Essential JavaScript

Java and Java Scripts are not the same. Java Script shares most of Java's expression syntax and basic control flow constructs. JavaScript is an interpreted scripting language, while Java compiles applications in a system of classes. The following table compares and contrasts JavaScript and Java.

1. Using Java Script in HTML (how to include your scripts in a page).

2. Creating simple JavaScripts (the fun stuff)

• Event-handling.
• A simple example.
• JavaScript for non-programmers. Click on each book number and you will see a small applet easy to copy.

3. Objects, properties and methods (the hard stuff).

• Objects in JavaScript have properties and methods. Figure 1 shows a graphical representation of an object.
  
  Figure 1

  An object in Java Script is an associative array (see Essential Perl). It can also be thought as a function that defines an object type. In order to make this clear, I will use an unortodox approach and define functions before I even introduce variables.

• Function: a set of statements or procedures, defined by the keyword function. The pattern is: function fname (a1, a2,...,an) {statements } , where fname is the name you give to the function, and a1 to an are the arguments, parameters used in the function. (I will be back to functions later).

• Object definition: objects are defined using the following pattern:

  function obj (v1, v2,..., vn) {

  this.v1=key1;

  this.v2=key2;

  ....................

  this.vn=keyn;

  }

  As I said before, objects are associative arrays. The object definition creates not only the object type but also the variables, and assigns the key (in the key=value pair) to the variables. These keys/variables become the properties of the object. Lets see an example of creating a student object.

  Function student (ss-no,name,gpa) {

         this.ss-no=ss-no;

         this.name=name;

         this.gpa=gpa;

  }

• Properties of an object are refered as fname.property . In the student example: student.gpa

  But how do we assign values to the properties? This happens when you instantiate (create an instance) of the object type. In the above example:

        marty = new student("123-45-6789", "Marty Hayes", "3.95") . Marty.gpa is the value 3.95.

• Methods. The last of the basic concepts regarding objects is method. A method is a function included as a property of an object. Then, use the following syntax to associate the function with an existing object: object.methodname = function_name;

  Example: let as consider the object home_computer. Its definition could be like:

  function home_computer(brand,memory,year) {

         this.brand=brand;

         this.memory=memory;

         this.year=year;

  }

  and for a student Marty it could be instantiated as:

         marty_computer = new home_computer ("Mac","8Meg","1994");

  The value of marty_computer.brand would be Mac.

  In our example we would add the new method computer to the object student using:

        student.computer = home_computer;

  And to add to marty (of the student object type) the value of the marty_computer (of the home_computer object type) we woul use:

         marty.computer = marty_computer;

  Finally, you use the pattern object.methodname.property; to obtain the value of the new method for the object. In our example it would be: marty.computer.brand and the value would be Mac.

• Values, Names, and Literals
• Expressions and Operators
• Statements
• Predefined Objects and Functions

5. Learning more JavaScript (the more involved stuff)

• Navigator objects.
• Using objects and forms
• Methods and functions - reference
• Properties - reference
• Introduction to Java Script by Vodoo (local mirror authorized by author).

This page is maintained by Al Bento who can be reached at abento@ubmail.edu. This page was last updated on May 17, 2007. Although we will attempt to keep this information accurate, we can not guarantee the accuracy of the information provided.