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appendix A

JavaScript Structure and Objects Reference


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.

Built-In Objects

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:



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.

The Math.random() method worked only on UNIX platforms until Netscape 3.0. Be sure you and your users use the latest version.


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:

The Date object will not work with dates before January 1st, 1970.

  • 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 JavaScript Object Hierarchy

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.
  • 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.
  • 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.

Creating and Customizing Objects

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."

Creating 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();

Customizing Objects

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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