Introduction to JavaScript
JavaScript is a programming language that can be included on web pages to make them more interactive. You can use it to check or modify the contents of forms, change images, open new windows and write dynamic page content. You can even use it with CSS to make DHTML (Dynamic HyperText Markup Language).
This allows you to make parts of your web pages appear or disappear or move around on the page. JavaScripts only execute on the page(s) that are on your browser window at any set time. When the user stops viewing that page, any scripts that were running on it are immediately stopped. The only exceptions are cookies or various client side storage APIs, which can be used by many pages to store and pass information between them, even after the pages have been closed.
JavaScript is a client side, interpreted, object oriented, high level scripting language, while Java is a client side, compiled, object oriented high level language. Now after that mouthful, here’s what it means.
- Client side
- Programs are passed to the computer that the browser is on, and that computer runs them. The alternative is server side, where the program is run on the server and only the results are passed to the computer that the browser is on. Examples of this would be PHP, Perl, ASP, JSP etc.
- Interpreted
- The program is passed as source code with all the programming language visible. It is then converted into machine code as it is being used. Compiled languages are converted into machine code first then passed around, so you never get to see the original programming language. Java is actually dual half compiled, meaning it is half compiled (to ‘byte code’) before it is passed, then executed in a virtual machine which converts it to fully compiled code just before use, in order to execute it on the computer’s processor. Interpreted languages are generally less fussy about syntax and if you have made mistakes in a part they never use, the mistake usually will not cause you any problems.
- Scripting
- This is a little harder to define. Scripting languages are often used for performing repetitive tasks. Although they may be complete programming languages, they do not usually go into the depths of complex programs, such as thread and memory management. They may use another program to do the work and simply tell it what to do. They often do not create their own user interfaces, and instead will rely on the other programs to create an interface for them. This is quite accurate for JavaScript. We do not have to tell the browser exactly what to put on the screen for every pixel (though there is a relatively new API known as canvas that makes this possible if needed), we just tell it that we want it to change the document, and it does it. The browser will also take care of the memory management and thread management, leaving JavaScript free to get on with the things it wants to do.
- High level
- Written in words that are as close to english as possible. The contrast would be with assembly code, where each command can be directly translated into machine code.
features of java script
- The basic part of a script is a variable, literal or object. A variable is a word that represents a piece of text, a number, a boolean true or false value or an object.
- A literal is the actual number or piece of text or boolean value that the variable represents.
- An object is a collection of variables held together by a parent variable, or a document component.
The next most important part of a script is an operator. Operators assign literal values to variables or say what type of tests to perform.
The next most important part of a script is a control structure. Control structures say what scripts should be run if a test is satisfied.
Functions collect control structures, actions and assignments together and can be told to run those pieces of script as and when necessary.
The most obvious parts of a script are the actions it performs. Some of these are done with operators but most are done using methods. Methods are a special kind of function and may do things like submitting forms, writing pages or displaying messages.
Events can be used to detect actions, usually created by the user, such as moving or clicking the mouse, pressing a key or resetting a form. When triggered, events can be used to run functions.
In contrast to Java’s compile-time system of classes built by declarations, JavaScript supports a runtime system based on a small number of data types representing numeric, Boolean, and string values. JavaScript has a prototype-based object model instead of the more common class-based object model. The prototype-based model provides dynamic inheritance; that is, what is inherited can vary for individual objects. JavaScript also supports functions without any special declarative requirements. Functions can be properties of objects, executing as loosely typed methods.
JavaScript is a very free-form language compared to Java. You do not have to declare all variables, classes, and methods. You do not have to be concerned with whether methods are public, private, or protected, and you do not have to implement interfaces. Variables, parameters, and function return types are not explicitly typed.
Java is a class-based programming language designed for fast execution and type safety. Type safety means, for instance, that you can’t cast a Java integer into an object reference or access private memory by corrupting Java bytecodes. Java’s class-based model means that programs consist exclusively of classes and their methods. Java’s class inheritance and strong typing generally require tightly coupled object hierarchies. These requirements make Java programming more complex than JavaScript programming.
In contrast, JavaScript descends in spirit from a line of smaller, dynamically typed languages such as HyperTalk and dBASE. These scripting languages offer programming tools to a much wider audience because of their easier syntax, specialized built-in functionality, and minimal requirements for object creation.