avaScript 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 semi-colon.
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 statements are available in JavaScript:
Note
new and this are not statements, but are
included in this section for convenience.
A statement that terminates the current while or
for loop and transfers program control to the statement following
the terminated loop.
break
Navigator 2.0
The following function has a break statement that
terminates the while loop when i 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
}
Notations by the author to 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 a /* and followed by a
*/.
1. // comment text
2. /* multiple line comment text */
Navigator 2.0
// 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. */
A statement that terminates execution of the block of
statements in a while or for loop, and continues execution
of the loop with the next iteration. 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.
continue
Navigator 2.0
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
}
A statement that 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
}
initial-expression is a statement or variable
declaration. It is typically used to initialize a counter variable. This
expression may optionally declare new variables with the var
keyword.
condition is evaluated on each pass through the
loop. If this condition evaluates to true, the statements in
statements are performed. This conditional test is optional. If omitted,
the condition always evaluates to true.
increment-expression is generally used to update
or increment the counter variable.
statements is a block of statements that are executed
as long as condition evaluates to true. This can be a single statement
or multiple statements. Although not required, it is good practice to indent
these statements from the beginning of the for statement.
Navigator 2.0
The following for statement starts by declaring
the variable i and initializing it to zero. It checks that i
is less than nine, performs the two succeeding statements, and increments
i by one after each pass through the loop.
for (var i = 0; i < 9; i++) {
n += i
myfunc(n)
}
A statement that 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 }
variable is the variable to iterate over every
property.
object is the object for which the properties are
iterated.
statements specifies the statements to execute
for each property.
Navigator 2.0
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
}
A statement that declares a JavaScript function
name with the specified parameters param. Acceptable parameters
include strings, numbers, and objects.
To return a value, the function must have a return
statement that specifies the value to return. You cannot nest a function
statement in another statement or in itself.
All parameters are passed to functions, by value.
In other words, the value is passed to the function, but if the function
changes the value of the parameter, this change is not reflected globally
or in the calling function.
In addition to defining functions as described here, you
can also define Function objects, as described in
"Function".
function name([param] [, param] [..., param]) {
statements }
name is the function name.
param is the name of an argument to be passed to
the function. A function can have up to 255 arguments.
Navigator 2.0
//This function 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
}
A statement that 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}]
condition can be any JavaScript expression that
evaluates to true or false. Parentheses are required around the condition.
If condition evaluates to true, the statements in statements1
are executed.
statements1 and statements2 can be any
JavaScript statements, including further nested if statements. Multiple
statements must be enclosed in braces.
Navigator 2.0
if ( cipher_char == from_char ) {
result = result + to_char
x++ }
else
result = result + clear_char
An operator that lets you 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.
Creating a user-defined object type requires two steps:
-
-
Define the object type by writing a function.
-
Create an instance of the object with new.
To define an object type, create a function for the object
type that specifies its name, properties, and methods. An object can have
a property that is itself another object. See the examples below.
You can always add a property to
a previously defined object. For example, the statement car1.color
= "black" adds a property color to car1, and assigns
it a value of "black". However, this does not affect any other objects. To
add the new property to all objects of the same type, you must add the property
to the definition of the car object type.
You can add a property to a previously defined object
type by using the prototype property. This defines a property that
is shared by all objects of the specified type, rather than by just one instance
of the object. The following code adds a color property to all objects
of type car, and then assigns a value to the color property
of the object car1. For more information, see
"prototype".
Car.prototype.color=null
car1.color="black"
birthday.description="The day you were born"
objectName = new objectType ( param1 [,param2] ...[,paramN] )
objectName is the name of the new object
instance.
objectType is the object type. It must be a function
that defines an object type.
param1...paramN are the property values for the
object. These properties are parameters defined for the objectType
function.
Navigator 2.0
Example 1: object type and object instance. Suppose
you want to create an object type for cars. You want this type of object
to be called car, and you want it to have properties for make, model,
and year. To do this, you would write the following function:
function car(make, model, year) {
this.make = make
this.model = model
this.year = year
}
Now you can create an object called mycar as
follows:
mycar = new car("Eagle", "Talon TSi", 1993)
This statement creates mycar and assigns it the
specified values for its properties. Then the value of mycar.make
is the string "Eagle", mycar.year is the integer 1993, and so on.
You can create any number of car objects by calls
to new. For example,
kenscar = new car("Nissan", "300ZX", 1992)
Example 2: object property that is itself another
object. Suppose you define an object called person as follows:
function person(name, age, sex) {
this.name = name
this.age = age
this.sex = sex
}
And then instantiate two new person objects as
follows:
rand = new person("Rand McNally", 33, "M")
ken = new person("Ken Jones", 39, "M")
Then you can rewrite the definition of car to include
an owner property that takes a person object, as follows:
function car(make, model, year, owner) {
this.make = make;
this.model = model;
this.year = year;
this.owner = owner;
}
To instantiate the new objects, you then use the
following:
car1 = new car("Eagle", "Talon TSi", 1993, rand);
car2 = new car("Nissan", "300ZX", 1992, ken)
Instead of passing a literal string or integer value when
creating the new objects, the above statements pass the objects rand
and ken as the parameters for the owners. To find out the name of
the owner of car2, you can access the following property:
car2.owner.name
A statement that specifies the value to be returned by
a function.
return expression
Navigator 2.0
The following function returns the square of its argument,
x, where x is a number.
function square( x ) {
return x * x
}
A keyword that you can use to refer to the current object.
In general, in a method this refers to the calling object.
this[.propertyName]
Navigator 2.0
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)">
A statement that declares a variable, optionally initializing
it to a 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 if a
global variable of the same name exists.
var varname [= value] [..., varname [= value] ]
varname is the variable name. It can be any legal
identifier.
value is the initial value of the variable and
can be any legal expression.
Navigator 2.0
var num_hits = 0, cust_no = 0
A statement that 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
}
condition is evaluated before each pass through
the loop. If this condition evaluates to true, the statements in the succeeding
block are performed. When condition evaluates to false, execution
continues with the statement following statements.
statements is a block of statements that are executed
as long as the condition evaluates to true. Although not required,
it is good practice to indent these statements from the beginning of the
while statement.
Navigator 2.0
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 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.
A statement that 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.
with (object){
statements
}
object specifies the default object to use for
the statements. The parentheses around object are required.
statements is any block of statements.
Navigator 2.0
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)
}
| file: /Techref/language/JAVA/SCRIPT/stmts.htm, 27KB, , updated: 2009/1/29 14:30, local time: 2024/11/25 10:46,
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