avaScript supports a compact set of statements that you can use to incorporate
a great deal of interactivity in Web pages. The statements fall into the
following categories:
The following sections provide a brief overview of each
statement. See the statements reference for details.
Use the if statement to perform certain statements
if a logical condition is true; use the optional else clause to perform
other statements if the condition is false. An if statement looks
as follows:
if (condition) {
statements1
[ } else {
statements2 ]
}
The condition can be any JavaScript expression that evaluates
to true or false. The statements to be executed can be any JavaScript statements,
including further nested if statements. If you want to use more than
one statement after an if or else statement, you must enclose
the statements in curly braces, {}.
Example. In the following example, the function
checkData returns true if the number of characters in a Text
object is three; otherwise, it displays an alert and returns false.
function checkData () {
if (document.form1.threeChar.value.length == 3) {
return true
} else {
alert("Enter exactly three characters. " +
document.form1.threeChar.value + " is not valid.")
return false
}
}
A loop is a set of commands that executes repeatedly until
a specified condition is met. JavaScript supports two loop statements:
for and while. In addition, you can use the break and
continue statements within loop statements.
Another statement, for...in, executes statements
repeatedly but is used for object manipulation. See
"Object manipulation statements and
operators".
A for loop repeats until a specified condition
evaluates to false. The JavaScript for loop is similar to the Java
and C for loop. A for statement looks as follows:
for ([initial-expression]; [condition]; [increment-expression]) {
statements
}
When a for loop executes, the following occurs:
-
-
The initializing expression initial-expression, if any, is executed.
This expression usually initializes one or more loop counters, but the syntax
allows an expression of any degree of complexity.
-
The condition expression is evaluated. If the value of
condition is true, the loop statements execute. If the value of
condition is false, the for loop terminates.
-
The update expression increment-expression executes.
-
The statements execute, and control returns to step 2.
Example. The following function contains a
for statement that counts the number of selected options in a scrolling
list (a Select object that allows multiple selections). The for
statement declares the variable i and initializes it to zero. It checks
that i is less than the number of options in the Select object,
performs the succeeding if statement, and increments i by one
after each pass through the loop.
<SCRIPT>
function howMany(selectObject) {
var numberSelected=0
for (var i=0; i < selectObject.options.length; i++) {
if (selectObject.options[i].selected==true)
numberSelected++
}
return numberSelected
}
</SCRIPT>
<FORM NAME="selectForm">
<P><B>Choose some music types, then click the button below:</B>
<BR><SELECT NAME="musicTypes" MULTIPLE>
<OPTION SELECTED> R&B
<OPTION> Jazz
<OPTION> Blues
<OPTION> New Age
<OPTION> Classical
<OPTION> Opera
</SELECT>
<P><INPUT TYPE="button" VALUE="How many are selected?"
onClick="alert ('Number of options selected: ' + howMany(document.selectForm.musicTypes))">
</FORM>
A while statement repeats a loop as long as a specified
condition evaluates to true. A while statement looks as follows:
while (condition) {
statements
}
If the condition becomes false, the statements within
the loop stop executing and control passes to the statement following the
loop.
The condition test occurs only
when the statements in the loop have been executed and the loop is about
to be repeated. That is, the condition test is not continuous but is performed
once at the beginning of the loop and again just following the last statement
in statements, each time control passes through the loop.
Example 1. The following while loop iterates
as long as n is less than three:
n = 0
x = 0
while( n < 3 ) {
n ++
x += n
}
With each iteration, the loop increments n and
adds that value to x. Therefore, x and n take on the
following values:
-
-
After the first pass: n = 1 and x = 1
-
After the second pass: n = 2 and x = 3
-
After the third pass: n = 3 and x = 6
After completing the third pass, the condition n
< 3 is no longer true, so the loop terminates.
Example 2: infinite loop. Make sure the condition
in a loop eventually becomes false; otherwise, the loop will never terminate.
The statements in the following while loop execute forever because
the condition never becomes false:
while (true) {
alert("Hello, world") }
The break statement terminates the current
while or for loop and transfers program control to the statement
following the terminated loop. A break statement looks as follows:
break
Example. The following function has a break
statement that terminates the while loop when i is three, and
then returns the value 3 * x.
function testBreak(x) {
var i = 0
while (i < 6) {
if (i == 3)
break
i++
}
return i*x
}
A continue statement terminates execution of the
block of statements in a while or for loop and continues execution
of the loop with the next iteration. A continue statement looks as
follows:
continue
In contrast to the break statement, continue
does not terminate the execution of the loop entirely. Instead,
-
-
In a while loop, it jumps back to the condition.
-
In a for loop, it jumps to the increment-expression.
Example. The following example shows a while
loop with a continue statement that executes when the value of
i is three. Thus, n takes on the values one, three, seven,
and twelve.
i = 0
n = 0
while (i < 5) {
i++
if (i == 3)
continue
n += i
}
JavaScript has several ways of manipulating objects:
new operator, this keyword, for...in statement, and
with statement.
You can use the new operator to create an instance
of a user-defined object type or of one of the built-in object types Array,
Boolean, Date, Function, Math, Number, or String. Use new
as follows:
objectName = new objectType ( param1 [,param2] ...[,paramN] )
The following example creates an Array object with
25 elements, then assigns values to the first three elements:
musicTypes = new Array(25)
musicTypes[0] = "R&B"
musicTypes[1] = "Blues"
musicTypes[2] = "Jazz"
The following examples create several Date
objects:
today = new Date()
birthday = new Date("December 17, 1995 03:24:00")
birthday = new Date(95,12,17)
The following example creates a user-define object type
car, with properties for make, model, and year. The example then creates
an object called mycar and assigns values to its properties. The value
of mycar.make is the string "Eagle", mycar.year is the integer
1993, and so on.
function car(make, model, year) {
this.make = make
this.model = model
this.year = year
}
mycar = new car("Eagle", "Talon TSi", 1993)
For more information on new, see
"new".
Use the this keyword to refer to the current object.
In general, this refers to the calling object in a method. Use
this as follows:
this[.propertyName]
Example 1. Suppose a function called
validate validates an object's value property, given the object
and the high and low values:
function validate(obj, lowval, hival) {
if ((obj.value < lowval) || (obj.value > hival))
alert("Invalid Value!")
}
You could call validate in each form element's
onChange event handler, using this to pass it the form element, as
in the following example:
<B>Enter a number between 18 and 99:</B>
<INPUT TYPE = "text" NAME = "age" SIZE = 3
onChange="validate(this, 18, 99)">
Example 2. When combined with the form property,
this can refer to the current object's parent form. In the following
example, the form myForm contains a Text object and a button.
When the user clicks the button, the value of the Text object is set
to the form's name. The button's onClick event handler uses
this.form to refer to the parent form, myForm.
<FORM NAME="myForm">
Form name:<INPUT TYPE="text" NAME="text1" VALUE="Beluga">
<P>
<INPUT NAME="button1" TYPE="button" VALUE="Show Form Name"
onClick="this.form.text1.value=this.form.name">
</FORM>
The for...in statement iterates a specified variable
over all the properties of an object. For each distinct property, JavaScript
executes the specified statements. A for...in statement looks as
follows:
for (variable in object) {
statements }
Example. The following function takes as its argument
an object and the object's name. It then iterates over all the object's
properties and returns a string that lists the property names and their
values.
function dump_props(obj, obj_name) {
var result = ""
for (var i in obj) {
result += obj_name + "." + i + " = " + obj[i] + "<BR>"
}
result += "<HR>"
return result
}
For an object car with properties make and
model, result would be:
car.make = Ford
car.model = Mustang
The with statement establishes the default object
for a set of statements. Within the set of statements, any property references
that do not specify an object are assumed to be for the default object. A
with statement looks as follows:
with (object){
statements
}
Example. The following with statement specifies
that the Math object is the default object. The statements following
the with statement refer to the PI property and the cos
and sin methods, without specifying an object. JavaScript assumes
the Math object for these references.
var a, x, y
var r=10
with (Math) {
a = PI * r * r
x = r * cos(PI)
y = r * sin(PI/2)
}
Comments are author notations that explain what a script
does. Comments are ignored by the interpreter. JavaScript supports Java-style
comments:
-
-
Comments on a single line are preceded by a double-slash (//).
-
Comments that span multiple lines are preceded by /* and followed by */:
Example. The following example shows two
comments:
// This is a single-line comment.
/* This is a multiple-line comment. It can be of any length, and
you can put whatever you want here. */