References Used

JavaScript Demystified: A Self-Teaching Guide by Jim Keogh. McGraw Hill, 2005.

History

Developed by Netscape
Originally called LiveScript
Meant as a scripting language that would enhance the capabilities of the Netscape browser
Renamed JavaScript after the introduction of Java, with the hope that Java's popularity would induce developers to adopt JavaScript for client-side scripting.

Uses and Limitations

JavaScript is not a full-featured programming language and its syntax differs from Java in some significant ways. Its most significant difference may be in its object model, which is more akin to a prototype-instance model than to a class-instance model. In a prototype-instance model the prototype is itself an object that may be changed at run-time (e.g., instance variables, also called properties, may be dynamically added or delete) and all instances will inherit the changes. In a class-instance model, the class is not a run-time object, but instead an immutable template from which instances are created.
Uses of JavaScript
1. Handle graphical interface of a browser application
2. Perform form validation
3. Perform simple calculations for a form
4. Perform custom actions in response to key strokes or mouse events
Limitations of JavaScript include:
1. Cannot read or write to the hard-drive, except to read cookies--a JavaScript application gets downloaded from a server and is not a trusted application as far as the client is concerned so the application should not be able to modify the client's hard drive.
2. Cannot be used to create server-side applications
3. Lack of formatted output: Javascript does not have a function comparable to C's printf function. The reason is that Javascript was designed to work with graphical interfaces and hence its output was expected to be in the form of html markup or dialog boxes.
4. Lack of console input: Javascript was designed to work with the graphical interface of a browser, not the console. As a result Javascript gets its input from form elements or dialog boxes, but not from a console window or a file.

JavaScript Terminology

JavaScript typically deals with objects, especially form objects. The form elements that you create on an html page are treated as objects by JavaScript.
Instance variables in JavaScript objects are called properties
As in Java or C++, JavaScript objects have methods that perform actions on JavaScript objects
Properties and methods are accessed using the . (dot) operator. For example, document.bgColor would access a document's background color and dcoument.write() would write some text.

Embedding JavaScript in an HTML Page

Use the script form element to surround JavaScript code. For example: A more interesting thing to do would be to pop-up an alert box:
Functions that you may want to execute in the body of the html page should be declared and placed in the head section of the html document
JavaScript code that should produce dynamic output for the web page should be embedded in the body section of the html document.
JavaScript code can be imported from a source file using the src attribute:
The functions defined in the head section can be either called directly from the body or in response to events that occur on forms (such as a click event). Events will be discussed further at a later time.

Debugging JavaScript

If your script has an error, your browser may either print an error message or give you a blank screen.
Ways to get your browser to display error messages
1. Safari/Firefox: type javascript: into the browser's URL window to display a JavaScript Console. This console should display both error messages and give you a one line text field where you can execute sample JavaScript commands. This doesn't work on my version of Safari.
2. Internet Explorer: select Tooks, Internet Options. On the Advanced page, uncheck the Disable Script Debugging box and check the Display a Notification About Every Script Error box.

Variables, Operators, and Expressions

Variables
1. Variable names have the same rules for construction as C or C++ variables and are not prefixed by a special character.
2. Variables are dynamically typed and declaring them is optional. You can declare a variable using the var keyword . For example:
```
	  var total
	  var purchase_price = 100
	  var sales_tax = purchase_price * .06
	  total = purchase_price + sales_tax
	  
```
  However, it is equally valid to not declare any of the variables, in which case you would write:
```
	  purchase_price = 100
	  sales_tax = purchase_price *.06
	  total = purchase_price + sales_tax
	  
```
  Notice that statements do not have to end with a semi-colon. The newline is interpreted as terminating the statement. If you want to put multiple statements on the same line, then they should be separated with a semi-colon.
Data types
1. integers: The parseInt(string) function will convert a string to an integer or return "NaN" if the string cannot be converted to an integer.
2. floating point numbers: The parseFloat(string) function will convert a string to an integer or return "NaN" if the string cannot be converted to a float.
3. booleans: true or false
4. strings: either single or doubly quoted
  1. .length: returns the length of the string
  2. substring(begin,end): returns the substring starting at index begin and going up to, but not including, end. Indices start at 0.
  3. charAt(index): returns the character at that index or an empty string if you are past the end of the string
  4. indexOf(substring, [start]): returns the index in the string where the substring starts or -1 if the substring cannot be found. If a start index is specified then the search starts at start.
5. objects
6. null: a special keyword that indicates that a variable has no value
Operators
1. JavaScript's arithmetic operators are the same as the C/C++ arithmetic operators and use the same syntax
2. JavaScript's logical operators are the same as C/C++'s logical operators and use the same syntax
3. The + operator can be used to concatenate two strings: In a mixed expression that involves both a string and a number, the number will be converted to a string, regardless of whether the string is the first or the second operator
4. The assignment operators (e.g., =, +=, -=) are the same as the C/C++ assignment operators and have the same syntax.
5. The comparison operators (e.g., <, <=, ==) are the same as the C/C++ comparison operators and have the same syntax.
6. Conditional operator: Like C/C++, JavaScript has the following conditional operator
```
	 booleanExpression ? trueExpression : falseExpression
	 
```
  The booleanExpression should evaluate to either true or false and either the trueExpression or falseExpression will be evaluated and its value returned as the result of the operator.

Comments

//: Single line comment
/* ... */: Multi-line comments

Input and Output

Input can be obtained in one of two ways:

via form elements (to be discussed later)
via the prompt function
1. the prompt function pops up a dialog box with a text box and allows the user to enter a string of input
2. the prompt function returns the string as its return value
3. syntax: prompt("prompt message", default value)
4. example: prompt("please enter sales tax rate", ".05");
5. it is ok to provide an empty string, '', as the default value, or to simply omit the default value, in which case it will default to the empty string
6. it is ok to provide any evaluable expression as the default value--the resulting value will be converted to a string
7. if the user presses the "Cancel" button then prompt returns null

Output can be provided in a number of ways

document.write(string): document is a pre-defined object and write is a method that writes a string. You can use the string concatenation operator, +, to concatenate several string fragments. You can write html markup in order to obtain formatted output.
1. document.writeln will write a string with a newline attached to it but it is of limited utility because 1) the newline does not show up in the html output--<br> would be needed to do that, and 2) the source view will show the script, not the dynamically generated html so the string will not show up in the source view.
alert(string): alert will pop up a dialog box with the given string value. Execution of the script will be suspended until the user presses the ok button.

Ifs and Loops

If statements are exactly the same as in C/C++ and have the same syntax
1. Single statements do not require {} braces
2. if-else is always used: there is no elsif, elif, or other abbreviations for elses that are used in other scripting languages
3. The pre-defined navigator object can be used to determine the browser and version number. This information can be used to determine whether or not the browser supports various features needed by the script.
  1. navigator.appName: The browser type (e.g., Internet Explorer, Safari)
  2. navigator.userAgent: The browser version
  The following script will pop up an alert box with your browser's name and version number. You can view the html page source to see what the script looks like.
Switch statements: The syntax for switch statements is similar to C/C++
```
     switch (expression) {
       case label1, label2, ... label_n:
         statements;
	 break;
       case ...
       case ...
       default:
         statements to execute if none of the cases match
     }
     
```
There are a few enhancements to C's switch statement in that labels can be one of:
1. a number
2. a string
3. a variable which will be evaluated and should yield a number or string
Loops
1. For loop: syntax exactly like C for loop
2. While loop: syntax exactly like C while loop
3. Do..While loop: syntax exactly like C do..while loop
4. For..In loop: The for..in loop iterates through an array or object and produces a list of numeric indices or property names.
  1. Example: Iterating through the elements of a numeric array
  2. Example: Iterating through the properties in an object: Note that I can use an object like an associative array to access its properties