appendix A
JavaScript Structure and Objects
Reference
CONTENTS
This appendix is a quick reference for JavaScript objects. It
includes the built-in objects and the objects in the object hierarchy.
For further information, refer to Part II, "Using JavaScript
Objects and Forms," of this guide.
The following objects are built in to JavaScript. Some can be
used to create objects of your own; others can be used only as
they are. Each is summarized in the following sections. See Chapter
4, "Using Built-In Objects and Custom Objects," for
details and examples.
You can create a new Array
object to define an array-a numbered list of variables. (Unlike
other variables, arrays must be declared.) Use the new
keyword to define an array, as in this example:
students = new Array(30)
Items in the array are indexed beginning with 0.
Refer to items in the array with brackets:
fifth = students[4];
Arrays have a single property, length,
which gives the current number of elements in the array. They
have the following methods:
- join quickly joins all
the array's elements together, resulting in a string. The elements
are separated by commas, or by the separator you specify.
- reverse returns a reversed
version of the array.
- sort returns a sorted
version of the array. Normally, this is an alphabetical sort;
however, you can use a custom sort method by specifying a comparison
routine.
Any string of characters in JavaScript is a string
object. The following statement assigns a variable to a string
value:
text = "This is a test."
Because strings are objects, you can also create a new string
with the new keyword:
text = new String("This is a test.");
string objects have a single
property, length, which reflects
the current length of the string. There are a variety of methods
available to work with strings:
- anchor() creates an HTML
anchor within the current page.
- indexOf() finds an occurrence
of a string within the string.
- lastIndexOf() finds an
occurrence of a string within the string, starting at the end
of the string.
- link() creates an HTML
link using the string's text.
- split() splits the string
into an array based on a separator.
- substring() returns a
portion of the string.
- toString() can be used
on non-string values and converts them to strings.
- toUpperCase() converts
all characters in the string to uppercase.
- toLowerCase() converts
all characters in the string to lowercase.
There are also a few methods that enable you to change a string's
appearance when it appears in an HTML document:
- big() displays big text,
using the <BIG> tag
in HTML 3.0.
- blink() displays blinking
text, using the <BLINK>
tag in Netscape.
- bold() displays bold
tag, using the <B>
tag.
- fixed() displays fixed-font
text, using the <TT>
tag.
- fontcolor() displays
the string in a colored font, equivalent to the <FONTCOLOR>
tag in Netscape.
- fontsize() changes the
font size, using the <FONTSIZE>
tag in Netscape.
- italics() displays the
string in italics, using the <I>
tag.
- small() displays the
string in small letters using the <SMALL>
tag in HTML 3.0.
- strike() displays the
string in a strikethrough font, using the <STRIKE>
tag.
- sub() displays subscript
text, equivalent to the <SUB>
tag in HTML 3.0.
- sup() displays superscript
text, equivalent to the <SUP>
tag in HTML 3.0.
As an example, this statement prints the value of the text
string in italics:
document.write(text.italics());
The Math object is not a
"real" object, because you can't create your own objects.
Each property or method uses the built-in Math
object. A variety of mathematical constants are available as properties
of the Math object:
- Math.E is the base of
natural logarithms (approximately 2.718).
- Math.LN2 is the natural
logarithm of two (approximately 0.693).
- Math.LN10 is the natural
logarithm of 10 (approximately 2.302).
- Math.LOG2E is the base
2 logarithm of e (approximately 1.442).
- Math.LOG10E is the base
10 logarithm of e (approximately 0.434).
- Math.PI is the ratio
of a circle's circumference to its diameter (approximately 3.14159).
- Math.SQRT1_2 is the square
root of one-half (approximately 0.707).
- Math.SQRT2 is the square
root of two (approximately 2.7178).
The methods of the Math object
enable you to perform mathematical functions. The methods are
listed in the following sections in categories.
Algebraic Functions
- Math.acos() calculates
the arc cosine of a number, in radians.
- Math.asin() calculates
the arc sine of a number.
- Math.atan() calculates
the arc tangent of a number.
- Math.atan2() calculates
the polar coordinate angle (theta) for an x,
y coordinate pair.
- Math.cos() calculates
the cosine of a number.
- Math.sin() returns the
sine of a number.
- Math.tan() calculates
the tangent of a number.
Statistical and Logarithmic Functions
- Math.exp() returns e
(the base of natural logarithms) raised to a power.
- Math.log() returns the
natural logarithm of a number.
- Math.max() accepts two
numbers and returns whichever is greater.
- Math.min() accepts two
numbers and returns the smaller of the two.
For example, this statement assigns the big
variable to the larger of x
and y:
big = Math.max(x,y);
Basic Math and Rounding
- Math.abs() calculates
the absolute value of a number.
- Math.ceil() rounds a
number up to the nearest integer.
- Math.floor() rounds a
number down to the nearest integer.
- Math.pow() calculates
one number to the power of another.
- Math.round() rounds a
number to the nearest integer.
- Math.sqrt() calculates
the square root of a number.
As an example, the following statement assigns the x
variable to the square root
of 35:
x = Math.sqrt(25);
Random Numbers
Math.random() returns a random
number between 0 and 1.
Note |
The Math.random() method worked only on UNIX platforms until Netscape 3.0. Be sure you and your users use the latest version.
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The Date object is a built-in
JavaScript object that enables you to work conveniently with dates
and times. You can create a Date
object any time you need to store a date and use the Date
object's methods to work with the date:
Note |
The Date object will not work with dates before January 1st, 1970.
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- setDate() sets the day
of the month.
- setMonth() sets the month.
JavaScript numbers the months from 0 to 11, starting with January
(0).
- setYear() sets the year.
- setTime() sets the time
(and the date) by specifying the number of milliseconds since
January 1st, 1970.
- setHours(), setMinutes(),
and setSeconds() set the
time.
- getDate() gets the day
of the month.
- getMonth() gets the month.
- getYear() gets the year.
- getTime() gets the time
(and the date) as the number of milliseconds since January 1st,
1970.
- getHours(), getMinutes(),
and getSeconds() get the
time.
- getTimeZoneOffset() gives
you the local time zone's offset from GMT.
- toGMTString() converts
the date object's time value to text, using GMT (Greenwich Mean
Time, also known as UTC).
- toLocalString() converts
the date object's time value to text, using the user's local time.
- Date.parse() converts
a date string, such as "June 20,
1996" to a Date
object (number of milliseconds since 1/1/1970).
- Date.UTC() is the opposite;
it converts a Date object
value (number of milliseconds) to a UTC (GMT) time.
The navigator object includes
information about the current browser version. At present, it
works only with Netscape browsers. Its properties include the
following:
- navigator.appcodeName
is the browser's code name, usually "Mozilla".
- navigator.appName is
the browser's name, usually "Netscape".
- navigator.appVersion
is the version of Netscape being used. Example: "3.0(Win95;I)".
- navigator.userAgent is
the user-agent header, which is sent to the host when requesting
a Web page. It includes the entire version information-for example,
"Mozilla/2.0(Win95;I)".
- navigator.javaEnabled
is either true or false,
indicating whether Java (not JavaScript) is enabled on the browser.
- navigator.plugins is
an array that contains information about each currently available
plug-in (see Chapter 13, "Working with Multimedia and Plug-Ins).
- navigator.mimeTypes is
an array containing an element for each of the available MIME
types (see Chapter 13).
The object hierarchy includes objects that represent the browser
window, the current document, and its contents. These objects
are summarized here and explained in detail in Chapter 5 "Accessing
Window Elements as Objects."
The window object represents
the current browser window. If multiple windows are open or frames
are used, there may be more than one window object. These are
given aliases to distinguish them:
- self is the current window,
as is window. This is the
window containing the current JavaScript document.
- top is the window currently
on top (active) on the screen.
- parent is a window that
contains frames. Each frame is also a window
object under parent.
- Within a window you have created, opener
refers to the window that opened the window.
- The frames array contains
the window object for each
frame. These can be addressed as parent.frames[0]
through the number of frames, or with their individual names,
as in parent.docframe.
Each window object includes
the following properties:
- defaultStatus is the
initial message displayed in the status line.
- length is the number
of frames within a parent window.
- name is the name of the
window.
- status is the current
value of the status line.
The window object also has
three child objects, which you'll look at in their own sections
later:
- The location object stores
the location (URL) of the document displayed in the window.
- The document object holds
the Web page itself.
- The history object contains
a list of sites visited before or after the current site in the
window.
The window object includes
the following methods:
- alert() displays an alert
dialog.
- blur() removes focus
from the window, sending it to the background.
- close() closes a window
you have opened.
- confirm() displays a
confirmation dialog and returns true
or false.
- focus() gives the window
focus, moving it to the top.
- open() opens a new window.
- prompt() prompts the
user and returns the text entered.
- scroll() scrolls the
window, either horizontally or vertically. The parameters are
x and y
(column and row) offset in pixels.
- setTimeout() sets a timeout
to execute a statement and returns an identifier for the timeout.
- clearTimeout() clears
the timeout you specify.
Finally, window objects have
the following event handlers, which you can define in the document's
<BODY> or <FRAMESET>
tag:
- The onLoad event occurs
when the document in the window is finished loading.
- The onUnload event occurs
when another document starts to load, replacing the window's current
document.
- The onFocus event occurs
when the window receives focus.
- The onBlur event occurs
when the window loses focus.
- The onError event occurs
if the document in the window fails to load properly.
The location object contains
information about the current URL being displayed by the window.
It has a set of properties to hold the different components of
the URL:
- location.protocol is
the protocol (or method) of the URL.
- location.hostname specifies
the host name.
- location.port specifies
the communication port.
- location.host is a combination
of the host name and port.
- location.pathname is
the directory to find the document on the host, and the name of
the file.
- location.hash is the
name of an anchor within the document, if specified.
- location.target specifies
the TARGET attribute of the
link that was used to reach the current location.
- location.query specifies
a query string.
- location.href is the
entire URL.
The location object also
has two methods:
- location.reload() reloads
the current document; this is the same as the reload button on
Netscape's toolbar.
- location.replace() replaces
the current location with a new one; this is similar to setting
the location object's properties.
The history object holds
information about the URLs that have been visited before and after
the current one in the window, and it includes methods to go to
previous or next locations:
- history.back() goes back
to the previous location.
- history.forward() goes
forward to the next location.
- history.go() goes to
a specified offset in the history list (negative numbers go back,
positive numbers go forward).
The document object represents
the current document in the window and is a child of the window
object. It includes the following properties:
- bgColor is the background
color, specified with the BGCOLOR
attribute.
- fgColor is the foreground
(text) color, specified with the TEXT
attribute.
- lastModified is the date
the document was last modified. This date is sent from the server
along with the page.
- linkColor is the color
used for nonvisited links, specified with the LINK
attribute.
- location specifies the
document's URL. Don't confuse this with the window.location
object.
- referrer is the URL of
the page the user was viewing prior to the current page-usually,
the page with a link to the current page.
- title is the title of
the current page, defined by the HTML <TITLE>
tag.
- vlinkColor is the color
for visited links, specified with the VLINK
attribute.
The document object also
includes the following child objects as properties:
- document.forms is an
array with an element for each form in the document. These can
also be addressed by name, as in document.regform.
Form elements are child objects of the form object and are described
in Chapter 6 "Using Interactive Forms."
- document.links is an
array containing elements for each of the links in the document.
It can also contain area
objects, used for client-side image maps.
- document.anchors is an
array with elements for each of the anchors in the document.
- document.images contains
an element for each of the images in the current document. Chapter
12, "Working with Graphics and Games," explains this
object in detail.
- document.applets is an
array with references to each embedded Java applet in the document.
This object is explained in Chapter 16, "Integrating JavaScript
with Java."
The document object has no
event handlers. It includes the following methods:
- clear() clears a document
you have closed.
- close() closes a stream
and displays any text you have written.
- open() opens a stream
and clears the current document.
- write() writes text to
the document window.
- writeln() writes text
to the document window and adds a carriage return.
This is a brief summary of the keywords you can use to create
your own objects and customize existing objects. These are documented
in detail in Chapter 4 "Using Built-In Objects and Custom
Objects."
There are three JavaScript keywords used to create and refer to
objects:
- new is used to create
a new object.
- this is used to refer
to the current object. this
can be used in an object's constructor function or in an event
handler.
- with makes an object
the default for a group of statements. Properties without complete
object references will refer to this object.
To create a new object, you need an object constructor function.
This simply assigns values to the object's properties using this:
function Name(first,last) {
this.first = first;
this.last = last;
}
You can then create a new object using new:
Fred = new Name("Fred","Smith");
You can also create a generic object using the Object()
constructor and define its properties later:
values = new Object();
You can add additional properties to an object you have created
just by assigning them:
Fred.middle = "Clarence";
Properties you add this way apply only to that instance of the
object, not to all objects of the type. A more permanent approach
is to use the prototype keyword,
which adds a property to an object's prototype (definition). This
means that any future object of the type will include this property.
You can include a default value:
Name.prototype.title = "Citizen";
You can use this technique to add properties to the definitions
of built-in objects as well. For example, this statement adds
a property called num to
all existing and future string objects, with a default value of
10:
string.prototype.num = 10;
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