FIT5900 : JavaScript, Part II

In the previous lecture:

JavaScript is a client side programming language that may be embedded within an HTML document.
JavaScript allows for user interaction, dynamic generation of content, form checking, and much more...

In this lecture:

JavaScript syntax

References:

Flanagan, D. JavaScript: The Definitive Guide
2nd edn, O'Reilly & Associates, Inc.
Stein, L.D. How To Set Up and Maintain a Web Site
2nd edn, Addison Wesley 1997, Chpt10.
Goodman, D. Dynamic HTML: The Definitive Reference
O'Reilly & Associates, Inc.

It is essential that one or more of these texts, or some other reference (eg. the WWW) be consulted on this topic.

This page is a summary only of a few important JavaScript language features after Stein. It is not intended to be a comprehensive explanation of JavaScript.

Strings

"This is a string" and 'This is a string'
'T' 'h' "e" "s" "e" "are strings too!"
There is no distinction between single characters and strings
Single quotes are useful:

onClick = "confirm('Are you sure you want to become a potato Dave?')"
You can escape a quotation mark using backslash:

quote = "\"Eat my shorts HAL!\", muttered Dave."
\n linefeed (UNIX newline), \r carriage return (Macintosh newline) \t (tab)
Strings may be concatenated using the + operator.
The escape() and unescape() global functions convert strings to URL encoded versions and visa-versa. For example:

var s = escape("hello dave");

// s now has the value: "hello%20dave"

Numbers

2001
2001.0
-2001.0
+2001.0 Does not work!
2.01E3
0x2A (hexadecimal numbers preceded by a 0x, digits from 0-F)
0743 (octal numbers preceded by a 0, digits from 0-7)

Booleans

Keywords true and false
These are not the same as 1 and 0!

Comments

// This is a comment to the end of a line
/* This is a comment
which spans more
than one line */

Type Conversion

JavaScript has automatic type conversion
eval (string) forces conversion of a string to a numeric value.

var aString = "2001";
var aNumber = eval(aString);
parseInt (string, [radix]) returns an integer value up to the first non-numeric character in the string. Base 10 is assumed if no radix is specified.
parseFloat (string) returns a floating point value or NaN (not a number) if the string contains any non-numeric characters.
isNaN(expression) returns true or false if number is NaN. (Windo$ implementation of parseFloat() may return 0 instead of NaN!)
isFinite(expression) returns true or false if the result of expression is anything within the range of Number.MIN_VALUE and Number.MAX_VALUE

Variables & Assignment

JavaScript is weakly typed hence the statements...

myVariable = "Dave";
myVariable = 2001;

...work just fine one after another even though one assigns myVariable to a string and the other to a number!
Strings are automatically converted to numbers if you try to perform arithmetic with them. (& vice versa)
Variable names must be alphanumeric characters starting with a letter or underscore. (Case is significant)

Variables do not need to be declared before use but it is important to declare local variables in a function to avoid overwriting global variables of the same name!

aVariable = "HAL";

function eliminateDave () // Function to keep HAL happy
{
var aVariable = "Dave";
eliminate(aVariable);
}

Operators & Expressions

The following operators and expressions are valid in JavaScript, just as they are in the C programming language.

x = a + (b - 2001 * (c % 2)) / 10.1
x++; x--; y = ++x; y = x--; x += 2001 etc.
(x && y); (x || y); y = !x;

Comparisons & Conditional Statements

The following are valid in JavaScript, just as they are in the C programming language.

if (x > 2001){ ... }
else if (x < 2001){ ... }
else { ... }
result = (x > 2001) ? ( ... ) : ( ... );
x >= y; x <= y; x > y; x < y; x == y; x != y
Early versions of JavaScipt do not support C's switch() statement.
Unlike in C, you can compare strings using the comparison operators.

if (name < "D")
{ alert("Your name comes before Dave's alphabetically"); }

Loops

The following are valid in JavaScript, just as they are in the C programming language.

var year=0;
while (year < 2001) { ...; year++; }
for (year=0; year < 2001; year++) { ... }
break; continue;
Also see for ... in loops.

Functions

JavaScript function definitions are made using the function keyword.
JavaScript function definitions do not include the return type.

Sample function definitions

function blah() { ... }
function bloo(myVar) { ... }
function blod(myVar1, myVar2) { return myVar1 + myVar2; }

Functions are called using C syntax:

machineName = blod("Hal", "9000");
Remember to declare local variables to avoid over-writing globals!

Functions can be assigned to variables. (The variable is a reference to the function)

Arrays

JavaScript has some pre-defined arrays:
- browser history list
- current document list of hyper links
- list of frames in the current window
You cannot (strictly speaking) create your own JavaScript arrays
Array indices begin at 0.
Arrays are indexed using square brackets [ and ].
Arrays have a read only length property which holds the size of the array.
Sample array access:

for (var i = 0; i < document.links.length; i++)
{ link = document.links[i]; }

Objects

JavaScript is not truly object oriented.
JavaScript "objects" contain data parts (properties) and functions (methods).

Creating a JavaScript Object

Create a constructor function with the same name as the object type.

The constructor can have any number of arguments.

Constructors return no result.

Constructors are responsible for defining the data and method members of an object.

function spaceShip(name, captainName, numberOfEngines)
{
this.shipName = name;
this.pilotName = captainName;
this.engines = numberOfEngines;
}

Remember that variables can be references to functions too!

Call the new operator to create instances of the object.

discovery = new spaceShip("Discovery", "Dave", 1);
enterprise = new spaceShip("Enterprise", "Kirk", 3);

Accessing Object Members

Access works just like access to a C struct's fields.

discovery.engines = 0; // Discovery's engine is not operational
enterprise.shipName = "klingonStarshipOne"; // Klingons hijack the Enterprise

JavaScript objects are really just arrays indexed by strings (rather than integers).
A special for ... in loop runs through the properties of an object in the order they were created.

var prop; for (prop in discovery) { thePropHolder = discovery[prop]; }

Some Web based Information:

Lloyd's JavaScript reference.

Netscape JavaScript reference.

This lecture's key point(s):

JavaScript is a full featured programming language, with a syntax reminiscent of C.
JavaScript is, however, weakly typed and has limited support for Object-Oriented programming.

FIT5900 courseware | FIT5900 lecture notes