JavaScript is cross-platform, object-oriented scripting language, which was developed by Netscape. With JavaScript you can easily create interactive web-pages.

JavaScript is not Java! Although it has a similar name, it's totally different language!

There are 3 types of JavaScript:

• Core JavaScript - It's the common part for all JavaScript implementations. Core JavaScript contains a core set of objects, such as Array, Date, and Math, and a core set of language elements such as operators, control structures, and statements.
• Client-Side JavaScript - This is the JavaScript, which is working in the browser. It includes core JavaScript and extentions to use within the browser (like window object etc.). This type of JavaScript supported by most of the modern browsers.
• Server-Side JavaScript - This is the JavaScript which runs on the server, which supports it (Netscape LiveWire). It includes core JavaScript and server extentions. It's supported by Netscape server. For more info on this see Netscape LiveWire part of this course.

We'll talk about the Client-Side JavaScript. The way you add JavaScript to the HTML page is inside:
<SCRIPT LANGUAGE=JavaScript>... </SCRIPT> .
Or you can import the external file with JavaScript:
<SCRIPT LANGUAGE="JavaScript" SRC="filename.js"></SCRIPT> .
Here is a simple example:

Source	Result
<SCRIPT LANGUAGE="JavaScript"> var msg = "Hello World!!"; function say_hi() { alert(msg); } </SCRIPT> <FORM> <INPUT TYPE=BUTTON NAME=hi VALUE="Say Hi!" OnClick="say_hi();"> </FORM>

I don't want to describe here the syntax of JavaScript, you can find it in any JavaScript tutorial (see references or learn from examples.

Some examples of the standard objects in Client-Side Javascript:
Note, that it's far from being a full reference, please consult Netscape Client-Side JavaScript Reference for the full list of available methods and objects.

Core JavaScript:
• Array - Provides the functionality to work with arrays (have a properties like length, methods like sort, reverse, concat etc.)
• Date - Lets you work with dates and times.
• Math - Has properties and methods for mathematical constants and functions. For example have a properties like E, PI and methods like abs, log, sin etc.

  Client-Side JavaScript:

• Button - Allows to work with buttons on an HTML form. Properties like form, name, value and methods like click, focus etc.
• document - Contains information about the current document, and provides methods for displaying HTML output to the user. Properties like forms - array of forms on this document (objects), cookie, images - array of images etc. You can do all kinds of things with this object, e.g. if you want to change the first image on your document from the empty_cart.gif to full_car.gif: document.images[0].src = "full_car.gif";
• Form - Lets you work with forms in the document. Properties like action, elements - array with all form elements (buttons, checkboxes, text fields etc.). Methods like submit - submits the form, reset etc. Example: if you need to submit the form from your function, just do form.submit()
• History - Contains an array of information on the URLs that the client has visited within a window. This information is stored in a history list and is accessible through the browser's Go menu. Properties like current, previous, methods like back, forward. e.g useful if you want to send a user back ( window.history.back() )
• Image - An image on your document. Useful if you want to change the image from your script, e.g. rollover.
• Location - Contains information on the current URL. Allows you to access different parts of URL, doing reload etc. e.g. if you want to send user to the new URL you can do something like: window.location.href="http://www.yahoo.com/";
• window - Represents a browser window or frame. This is the top-level object for each document. Have a lot of useful properties & methods. Examples of usage:
  window2 = window.open(...); - opens the new window
  window.close(); - closes window
  window.confirm("Are you sure?"); - gets the confirmation from the user.
  Note, since window is a top-level object for the client hierarchy, many methods can be called directly, it'll be assumed, that these are the methods of current window. e.g. confirm("Are you sure?"); will work just fine.

Some examples of useful things you can do with JavaScript can be found here.

Summary:

• JavaScript is extremely useful for building the interactive Web interfaces.
• It is also very useful in verifying the user's input.
  It can be pretty frustrating for the user to submit the form with 10 fields just to find out, that he forgot to put the zip code, press the back button and then see, that he has to enter everything again... - Avoid it by checking such things on the browser side using the JavaScript. Note here, that it's for the user's convenience only, on the server side never trust the browser! See more in security part for the reason 'why'.
• Sometimes you can even get some work done on the client-side.
  e.g. if you have a screen, where user chooses the items from the list of products, you can calculate a total and update it each time, he/she adds or deletes an item. Although, as it was said in the previous paragraph, don't trust these numbers for the real transactions! Also remember, that JavaScript is the scripting language and is not intended for the huge programs, so even though you can write some big client-side application with this, it's not such a good idea :-)

More info on JavaScript can be found here.