a very simple syntax! Almost anyone can put small "snippets" of code into their HTML pages
  • JavaScript can put dynamic text into an HTML page - A JavaScript statement like this: document.write("<h1>" + name + "</h1>") can write a variable text into an HTML page
  • JavaScript can react to events - A JavaScript can be set to execute when something happens, like when a page has finished loading or when a user clicks on an HTML element
  • JavaScript can read and write HTML elements - A JavaScript can read and change the content of an HTML element
  • JavaScript can be used to validate data - A JavaScript can be used to validate form data before it is submitted to a server. This saves the server from extra processing
  • JavaScript can be used to detect the visitor's browser - A JavaScript can be used to detect the visitor's browser, and - depending on the browser - load another page specifically designed for that browser
  • JavaScript can be used to create cookies - A JavaScript can be used to store and retrieve information on the visitor's computer

JavaScript Blocks

JavaScript statements can be grouped together in blocks.

Blocks start with a left curly bracket {, and ends with a right curly bracket }.

The purpose of a block is to make the sequence of statements execute together.

JavaScript Comments

Comments can be added to explain the JavaScript, or to make the code more readable.

Single line comments start with //.

JavaScript Multi-Line Comments

Multi line comments start with /* and end with */.


The Real Name is ECMAScript

JavaScript's official name is ECMAScript.

ECMAScript is developed and maintained by the ECMA organization. 

ECMA-262 is the official JavaScript standard.

The language was invented by Brendan Eich at Netscape (with Navigator 2.0), and has appeared in all Netscape and Microsoft browsers since 1996.

The development of ECMA-262 started in 1996, and the first edition of was adopted by the ECMA General Assembly in June 1997.

The standard was approved as an international ISO (ISO/IEC 16262) standard in 1998.

The development of the standard is still in progress.

Where to Put the JavaScript

JavaScripts in a page will be executed immediately while the page loads into the browser. This is not always what we want. Sometimes we want to execute a script when a page loads, other times when a user triggers an event.

Scripts in <head>

Scripts to be executed when they are called, or when an event is triggered, go in the head section.

If you place a script in the head section, you will ensure that the script is loaded before anyone uses it.

JavaScript Variables

As with algebra, JavaScript variables are used to hold values or expressions.

A variable can have a short name, like x, or a more descriptive name, like carname.

Rules for JavaScript variable names:

  • Variable names are case sensitive (y and Y are two different variables)
  • Variable names must begin with a letter or the underscore character

    Declaring (Creating) JavaScript Variables

    Creating variables in JavaScript is most often referred to as "declaring" variables.

    You can declare JavaScript variables with the var statement: