Chapter 20
Using the History List

What is a History List?

As you surf the Web, you will load many different pages, each with its own URL. The browser maintains a list of the most recent URLs, which can be viewed with ease in both Navigator and IE, as demonstrated in the following figures:

Figure 20-1.  IE’s history list.

Figure 20-2.  Navigator’s history list.

The history list behaves like a LIFO (Last In First Out) queue, where the Back button climbs up the list so URLs loaded by the Back button are not entered into the history list. Therefore, the history list does not always contain all the recently visited pages. For example, if you reach a page named “a.html” and you press the Back button to load a page named “b.html,” its URL replaces the URL of “a.html.”

The history Object

The history list is represented in JavaScript by the window.history object. This object lets you deal with the history list but not with its exact data. That is, actual URLs maintained in that list cannot be extracted or otherwise modified by a script. The only property of this object is its length. Its methods enable you to load the list’s entries but not to manipulate the URL explicitly.

You can take advantage of this object to automatically navigate the user’s browser backwards, as if possessed by spirits. Another possible application is to create the equivalent of the browser’s Back button directly in the document.

Since the history object is a property of the topmost window object, you have the option to refer to it as window.history or simply history.

History List Length

You can access the number of entries in the history list via the history.length property. It works exactly like the strings’ and arrays’ length property. You can use this property to find how many pages the user has visited lately:

// display message according to number of entries
if (history.length > 10)
alert("You've already accessed " + history.length + " Web
pages this session")
else
alert("You've only accessed " + history.length + " Web pages
this session")

This script displays an alert message which depends on the number of entries in the history list.

History List Entry Indexing

Like in arrays, each entry of the history list has an index, which differentiates it from the other elements of the list. However, the indexing method is quite different from character indexing in strings, or element indexing in arrays. As opposed to these indexing algorithms, the history list indexing scheme does not feature a minimum value. The index of the document currently loaded into the browser’s window is 0. The index of the document that was loaded before the current document, the one that can be reached by pressing the Back button, is –1. The document before that is indexed at –2, and so on. Similarly, documents that were first loaded after the current document are indexed positively. The index of the first document loaded after the current one, the one that can be retrieved via the Forward button, is 1. The following one is indexed at 2, and so on. The complete index resembles an axis with no limit at both ends.

The history list is dynamic (changes rapidly) because whenever the page in the browser’s window is replaced by a new document, the current entry becomes the previous one, and a new document takes its place. The desired shifting in terms of indexing is performed automatically by the browser, so you don’t have to worry about it.

Since most people tend to surf different places at different times, the content of the history list almost never repeats itself. You might think that by creating a very structured site, you can control the way the user surfs your site and thus be able to forecast the content of the history list. This is generally impossible, and you should not even try to do it.