Javascript

This language was originally developed at Netscape and first appeared in Netscape2.
It has gone through versions 1.0 to 1.8.1 and has been standardized as ECMA-262 and ISO-16262. Up to version 1.5 is pretty cross-platform.
The standardized version is sometimes called ECMAscript; the Microsoft version is sometimes called JScript.
It can be used on both the client and on the server.
We will use it mainly on the client except for a little bit at the end of the semester.
Only the name is similar to Java. Unlike Java:
- It is a loosely rather than strongly type language.
- The object model is simpler
- It is interpreted

Uses of Javascript

To do some of the processing of web pages on the client-side rather than make the server do it. For instance one might want to dynamically change the look of a page without going back to the server.
To perform actions when certain events occur on the client-side. For instance, when a web-page is submitted it could be used to validate the page.
To do simple networking in the background transparently to the user.
It can be often used as a smaller alternative to a Java applet.

Objects in Javascript

Javascript objects are collections of properties.
Each property is either a data property or a method property (a method).
Data properties appear in two categories: primitive values and references to other objects.
We access objects in Javascript by using variables by reference.
All primitive values are accessed directly by value.
There is a root object in Javascript called Object. It is the ancestor through something known as prototype inheritance of all other objects in Javascript.

General Overview

Many details on Javascript can be found at:
http://www.webreference.com/javascript/reference/core_ref/contents.html
Javascripts can be either directly or indirectly embedded in a document.

Directly:

<script type="text/javascript">
<!-- hide from old browsers
-- code --
// -->
</script>

Indirectly:

<script type="text/javascript" src="myscript.js" />

Lexicon

Identifiers (i.e., names) must begin with a letter, an underscore, or a $ and subsequent characters may be one of these three or numbers.
Javascript has 25 reserved words: break, case, catch, continue, default, delete, do, else, finally, for, function, if, in, instanceof, new, return, switch, this, throw, try, typeof, var, void, while, with.
Javascript has additional keywords which are reserved for future use, as well as predefined words such as alert, open, java, and self.
Javascript supports /*...*/ and // for comments.
Semicolons to end lines in Javascript are optional. Beware:
```
return 
x; // has the effect of just return!
```

Example Javascript and HTML document

<html>
<head><title>test</title>
<meta name="description" value="this example illustrates how Javascripts are executed both when the document is loaded and on the occurrence of events" />
<script type="text/javascript" ><!--
   function sayHello()
   {
		alert("hi there");
   } 
//-->
</script>
</head><body><form><input type="button" value="test" onClick="return sayHello();" 
/><!-- responds to events --></form>
<script type="text/javascript" ><!--
for( i = 0; i<100; i++)
{ 
	document.writeln("<p>hi"+i+"</p>");
} // run when document loads
-->
</script></body>
</html>

Primitives

Javascript has 5 primitive types: Number, String, Boolean, Undefined and Null.
Javascript has predefined objects corresponding to Number, String and Boolean. Each of these is wrappers for a value of the corresponding primitive type.
Javascript will do type coercion between objects of type String and Number.
A number literal can be either a integer or a float. You can use hex for integers. If it is a float it can use scientific notation:
7, 0xff, 0XFF, 7.2, .73, -.23, 7E2, 7e2, 7.2e-2, etc.
A string literal can be delimited either by a single or double quote.
"Hi there", 'hi there', '"', "\n", '\'\\', etc.
There are two Boolean literals: true or false.
The only Null literal is null which can be coerced to false as a Boolean and 0 as an Number.
The only literal of type Undefined is undefined. It can be coerced to false as a Boolean and NaN as a Number.

Quiz

Which of the following statements is true?

git revision numbers count up beginning with revision 0
A git commit always affects the github repository.
svn can use http/webdav to checkout/checkin to a repository.

Variables

The Javascript interpreter determines the type of a variable as needed by the circumstance.
Variables can be declared either by assigning it a value or by explicitly declaring it:
```
var myVariable, pi=3.14;
// explicit declarations effect the scope of the variable
```

Numeric Operators and Objects

Numeric operators in Javascript are similar to Java or C: +, -, *, /, %, ++, --, etc.
The Math object has many useful built-in methods and properties for Number objects.
For example, Math.sin(x), Math.PI, Math.random, Math.abs, etc.
The Number object has a useful collection of Number properties. Number.MIN_VALUE, Number.MAX_VALUE, Number.NaN, etc. It also has the toString method.
For example, x=10; y= x.toString(); z=x.toString(2); //binary representation

Strings and Type Conversion

The + sign is used to concatenate strings.

first = "hello"
second = first + "bye" // "hellobye"

The String object also has useful methods such as: charAt, indexOf, substring, toLowerCase, toUpperCase, etc. A String object also always has a length property
When the value of one type is used in a situation where another type is required, Javascript tries to implicitly coerce the type into the required one. For example,
```
"August" + 1977 // "August1977"
1977 + "August" // "1977August"
7*"3" =21
```

One can also explicitly, do type conversion.

var str_value = String(value); // Might want to use toString
var number = Number(aString); // Might want to use parseInt or parseFloat

Javascript

Outline

Javascript

Uses of Javascript

Objects in Javascript

General Overview

Lexicon

Example Javascript and HTML document

Primitives

Quiz

Variables

Numeric Operators and Objects

Strings and Type Conversion