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This chapter describes all JavaScript statements. JavaScript statements consist of keywords used with the appropriate syntax. A single statement may span multiple lines. Multiple statements may occur on a single line if each statement is separated by a semicolon.
Syntax conventions: All keywords in syntax statements are in bold. Words in italics represent user-defined names or statements. Any portions enclosed in square brackets, [ ], are optional. {statements} indicates a block of statements, which can consist of a single statement or multiple statements delimited by a curly braces { }.
The following table lists statements available in JavaScript.
Table 4.1 JavaScript statements.
break
|
Terminates the current while or for loop and transfers program control to the statement following the terminated loop.
|
comment
|
Notations by the author to explain what a script does. Comments are ignored by the interpreter.
|
continue
|
Terminates execution of the block of statements in a while or for loop, and continues execution of the loop with the next iteration.
|
do...while
|
Executes the specified statements until the test condition evaluates to false. Statements execute at least once.
|
export
|
Allows a signed script to provide properties, functions, and objects to other signed or unsigned scripts.
|
for
|
Creates a loop that consists of three optional expressions, enclosed in parentheses and separated by semicolons, followed by a block of statements executed in the loop.
|
for...in
|
Iterates a specified variable over all the properties of an object. For each distinct property, JavaScript executes the specified statements.
|
function
|
Declares a function with the specified parameters. Acceptable parameters include strings, numbers, and objects.
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if...else
|
Executes a set of statements if a specified condition is true. If the condition is false, another set of statements can be executed.
|
import
|
Allows a script to import properties, functions, and objects from a signed script that has exported the information.
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label
|
Provides an identifier that can be used with break or continue to indicate where the program should continue execution.
|
return
|
Specifies the value to be returned by a function.
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switch
|
Allows a program to evaluate an expression and attempt to match the expression's value to a case label.
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var
|
Declares a variable, optionally initializing it to a value.
|
while
|
Creates a loop that evaluates an expression, and if it is true, executes a block of statements. The loop then repeats, as long as the specified condition is true.
|
with
|
Establishes the default object for a set of statements.
|
Use the break statement to terminate a loop, switch, or label statement.
Terminates the current loop, switch, or label statement and transfers program control to the statement following the terminated loop.
break [label]
The break statement includes an optional label that allows the program to break out of a labeled statement. The statements in a labeled statement can be of any type.
Example 1. The following function has a break statement that terminates the while loop when e is 3, and then returns the value 3 * x.
function testBreak(x) {
var i = 0
while (i < 6) {
if (i == 3)
break
i++
}
return i*x
}
Example 2. In the following example, a statement labeled checkiandj contains a statement labeled checkj. If break is encountered, the program breaks out of the checkj statement and continues with the remainder of the checkiandj statement. If break had a label of checkiandj, the program would break out of the checkiandj statement and continue at the statement following checkiandj.
checkiandj :
if (4==i) {
document.write("You've entered " + i + ".<BR>");
checkj :
if (2==j) {
document.write("You've entered " + j + ".<BR>");
break checkj;
document.write("The sum is " + (i+j) + ".<BR>");
}
document.write(i + "-" + j + "=" + (i-j) + ".<BR>");
}
continue, label, switch
Notations by the author to explain what a script does. Comments are ignored by the interpreter.
// comment text
/* multiple line comment text */
JavaScript supports Java-style 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. */
Restarts a while, do-while, for, or label statement.
continue [label]
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 update expression.
The continue statement can now include an optional label that allows the program to terminate execution of a labeled statement and continue to the specified labeled statement. This type of continue must be in a looping statement identified by the label used by continue.
Example 1. The following example shows a while loop that has a continue statement that executes when the value of i is 3. Thus, n takes on the values 1, 3, 7, and 12.
i = 0
n = 0
while (i < 5) {
i++
if (i == 3)
continue
n += i
}
Example 2. In the following example, a statement labeled checkiandj contains a statement labeled checkj. If continue is encountered, the program continues at the top of the checkj statement. Each time continue is encountered, checkj reiterates until its condition returns false. When false is returned, the remainder of the checkiandj statement is completed. checkiandj reiterates until its condition returns false. When false is returned, the program continues at the statement following checkiandj.
If continue had a label of checkiandj, the program would continue at the top of the checkiandj statement.
checkiandj :
while (i<4) {
document.write(i + "<BR>");
i+=1;
checkj :
while (j>4) {
document.write(j + "<BR>");
j-=1;
if ((j%2)==0)
continue checkj;
document.write(j + " is odd.<BR>");
}
document.write("i = " + i + "<br>");
document.write("j = " + j + "<br>");
}
break, label
Executes the specified statements until the test condition evaluates to false. Statements execute at least once.
do
statements
while (condition);
In the following example, the do loop iterates at least once and reiterates until i is no longer less than 5.
do {
i+=1
document.write(i);
while (i<5);
Allows a signed script to provide properties, functions, and objects to other signed or unsigned scripts.
export name1, name2, ..., nameN
export *
Typically, information in a signed script is available only to scripts signed by the same principals. By exporting properties, functions, or objects, a signed script makes this information available to any script (signed or unsigned). The receiving script uses the companion import statement to access the information.
import
Creates a loop that consists of three optional expressions, enclosed in parentheses and separated by semicolons, followed by a block of statements executed in the loop.
for ([initial-expression]; [condition]; [increment-expression]) {
statements
}
The following for statement starts by declaring the variable i and initializing it to 0. It checks that i is less than nine, performs the two succeeding statements, and increments i by 1 after each pass through the loop.
for (var i = 0; i < 9; i++) {
n += i
myfunc(n)
}
Iterates a specified variable over all the properties of an object. For each distinct property, JavaScript executes the specified statements.
for (variable in object) {
statements
}
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 show_props(obj, objName) {
var result = ""
for (var i in obj) {
result += objName + "." + i + " = " + obj[i] + "\n"
}
return result
}
Declares a function with the specified parameters. Acceptable parameters include strings, numbers, and objects.
function name([param] [, param] [..., param]) {
statements
}
You can also define functions using the Function constructor; see "Function" on page 169.
To return a value, the function must have a return statement that specifies the value to return.
A function created with the function statement is a Function object and has all the properties, methods, and behavior of Function objects. See "Function" on page 169 for detailed information on functions.
The following code declares a function that returns the total dollar amount of sales, when given the number of units sold of products a, b, and c.
function calc_sales(units_a, units_b, units_c) {
return units_a*79 + units_b*129 + units_c*699
}
"Function" on page 169
Executes a set of statements if a specified condition is true. If the condition is false, another set of statements can be executed.
if (condition) {
statements1
}
[else {
statements2
}]
You should not use simple assignments in a conditional statement. For example, do not use the following code:
if(x = y)
{
/* do the right thing */
}
If you need to use an assignment in a conditional statement, put additional parentheses around the assignment. For example, use if( (x = y) ).
JavaScript 1.2 and earlier versions.
You can use simple assignments in a conditional statement. An assignment operator in a conditional statement is converted to an equality operator. For example, if(x = y) is converted to if(x == y). In Navigator, this expression also displays a dialog box with the message "Test for equality (==) mistyped as assignment (=)? Assuming equality test."
if (cipher_char == from_char) {
result = result + to_char
x++}
else
result = result + clear_char
Allows a script to import properties, functions, and objects from a signed script that has exported the information.
import objectName.name1, objectName.name2, ..., objectName.nameN
import objectName.*
The objectName parameter is the name of the object that will receive the imported names. For example, if f and p have been exported, and if obj is an object from the importing script, the following code makes f and p accessible in the importing script as properties of obj.
import obj.f, obj.p
Typically, information in a signed script is available only to scripts signed by the same principals. By exporting (using the export statement) properties, functions, or objects, a signed script makes this information available to any script (signed or unsigned). The receiving script uses the import statement to access the information.
The script must load the export script into a window, frame, or layer before it can import and use any exported properties, functions, and objects.
export
Provides a statement with an identifier that lets you refer to it elsewhere in your program.
For example, you can use a label to identify a loop, and then use the break or continue statements to indicate whether a program should interrupt the loop or continue its execution.
label :
statements
For an example of a label statement using break, see break. For an example of a label statement using continue, see continue.
break, continue
Specifies the value to be returned by a function.
return expression
The following function returns the square of its argument, x, where x is a number.
function square(x) {
return x * x
}
Allows a program to evaluate an expression and attempt to match the expression's value to a case label.
switch (expression){
case label :
statements;
break;
case label :
statements;
break;
...
default : statements;
}
If a match is found, the program executes the associated statement. If multiple cases match the provided value, the first case that matches is selected, even if the cases are not equal to each other.
The program first looks for a label matching the value of expression and then executes the associated statement. If no matching label is found, the program looks for the optional default statement, and if found, executes the associated statement. If no default statement is found, the program continues execution at the statement following the end of switch.
The optional break statement associated with each case label ensures that the program breaks out of switch once the matched statement is executed and continues execution at the statement following switch. If break is omitted, the program continues execution at the next statement in the switch statement.
In the following example, if expression evaluates to "Bananas", the program matches the value with case "Bananas" and executes the associated statement. When break is encountered, the program breaks out of switch and executes the statement following switch. If break were omitted, the statement for case "Cherries" would also be executed.
switch (i) {
case "Oranges" :
document.write("Oranges are $0.59 a pound.<BR>");
break;
case "Apples" :
document.write("Apples are $0.32 a pound.<BR>");
break;
case "Bananas" :
document.write("Bananas are $0.48 a pound.<BR>");
break;
case "Cherries" :
document.write("Cherries are $3.00 a pound.<BR>");
break;
default :
document.write("Sorry, we are out of " + i + ".<BR>");
}
document.write("Is there anything else you'd like?<BR>");
Declares a variable, optionally initializing it to a value.
var varname [= value] [..., varname [= value] ]
The scope of a variable is the current function or, for variables declared outside a function, the current application.
Using var outside a function is optional; you can declare a variable by simply assigning it a value. However, it is good style to use var, and it is necessary in functions in the following situations:
var num_hits = 0, cust_no = 0
Creates a loop that evaluates an expression, and if it is true, executes a block of statements. The loop then repeats, as long as the specified condition is true.
while (condition) {
statements
}
The following while loop iterates as long as n is less than three.
n = 0
x = 0
while(n < 3) {
n ++
x += n
}
Each iteration, the loop increments n and adds it to x. Therefore, x and n take on the following values:
After completing the third pass, the condition n < 3 is no longer true, so the loop terminates.
Establishes the default object for a set of statements.
with (object){
statements
}
JavaScript looks up any unqualified names within the set of statements to determine if the names are properties of the default object. If an unqualified name matches a property, then the property is used in the statement; otherwise, a local or global variable is used.
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)
}
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Last Updated: 05/28/99 12:01:10
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Netscape Communications Corporation