Event Handlers

JavaScript programs are typically event-driven. Events are actions that occur on the Web page, usually as a result of something the user does, although not always. For example, a button click is an event, as is giving focus to a form element; resizing the page is an event, as is submitting a form. It is these events which cause JavaScript programs to spring into action. For example, if you move your mouse over this phrase, a message will pop-up, courtesy of JavaScript.

An event, then, is the action which triggers an event handler. The event handler specifies which JavaScript code to execute. Often, event handlers are placed within the HTML tag which creates the object on which the event acts:





For example, a hyperlink is subject to a MouseOver event, meaning that its event handler will be triggered when the mouse passes over the link. Therefore, you place the event handler for a hyperlink's MouseOver inside the A tag:





The JavaScript which is called by the event handler may be any valid JavaScript code: a single statement or a series of statements, although most often it is a function call.

The set of all events which may occur, and the particular page elements on which they can occur, is part of the Document Object Model (DOM), and not JavaScript itself (see the earlier section "Document Object Model"). As a result, Netscape and Microsoft do not share the exact same set of events, nor are all page elements subject to the same events between browsers. For example, Internet Explorer 4 supports a MouseOver event for an image while Navigator 4 does not.

The table below illustrates some of the most commonly used events supported in both DOM's. Because the DOM's differ in their event support, the following documents are recommended as an overview of each browser's event support:

Navigator 4: Event Handlers Summary
MSIE 4: DHTML Event Summary


Common Events Event

Occurs when...

Event Handler

click User clicks on form element or link onClick
change User changes value of text, textarea, or select element onChange
focus User gives form element input focus onFocus
blur User removes input focus from form element onBlur
mouseover User moves mouse pointer over a link or anchor onMouseOver
mouseout User moves mouse pointer off of link or anchor onMouseOut
select User selects form element's input field onSelect
submit User submits a form onSubmit
resize User resizes the browser window onResize
load User loads the page in the Navigator onLoad
unload User exits the page onUnload

Objects

An object is a "package" of data; a collection of properties (variables) and methods (functions) all classed under a single name. For example, imagine that there was an object named car. We could say that the car object possesses several properties: make, model, year, and color, for example. We might even say that car possesses some methods: go(), stop(), and reverse(). Although car is obviously fictional, you can see that its properties and methods all relate to a common theme.

In JavaScript you may create your own objects for storing data. More commonly, though, you will use the many "built-in" objects which allow you to work with, manipulate, and access the Web page and Web browser. This set of pre-existing objects is known as the "Document Object Model".

Document Object Model
Often referred to as the DOM, this object model is a hierarchy of all objects "built in" to JavaScript. Most of these objects are directly related to characteristics of the Web page or browser. The reason we qualify the term "built in" is because the DOM is technically separate from JavaScript itself. That is, the JavaScript language specification, standardized by the ECMA, does not actually specify the nature or specifics of the DOM. Consequently, Netscape and Microsoft have developed their own individual DOM's which are not entirely compatible. Additionally, the DOM stands apart from JavaScript because it could theoretically be accessed by other scripting languages as well.

In summary, then, what we refer to as "JavaScript" is actually made up of both JavaScript, the language, and the DOM, or object model which JavaScript can access. In a future WDVL article we will take a closer look at the DOM and its current and future role.

Below is a graphical chart illustrating a high-level view of Netscape's DOM. Microsoft's DOM is actually a superset of Netscape's, and so the chart below actually represents a subset of Microsoft's own DOM.


Reprinted from Netscape's JavaScript Guide



Properties
Access the properties of an object with a simple notation: objectName.propertyName. Both the object name and property name are case sensitive, so watch your typing. Because a property is essentially a variable, you can create new properties by simply assigning it a value. Assuming, for instance, that carObj already exists (we'll learn to create a new object shortly), you can give it properties named make, model, and year as follows:


carObj.make="Toyota";
carObj.model="Camry";
carObj.year=1990;
document.write(carObj.year);


A JavaScript object, basically, is an array. If you're familiar with other languages you probably recognize an array as a collection of values residing within a single named data structure. You can access an object's properties either using the objectName.propertyName syntax illustrated above, or by using an array syntax:


carObj["make"]="Toyota";
carObj["model"]="Camry";
document.write(carObj["year"]);


Methods
Unlike a basic data array, an object can also contain functions, which are known as methods when part of an object. You call a method using the basic syntax: objectName.methodName(). Any arguments required for the method are passed between the parentheses, just like a normal function call.

For example, the window object possesses a method named close(), which simply closes the specified browser window:


window.close();


Creating Objects
Most of the time you will be referencing objects which are built-in to the DOM. However, you may want to create your own objects for storing data within a JavaScript program. There are several ways to create a new object, but we'll look at two: creating a direct instance of an object and creating an object prototype.

direct instance of an object
Despite the awkward sound name, a "direct instance of an object" simply means creating a new single object, such as myPetDog:


myPetDog=new Object();
myPetDog.name="Barney";
myPetDog.breed="beagle";
myPetDog.year=1981;


Assigning a method to your new object is also simple. Assume that you already have coded a function named woof(), which causes a barking sound to play:


myPetDog.woof=woof;


prototype of an object
Sometimes, you'll want to create a "template" or prototype of an object. This does not create an actual instance of the object, but defines the structure of the object. In the future, then, you can quickly stamp out a particular instance of the object. Suppose that instead of myPetDog, you created a prototype object named petDog. This object could then be a template for a particular pet dog object. First, create a function which defines the petDog structure:


function petDog(name, breed, year)
{ this.name = name;
this.breed = breed;
this.year = year;
}


Now that the petDog prototype has been set, you can quickly create single instances of a new object based on the petDog structure:


myPetDog=new petDog("barney","beagle",1981);
yourPetDog=new petDog("max","terrier",1990);

Functions

A function groups together a set of statements under a named subroutine. This allows you to conveniently "call" the function whenever its action is required. Functions are a fundamental building block of most JavaScript programs, so you'll become quite familiar with their use. Before you can call on a function, of course, you must first create it. We can break down the use of functions, then, into two logical categories: defining functions and calling functions.

defining functions
The function definition is a statement which describes the function: its name, any values (known as "arguments") which it accepts incoming, and the statements of which the function is comprised.


function funcName(argument1,argument2,etc)
{ statements; }


A function doesn't necessarily require arguments, in which case you need only write out the parenthesis; e.g. funcName(). If you do specify arguments, those arguments will be variables within the function body (the statements which make up the function). The initial values of those variables will be any values passed on by the function call.

Generally it's best to define the functions for a page in the HEAD portion of a document. Since the HEAD is loaded first, this guarantees that functions are loaded before the user has a chance to do anything that might call a function. Alternately, some programmers place all of their functions into a separate file, and include them in a page using the SRC attribute of the SCRIPT tag. Either way, the key is to load the function definitions before any code is executed.

Consider, for example, a simple function which outputs an argument to the Web page, as a bold and blinking message:


function boldblink(message)
{ document.write(""+message+""); }



Some functions may return a value to the calling expression. The following function accepts two arguments, x and y, and returns the result of x raised to the y power:


function raiseP(x,y)
{ total=1;
for (j=0; j { total*=x; }
return total; //result of x raised to y power
}


calling functions
A function waits in the wings until it is called onto the stage. You call a function simply by specifying its name followed by a parenthetical list of arguments, if any:


clearPage();
boldblink("Call me gaudy!");


Functions which return a result should be called from within an expression:


total=raiseP(2,8);
if (raiseP(tax,2)<100) ...


Quite commonly, JavaScript functions are called from within event handlers, which we'll take a look at later in this article.

Statements

Statements define the flow of a script, known as "program flow." A statement, like a fully grammatical English sentence, is made up of smaller expressions which, altogether, evaluate into a cogent meaning. In JavaScript, statements are organized as either conditionals, loops, object manipulations, and comments.

Good practice suggests that each JavaScript statements should be terminated with a semicolon (;). This is often not strictly necessary, as a new line also serves to separate statements, but when multiple statements reside on the same line the semicolon delimiter is mandatory.

A set of statements that is surrounded by braces is called a block. Blocks of statements are used, for example, in functions and conditionals.

Normally statements are executed sequentially: x = 1; y = 2; z = x + y; but this can be altered by some statements which test a condition and branch or loop according to the result.

Conditionals
Conditional statements direct program flow in specified directions depending upon the outcomes of specified conditions. These tests are a major influence on the order of execution in a program.

if...else
As seen in many programming languages, if the condition evaluates to true then the block of statements1 is executed. Optionally, an else clause specifies a block of statements2 which are executed otherwise. You may omit the else clause if there are no statements which need to be executed if the condition is false.


if (condition)
{ statements1; }

else
{ statements2; }


switch (Netscape & MSIE 4)
Commonly known as a "case statement," switch matches an expression with a specified case, and executes the statements defined for that case. In essence, the switch statement is a sort of shorthand for combining many implied if statements together.


switch (expression){
case label :
statement;
break;
case label :
statement;
break;
...
default : statement;
}


For example, imagine that you wanted to execute different sets of statements depending on whether favoritePet was "dog," "cat," or "iguana." Note that the break; statement prevents any cases below the match from being executed. The default case is matched if none of the cases match the expression.


switch (favoritePet){
case "dog" :
statements;
break;
case "cat" :
statements;
break;
case "iguana" :
statements;
break;
default : statements;
}


Loops
for
The venerable for loop repeatedly cycles through a block of statements until a test condition is false. Typically, the number of times a loop is repeated depends on a counter. The JavaScript for syntax incorporates the counter and its increments:


for (initial-statement; test; increment)
{ statements; }


The initial-statement is executed first, and once only. Commonly, this statement is used to initialize a counter variable. Then the test is applied and if it succeeds then the statements are executed. The increment is applied to the counter variable and then the loop starts again. For instance, consider a loop which executes 10 times:


for (i=0; i<10; i++)
{ statements; }


do...while (Netscape & MSIE 4)
Another loop, a do...while statement executes a block of statements repeatedly until a condition becomes false. Due to its structure, this loop necessarily executes the statement at least once.


do
{ statements;}
while (condition)


while
In similar fashion as the do...while statement, the while statement executes its statement block as long as the condition is true. The main difference between while and do...while, aside from the fact that only while is supported in all JavaScript versions, is that a while loop may not execute the statements even once if the condition is initially false.


while (condition)
{ statements; }


break and continue
Both of these statements may be used to "jump the tracks" of an iterating loop. When used within the statement block of a loop, each statement behaves slightly differently:

break Aborts execution of the loop, drops out of loop to the next statement following the loop.
continue Aborts this single iteration of the loop, returns execution to the loop control, meaning the condition specified by the loop statement. Loop may execute again if condition is still true.


Object manipulation
for...in
The sometimes confusing for...in statement is used to cycle through each property of an object or each element of an array. The idea is that you may want to execute a statement block which operates on every property or element.


for (variable in object)
{ statements; }


Imagine, for example, that an object named wine1 has five properties: vineyard, year, varietal, alcohol, and color. You want to output the value of each property, as if producing a record from a database.


var record = "Wine 1

"
for (var prop in wine1)
{record += prop + " = " + wine1[prop] + "
"}
record += "
"
document.write(record)


with
The with statement serves as a sort of shorthand, allowing you to execute a series of statement who all assume a specified object as the reference. In other words, the object specified in the with statement is used as the default object whenever a property is encountered with no object specified.


with (object)
{ statements; }


Comments
Despite the fact that comments are purely optional, they can easily be a crucial part of your program. Comments can explain the action, like a color commentary, which can be a great help in understanding the code. Whether as a teaching tool or to simply remind yourself what the code does, comments are best sprinkled liberally throughout a program. Remember, comments are for humans, so write them that way!

Comments can also be used for debugging -- you can comment "out" sections of code to prevent them from being executed. In doing so you may learn more about why a certain problem is occurring in your program.

Because JavaScript must ignore comments, there is an appropriate syntax for demarcating text as a comment. For single line comments, simply precede the line with two backslashes. For multi-line comment blocks, begin the comment with /* and close with */.


//A lonely ol' single line comment
/* A dense thicket of commentary, spanning many captivating lines
of explanation and intrigue. */

Operators

Operators take one or more variables or values (operands) and return a new value; e.g. the '+' operator can add two numbers to produce a third. You use operators in expressions to relate values, whether to perform arithmetic or compare quantities. Operators are divided into several classes depending on the relation they perform:

arithmetic or computational
Arithmetic operators take numerical values (either literals or variables) as their operands and return a single numerical value. The standard arithmetic operators are:

+ Addition
- Subtraction
* Multiplication
/ Division
% Modulus: the remainder after division;
e.g. 10 % 3 yields 1.
++ Unary increment: this operator only takes one operand. The operand's value is increased by 1. The value returned depends on whether the ++ operator is placed before or after the operand; e.g. ++x will return the value of x following the increment whereas x++ will return the value of x prior to the increment.
-- Unary decrement: this operator only takes one operand. The operand's value is decreased by 1. The value returned depends on whether the -- operator is placed before or after the operand; e.g. --x will return the value of x following the decrement whereas x-- will return the value of x prior to the decrement.
- Unary negation: returns the negation of operand.


comparison
A comparison operator compares its operands and returns a logical value based on whether the comparison is true or not. The operands can be numerical or string values. When used on string values, the comparisons are based on the standard lexicographical (alphabetic) ordering.

== "Equal to" returns true if operands are equal.
!= "Not equal to" returns true if operands are not equal.
> "Greater than" returns true if left operand is greater than right operand.
>= "Greater than or equal to" returns true if left operand is greater than or equal to right operand.
< "Less than" returns true if left operand is less than right operand.
<= "Less than or equal to" returns true if left operand is less than or equal to right operand.


boolean
Boolean operators are typically used to combine multiple comparisons into a conditional expression. For example, you might want to test whether (total>100) AND (stateTax=true). A boolean operator takes two operands, each of which is a true or false value, and returns a true or false result.

&& "And" returns true if both operands are true.
|| "Or" returns true if either operand is true.
! "Not" returns true if the negation of the operand is true (e.g. the operand is false).


string
Strings can be compared using the comparison operators. Additionally, you can concatenate strings using the + operator.


"dog" + "bert"
yields
"dogbert"


assignment
The assignment operator (=) lets you assign a value to a variable. You can assign any value to a variable, including another variable (whose value will be assigned). Several shorthand assignment operators allow you to perform an operation and assign its result to a variable in one step.

= Assigns the value of the righthand operand to the variable on the left.
Example: total=100;
Example: total=(price+tax+shipping)
+=
(also -=, *=, /=) Adds the value of the righthand operand to the lefthand variable.
Example: total+=shipping (adds value of shipping to total and assigned result to total)
&=
(also |=) Assigns result of (lefthand operand && righthand operand) to lefthand operand.




special
Several JavaScript operators, rarely used, fall into no particular category. These operators are summarized below.

Conditional operator

(condition) ? trueVal : falseVal Assigns a specified value to a variable if a condition is true, otherwise assigns an alternate value if condition is false.
Example:
preferredPet = (cats > dogs) ? "felines" : "canines"

If (cats>dogs), preferredPet will be assigned the string value "felines," otherwise it will be assigned "canines".
typeof operand Returns the data type of operand.
Example -- test a variable to determine if it contains a number:
if (typeof total=="number") ...


regular expressions (Netscape & MSIE 4)
New to JavaScript 1.2 is support for regular expressions, which are defined patterns used to match character combinations appearing in string values. Regular expressions are very powerful, potentially allowing you to search for any conceivable character pattern. However, they can also be quite complex to construct. Because regular expressions are widely supported in all high-level development environments, it is advised that you consider learning about regular expressions as a subject unto itself.

Two detailed explanations of regular expressions can be found at

Netscape's JavaScript Guide: Chapter 9 Expressions and Operators
PERL5 Regular Expression Description

Variables and Data Types

Variables store and retrieve data, also known as "values". A variable can refer to a value which changes or is changed. Variables are referred to by name, although the name you give them must conform to certain rules. A JavaScript identifier, or name, must start with a letter or underscore ("_"); subsequent characters can also be digits (0-9). Because JavaScript is case sensitive, letters include the characters "A" through "Z" (uppercase) and the characters "a" through "z" (lowercase). Typically, variable names are chosen to be meaningful regarding the value they hold. For example, a good variable name for containing the total price of goods orders would be total.


scope

When you assign a new variable to an initial value, you must consider the issue of scope. A variable may be scoped as either global or local. A global variable may be accessed from any JavaScript on the page. A local variable may only be accessed from within the function in which it was assigned.

Commonly, you create a new global variable by simply assigning it a value:


newVariable=5;


However, if you are coding within a function and you want to create a local variable which only scopes within that function you must declare the new variable using the var statement:


function newFunction()
{ var loop=1;
total=0;
...additional statements...
}


In the example above, the variable loop will be local to newFunction(), while total will be global to the entire page.


type

A value, the data assigned to a variable, may consist of any sort of data. However, JavaScript considers data to fall into several possible types. Depending on the type of data, certain operations may or may not be able to be performed on the values. For example, you cannot arithmetically multiply two string values. Variables can be these types:

Numbers
3 or 7.987, Integer and floating-point numbers.

Integers can be positive, 0, or negative; Integers can be expressed in decimal (base 10), hexadecimal (base 16), and octal (base 8). A decimal integer literal consists of a sequence of digits without a leading 0 (zero). A leading 0 (zero) on an integer literal indicates it is in octal; a leading 0x (or 0X) indicates hexadecimal. Hexadecimal integers can include digits (0-9) and the letters a-f and A-F. Octal integers can include only the digits 0-7.
A floating-point number can contain either a decimal point, an "e" (uppercase or lowercase), which is used to represent "ten to the power of" in scientific notation, or both. The exponent part is an "e" or "E" followed by an integer, which can be signed (preceded by "+" or "-"). A floating-point literal must have at least one digit and either a decimal point or "e" (or "E").

Booleans
True or False. The possible Boolean values are true and false. These are special values, and are not usable as 1 and 0. In a comparison, any expression that evaluates to 0 is taken to be false, and any statement that evaluates to a number other than 0 is taken to be true.
Strings
"Hello World !" Strings are delineated by single or double quotation marks. (Use single quotes to type strings that contain quotation marks.)
Objects
myObj = new Object();
Null
Not the same as zero - no value at all. A null value is one that has no value and means nothing.
Undefined
A value that is undefined is a value held by a variable after it has been created, but before a value has been assigned to it.


That said, JavaScript is a loosely typed language -- you do not have to specify the data type of a variable when you declare it, and data types are converted automatically as needed during script execution. By and large, you may simply assign any type of data to any variable. The only time data typing matters is when you need to perform operations on the data. Certain operators behave differently depending on the type of data being deal with. For example, consider the + operator:


"5" + "10"
yields
"510" (string concatenation)

5 + 10
yields
15 (arithmetic sum)

JavaScript Grammar

JavaScript code, much like other programming languages, is made up of statements which serve to make assignments, compare values, and execute other sections of code. By and large, programmers will already be familiar with JavaScript's usage of variables, operators, and statements. Below is a chart summarizing the main elements of JavaScript grammar. Following, we will look at each element in detail.

Variables
Labels which refer to a changeable value.
Example: total may be possess a value of 100.
Operators
Actors which can be used to calculate or compare values.
Example: Two values may be summed using the addition operator (+); total+tax
Example: Two values may be compared using the greater-than operator (>); total>200
Expressions
Any combination of variables, operators, and statements which evaluate to some result. In English parlance this might be termed a "sentence" or even a "phrase", in that grammatical elements are combined into a cogent meaning.
Example: total=100;
Example: if (total>100)
Statements
As in English, a statement pulls all grammatical elements together into a full thought. JavaScript statements may take the form of conditionals, loops, or object manipulations. It is good form to separate statements by semicolons, although this is only mandatory if multiple statements reside on the same line.
Example: if (total>100) {statements;} else {statements;}
Example: while (clicks<10) {statements;}
Objects
Containing constructs which possess a set of values, each value reflected into an individual property of that object. Objects are a critical concept and feature of JavaScript. A single object may contain many properties, each property which acts like a variable reflecting a certain value. JavaScript can reference a large number of "built-in" objects which refer to characteristics of a Web document. For instance, the document object contains properties which reflect the background color of the current document, its title, and many more. For a fuller explanation of the built-in objects of JavaScript, see the section on "Document Object Model".
Functions and Methods
A JavaScript function is quite similar to a "procedure" or "subroutine" in other programming languages. A function is a discrete set of statements which perform some action. It may accept incoming values (parameters), and it may return an outgoing value. A function is "called" from a JavaScript statement to perform its duty. A method is simply a function which is contained in an object. For instance, a function which closes the current window, named close(), is part of the window object; thus, window.close() is known as a method.

JavaScript Tutorial for Programmers

JavaScript code is typically embedded into an HTML document using the SCRIPT tag. You are free to embed as many scripts into a single document as you like, using multiple SCRIPT tags. A script embedded in HTML with the SCRIPT tag uses the format:





The LANGUAGE attribute is optional, but recommended. You may specify that a section of code only be executed by browsers which support a particular version of JavaScript; for instance:





The external file is simply a text file containing JavaScript code, and whose filename ends with the extension ".js". Note that although some version 3 browsers support the SRC attribute, it only functions reliably across platforms in the version 4 browsers.

Scripts can be placed inside comment fields to ensure that your JavaScript code is not displayed by old browsers that do not recognize JavaScript. The markup to begin a comment field is . This practice is certainly optional, but considered good form when your page is likely to be visited by older browsers. Certainly, as older browsers fade away, this practice will likely become unnecessary.