Chapter 10

Strings, Math, and the History List

CONTENTS

• The string Object
  • The length Property
  • Methods of the string Object
• Creating Search and Replace Tools
  • The replace() Function
  • The search() Function
  • The space() Function
• The Math Object
  • The Trigonometric Methods
  • The log() and exp() Methods
  • The abs() Method
• Calculating Geometric Measurements
  • The circle() Function
  • Working with the Triangle
• Working with the History List
  • Properties and Methods of the history Object
• Summary
• Commands and Extensions Review
• Exercises
• Answers

Up to this point in the book, you have learned about the major objects and tools of JavaScript. Even so, this leaves several useful objects undiscovered.

In this chapter you are going to take a detailed look at some of the objects that you have been introduced to only briefly earlier in the book. These include the string object, the Math object, and the history object.

Every string in JavaScript is an object. The string object offers properties and methods to perform a variety of manipulations on a given string. These include methods for searching a string, extracting substrings, and applying HTML tags to the content of the string.

The Math object provides those functions and methods necessary to perform mathematical calculations. These range from the PI value to methods for all the trigonometric functions.

The history object is a bit different in that it doesn't involve the manipulation of information the way the string and Math objects do. The history object reflects the information in the browser's history list.

In this chapter you will learn the details of each of these object's properties and methods plus:

• How to manipulate the content of strings
• How to perform advanced mathematical calculations with the Math object
• How to build dynamic forward and back buttons-in any frame

The string Object

You already have considerable experience working with strings. You have used them throughout the book, you understand how to represent string literals, and you even know some of the basic techniques for examining the content of strings.

Even with the substring() and indexOf() methods which you saw earlier, though, you haven't reached the true possibilities of working with the string object.

The length Property

The string object has only one property: length. The length property is an integer value reflecting the number of characters in the string. Because the index of the first character in a string is zero, this means the length property is one greater than the index of the last character in the string.

For example, the string "Hello" has a length of five. The index of the first character ("H") is 0, and the index of the last character ("o") is 4.

Methods of the string Object

The flexibility and power of the string object rest in the wide variety of methods available to manipulate the content of the string. Table 10.1 outlines the methods available in the string object.

The HTML Methods

As you can see in Table 10.1, many of the methods of the string object are designed to add HTML tags to the content of the string so that when you display the string, it is suitably formatted. This can make the JavaScript code easier to read than if all the string assignments contained HTML tags, with the actual text to be displayed using document.write() or document.writeln().

The way these functions work is to return a new string containing the additional HTML tags. So, if you have a string variable named sample with the value "test", sample.big() returns "<BIG>test</BIG>" but sample still has a value of "test".

For instance, the following JavaScript commands output the text "Hello!" in large, blinking, bold letters:

var sample = "Hello!";
var sampleBig = sample.big();
var sampleBlink = sampleBig.blink();
var sampleBold = sampleBlink.bold();
document.write(sampleBold);

The following text displays the same word but as a hypertext link to the file: http://some.domain/some/file.html
var sample = "Hello!";
sample = sample.link("http://some.domain/some/file.html");
document.write(sample);

Because these methods return strings, you can also string together a series of methods and rewrite the first example as

var sample = "Hello!";
document.write(sample.big().blink().bold());

To give you a better idea of what these methods actually do to the content of your strings, the script in Listing 10.1 displays the actual content of the strings using the XMP tag to force the browser not to interpret any HTML in the output.

Listing 10.1. Applying HTML tags with JavaScript's string object.

<HTML>

<HEAD>
<TITLE>HTML method example</TITLE>
</HEAD>

<BODY>

<SCRIPT LANGUAGE="JavaScript">
<!-- HIDE FROM OTHER BROWSERS

var sample = "hello";

document.write("<XMP>" + sample.italics() + "</XMP>");
document.write(sample.italics());

document.write("<XMP>" + sample.blink() + "</XMP>");
document.write(sample.blink());

document.write("<XMP>" + sample.anchor("test") + "</XMP>");
document.write(sample.anchor("test"));

document.write("<XMP>" + sample.fontsize(7) + "</XMP>");
document.write(sample.fontsize(7));

document.write("<XMP>" + sample.bold().strike() + "</XMP>");
document.write(sample.bold().strike());

document.write("<XMP>" + sample.fontcolor("iceblue").big().sup() + "</XMP>");
document.write(sample.fontcolor("iceblue").big().sup());

// STOP HIDING FROM OTHER BROWSERS -->
</SCRIPT>

</BODY>

</HTML>

The script produces results like those in Figure 10.1.

Figure 10.1 : The HTML methods of the string object return new strings containing the appropriate HTML tags.

In this script, you are using various methods of the string object and the value returned by these methods. The script contains the examples in pairs of document.write() statements. The first output is the result of a method call surrounded by the XMP HTML container tags. The XMP tag ensures that any content inside the container is displayed without any processing. In this way, any HTML inside the container is simply displayed as regular text rather than treated as HTML.

The second line of each pair calls the same method but this time without the surrounding XMP tags so that the user can see what the result looks like when treated as HTML.

The substring() Method

You have seen the substring() method several times in previous chapters. You first saw the substring() method in Exercise 6.3 when you used it to verify input in a form.

To review, the method takes two integer arguments and returns the string starting at the first argument and ending at the character before the second argument. Where the first argument is larger, the process is reversed, and the substring starts at the second argument and continues until one character before the first argument. When both arguments are equal, an empty string is returned.

For instance, if you have a string named sample with a value "Hello!", then sample.substring(0,3) is "Hel", sample.substring(3,0) is "Hel", and sample.substring(2,4) has the value "ll".

The Case Methods

The string object has two methods for changing the case of characters in a string. toLowerCase() returns a new string with all characters in lowercase. Similarly, toUpperCase() returns a copy of the string with all characters uppercase.

For instance, if the variable sample is "tEsT", then sample.toLowerCase() is "test", and sample.toUpperCase() is "TEST".

Using a combination of these methods and the substring() method, you can achieve more interesting results. If you want to take a string and make the first character uppercase and the rest lowercase, you could use the following technique:

var sample = "tEsT";
var newSample = sample.substring(0,1).toUpperCase() +
Âsample.substring(1,sample.length).toLowerCase();

Other Methods

The string object has three other methods: indexOf(), lastIndexOf(), and charAt().

You saw the indexOf() method in Chapter 9, "Remember Where You've Been with Cookies." Simply, given two arguments (a string and an index) the method starts searching the string object from the index and looks for the first occurrence of the string that has been passed to it as an argument. It returns the index of this occurrence.

This is best understood by example: If you have a string named sample with the value "Greetings! Welcome to Navigator 3.0! Enjoy!", then sample.indexOf("Wel",2) would return a value of 13 and sample.lastIndexOf("!",sample.length - 3) would return a value of 35.

What happens in the first example is that the method starts searching the string sample from index 2 (the first "e" in "Greetings"). It checks if the phrase "Wel" starts at that index and if not, it moves to the next character (index 3) and tries again. This is repeated until the character at index 13, where a match is found.

The send example is similar, but it moves backwards through the string looking for a match. In this case it starts at the 3 character from the end ("o" in "Enjoy") and moves back until it finds a "!".

The other method, charAt(), is almost the reverse of this process. Given an index as an argument, it returns the character at that location. This is easier to use than the substring() method to extract a single character from a string.

For instance, with the above string, both sample.charAt(3) and sample.substring(3,4) have the value of "e".

With these methods, you can now develop tools to enable users to play with HTML to see how it looks. Using two frames, you will build an application that enables users to enter text in the left frame and select from a list of HTML attributes. They will see the text displayed with the combined attributes in the right frame, along with the actual HTML code needed to produce the results. Listings 10.2 through 10.4 contain the script files for the program.

In order to do this, you need a top-level frameset which looks like Listing 10.2

Listing 10.2. Top-level frameset.

<HTML>

<HEAD>
<TITLE>Listing 10.2</TITLE>
</HEAD>

<FRAMESET COLS="50%,*">
  <FRAME SRC="htmlform.html" NAME="choose">
  <FRAME SRC="sample.html" NAME="output">
</FRAMESET>

</HTML>

The htmlform.html file is where all the work is done.

Listing 10.3. The htmlform.html file.

<HTML>

<HEAD>

<SCRIPT LANGUAGE="JavaScript">
<!-- HIDE FROM OTHER BROWSERS

function display(form) {
  var format = form.toDisplay.value;
  var doc = parent.output;

  format = (form.big.checked) ? format.big() : format;
  format = (form.blink.checked) ? format.blink() : format;
  format = (form.bold.checked) ? format.bold() : format;
  format = (form.fixed.checked) ? format.fixed() : format;
  format = (form.italics.checked) ? format.italics() : format;
  format = (form.small.checked) ? format.small() : format;
  format = (form.strike.checked) ? format.strike() : format;
  format = (form.sup.checked) ? format.sup() : format;
  format = (form.sub.checked) ? format.sub() : format;
  format = (form.color.value == "") ? format.fontcolor("black") :
Âformat.fontcolor(form.color.value);
  format = (form.size.value == "") ? format.fontsize(3) :
Âformat.fontsize(form.size.value);

  var result = "<CENTER>The HTML code: <XMP>";
  result += format;
  result += "</XMP> looks like:<P>"
  result += format;
  result += "</CENTER>";

  doc.document.open("text/html");
  doc.document.write(result);
  doc.document.close();

}

// STOP HIDING -->
</SCRIPT>

<BODY BGCOLOR="aquamarine">

<CENTER>
<H1>The HTML tester page</H1>
Please enter some text, select some attributes and
enter a color and size (from 1 to 7).
The display will update dynamically.
<BR>
</CENTER>
<FORM METHOD=POST>

<TEXTAREA NAME="toDisplay" ROWS=10 COLS=35 WRAP=SOFT
onChange="display(this.form);">
Enter Text Here
</TEXTAREA><BR>
<INPUT TYPE="checkbox" NAME="big" onClick="display(this.form);">Big<BR>
<INPUT TYPE="checkbox" NAME="blink" onClick="display(this.form);">Blinking<BR>
<INPUT TYPE="checkbox" NAME="bold" onClick="display(this.form);">Bold<BR>
<INPUT TYPE="checkbox" NAME="fixed"
onClick="display(this.form);">Fixed Width<BR>
<INPUT TYPE="checkbox" NAME="italics" onClick="display(this.form);">Italics<BR>
<INPUT TYPE="checkbox" NAME="small" onClick="display(this.form);">Small<BR>
<INPUT TYPE="checkbox" NAME="strike"
onClick="display(this.form);">Striked Out<BR>
<INPUT TYPE="checkbox" NAME="sub" onClick="display(this.form);">Subscript<BR>
<INPUT TYPE="checkbox" NAME="sup" onClick="display(this.form);">SuperScript<BR>
Font Color: <INPUT TYPE="text" NAME="color" VALUE="black"
onChange="display(this.form);"><BR>
Font Size (1 to 7): <INPUT TYPE="text" NAME="size" VALUE="3"
onChange="display(this.form);">

</FORM>

<SCRIPT LANGUAGE="JavaScript">
<!-- HIDE FROM OTHER BROWSERS

display(document.forms[0]);

// STOP HIDING -->
</SCRIPT>

</BODY>

</HTML>

The file sample.html is just a blank HTML file to fill the window when the frame is initially loaded.

Listing 10.4. The source code for sample.html.

<HTML>

<BODY BGCOLOR="#FFFFFF">
</BODY>

</HTML>

This script produces results like those in Figure 10.2.

Figure 10.2 : Using methods from the string object to dynamically test different combinations of HTML attributes.

This program not only highlights the effect of the previous example but also some of the methods and techniques you learned in previous chapters.

All the work is done in the file htmlform.html (Listing 10.3). The program only really does one task-displays information based on the content of a form-so only one function, display(), is necessary.

function display(form) {
  var format = form.toDisplay.value;
  var doc = parent.output;

You start by setting up the global variable format. You use this variable to hold the entire text and HTML tags to be tested. You start by assigning the content of the textarea input field, which contains the text you are going to test.

You also define doc to be equal to the object parent.output. parent.output refers to the second frame in the frameset and is effectively the window object for that frame. In this way, you can use doc instead of parent.output. For instance, doc.document.write() is the same as parent.output.document.write().

  format = (form.big.checked) ? format.big() : format;
  format = (form.blink.checked) ? format.blink() : format;
  format = (form.bold.checked) ? format.bold() : format;
  format = (form.fixed.checked) ? format.fixed() : format;
  format = (form.italics.checked) ? format.italics() : format;
  format = (form.small.checked) ? format.small() : format;
  format = (form.strike.checked) ? format.strike() : format;
  format = (form.sup.checked) ? format.sup() : format;
  format = (form.sub.checked) ? format.sub() : format;
  format = (form.color.value == "") ? format.fontcolor("black") :
format.fontcolor(form.color.value);
  format = (form.size.value == "") ? format.fontsize(3) :
format.fontsize(form.size.value);

This section of code looks complex at first glance, but in reality, it is simple. You start by checking whether any of the checkboxes are checked. Because the checkbox object's checked property is a Boolean value, you can use it as the condition for a conditional expression, which performs the appropriate method and then assigns the result back to format.

Next, you apply the appropriate fontcolor() and fontsize() methods based on the form content. If either field is empty, you use a default value.

  var result = "<CENTER>The HTML code: <XMP>";
  result += format;
  result += "</XMP> looks like:<P>"
  result += format;
  result += "</CENTER>";

  doc.document.open("text/html");
  doc.document.write(result);
  doc.document.close();

The final task is to output the results. The string result holds the complete output for the second frame. Then you use document.open() to open a new output stream in the second frame for the HTML MIME type. You write the results to the frame and close the stream with document.close().

<FORM METHOD=POST>

<TEXTAREA NAME="toDisplay" ROWS=10 COLS=35 WRAP=SOFT
onChange="display(this.form);">
Enter Text Here
</TEXTAREA><BR>
<INPUT TYPE="checkbox" NAME="big" onClick="display(this.form);">Big<BR>
<INPUT TYPE="checkbox" NAME="blink" onClick="display(this.form);">Blinking<BR>
<INPUT TYPE="checkbox" NAME="bold" onClick="display(this.form);">Bold<BR>
<INPUT TYPE="checkbox" NAME="fixed"
onClick="display(this.form);">Fixed Width<BR>
<INPUT TYPE="checkbox" NAME="italics" onClick="display(this.form);">Italics<BR>
<INPUT TYPE="checkbox" NAME="small" onClick="display(this.form);">Small<BR>
<INPUT TYPE="checkbox" NAME="strike"
onClick="display(this.form);">Striked Out<BR>
<INPUT TYPE="checkbox" NAME="sub" onClick="display(this.form);">Subscript<BR>
<INPUT TYPE="checkbox" NAME="sup" onClick="display(this.form);">SuperScript<BR>
Font Color: <INPUT TYPE="text" NAME="color" VALUE="black"
onChange="display(this.form);"><BR>
Font Size (1 to 7): <INPUT TYPE="text" NAME="size" VALUE="3"
onChange="display(this.form);">

The form is simple. When any text field changes, you call display() and when any checkbox is clicked, you also call display(). No buttons are needed.

<SCRIPT LANGUAGE="JavaScript">
<!-- HIDE FROM OTHER BROWSERS

display(document.forms[0]);

// STOP HIDING -->
</SCRIPT>

You end the body of the HTML file with a one-line script that calls display() for the first time to update the second frame with the contents of the form. This also could have been done in the onLoad event handler.

Creating Search and Replace Tools

Anyone familiar with UNIX will miss many of the powerful text searching and matching tools found in the operating system and in scripting languages such as Perl, Awk, and sed. Although JavaScript provides the indexOf(), lastIndexOf(), charAt(), and substring() methods to help manipulate string contents, it doesn't provide powerful search and replace capabilities.

In this example, you extend the functionality of JavaScript's text manipulation capabilities with simple search and replace functions.

The search function should be able to search for words both in a case sensitive and case insensitive manner and should be able to search for whole words or substrings in words. Likewise, the replace function should be able to replace a word or substring, paying attention to case in the original text or ignoring it.

The search function should return true or false, and the replace function should return a new string with the result of the replace.

Listing 10.5 is the source code for these search and replace functions.

Listing 10.5. Searching and replacing in JavaScript.

<SCRIPT LANGUAGE="JavaScript">
<!-- HIDE FROM OTHER BROWSERS

// SET UP ARGUMENTS FOR FUncTION CALLS
//
var caseSensitive = true;
var notCaseSensitive = false;
var wholeWords = true;
var anySubstring = false;


// SEARch FOR A TERM IN A TARGET STRING
//
// search(targetString,searchTerm,caseSensitive,wordOrSubstring)
//
// where caseSenstive is a boolean value and wordOrSubstring is a boolean
// value and true means whole words, false means substrings
//
function search(target,term,caseSens,wordOnly) {

  var ind = 0;
  var next = 0;

  if (!caseSens) {
    term = term.toLowerCase();
    target = target.toLowerCase();
  }

  while ((ind = target.indexOf(term,next)) >= 0) {
    if (wordOnly) {
      var before = ind - 1;
      var after = ind + term.length;
      if (!(space(target.charAt(before)) && space(target.charAt(after)))) {
        next = ind + term.length;
        continue;
      }
    }
    return true;
  }

  return false;

}

// SEARch FOR A TERM IN A TARGET STRING AND REPLACE IT
//
// replace(targetString,oldTerm,newTerm,caseSensitive,wordOrSubstring)
//
// where caseSenstive is a boolean value and wordOrSubstring is a boolean
// value and true means whole words, false means substrings
//
function replace(target,oldTerm,newTerm,caseSens,wordOnly) {

  var work = target;
  var ind = 0;
  var next = 0;

  if (!caseSens) {
    oldTerm = oldTerm.toLowerCase();
    work = target.toLowerCase();
  }

  while ((ind = work.indexOf(oldTerm,next)) >= 0) {
    if (wordOnly) {
      var before = ind - 1;
      var after = ind + oldTerm.length;
      if (!(space(work.charAt(before)) && space(work.charAt(after)))) {
        next = ind + oldTerm.length;
        continue;
      }
    }
    target = target.substring(0,ind) + newTerm +
    target.substring(ind+oldTerm.length,target.length);
    work = work.substring(0,ind) + newTerm +
    work.substring(ind+oldTerm.length,work.length);
    next = ind + newTerm.length;
    if (next >= work.length) { break; }
  }

  return target;

}

// chECK IF A chARACTER IS A WORD BREAK AND RETURN A BOOLEAN VALUE
//
function space(check) {

  var space = " .,/<>?!`';:@#$%^&*()=-|[]{}" + '"' + "\\\n\t";

  for (var i = 0; i < space.length; i++)
    if (check == space.charAt(i)) { return true; }

  if (check == "") { return true; }
  if (check == null) { return true; }

  return false;

}

// STOP HIDING -->
</SCRIPT>

To demonstrate how these functions work, you can set up a simple search and replace application using the functions in Listing 10.6.

Listing 10.6. Using the search and replace functions.

<HTML>

<HEAD>
<TITLE>Listing 10.6</TITLE>

<SCRIPT LANGUAGE="JavaScript">
<!-- HIDE FROM OTHER BROWSERS

// SET UP ARGUMENTS FOR FUncTION CALLS
//
var caseSensitive = true;
var notCaseSensitive = false;
var wholeWords = true;
var anySubstring = false;


// SEARch FOR A TERM IN A TARGET STRING
//
// search(targetString,searchTerm,caseSensitive,wordOrSubstring)
//
// where caseSenstive is a boolean value and wordOrSubstring is a boolean
// value and true means whole words, false means substrings
//
function search(target,term,caseSens,wordOnly) {

  var ind = 0;
  var next = 0;

  if (!caseSens) {
    term = term.toLowerCase();
    target = target.toLowerCase();
  }

  while ((ind = target.indexOf(term,next)) >= 0) {
    if (wordOnly) {
      var before = ind - 1;
      var after = ind + term.length;
      if (!(space(target.charAt(before)) && space(target.charAt(after)))) {
        next = ind + term.length;
        continue;
      }
    }
    return true;
  }

  return false;

}

// SEARch FOR A TERM IN A TARGET STRING AND REPLACE IT
//
// replace(targetString,oldTerm,newTerm,caseSensitive,wordOrSubstring)
//
// where caseSenstive is a boolean value and wordOrSubstring is a boolean
// value and true means whole words, false means substrings
//
function replace(target,oldTerm,newTerm,caseSens,wordOnly) {

  var work = target;
  var ind = 0;
  var next = 0;

  if (!caseSens) {
    oldTerm = oldTerm.toLowerCase();
    work = target.toLowerCase();
  }

  while ((ind = work.indexOf(oldTerm,next)) >= 0) {
    if (wordOnly) {
      var before = ind - 1;
      var after = ind + oldTerm.length;
      if (!(space(work.charAt(before)) && space(work.charAt(after)))) {
        next = ind + oldTerm.length;
        continue;
      }
    }
    target = target.substring(0,ind) + newTerm +
    target.substring(ind+oldTerm.length,target.length);
    work = work.substring(0,ind) + newTerm +
    work.substring(ind+oldTerm.length,work.length);
    next = ind + newTerm.length;
    if (next >= work.length) { break; }
  }

  return target;

}

// chECK IF A chARACTER IS A WORD BREAK AND RETURN A BOOLEAN VALUE
//
function space(check) {

  var space = " .,/<>?!`';:@#$%^&*()=-|[]{}" + '"' + "\\\n\t";

  for (var i = 0; i < space.length; i++)
    if (check == space.charAt(i)) { return true; }

  if (check == "") { return true; }
  if (check == null) { return true; }

  return false;

}

// STOP HIDING -->
</SCRIPT>

</HEAD>

<BODY>

<TABLE WIDTH=100%>

<TR>

<TD VALIGN=TOP>
<DIV ALIGN=CENTER>
<H1>Search</H1>

<FORM METHOD=POST>

<SCRIPT LANGUAGE="JavaScript">
<!- HIDE FROM OTHER BROWSERS

function doSearch(form) {
  var result = search(form.initial.value,form.term.value,
form.casesens.checked,form.word.checked);
  alert ((result) ? "Found!" : "Not Found!");
}

// STOP HIDING -->
</SCRIPT>

<TEXTAREA NAME="initial" ROWS=2 COLS=30>Search Text</TEXTAREA><BR>
Search For: <INPUT TYPE="text" NAME="term"><BR>
<INPUT TYPE="checkbox" NAME="casesens"> Case Sensitive
<INPUT TYPE="checkbox" NAME="word"> Whole Word Search<BR>
<INPUT TYPE="button" VALUE="SEARch" onClick="doSearch(this.form);">
</FORM>
</DIV>

</TD>

<TD VALIGN=TOP>
<DIV ALIGN=CENTER>
<H1>Replace</H1>

<FORM METHOD=POST>

<SCRIPT LANGUAGE="JavaScript">
<!-- HIDE FROM OTHER BROWSERS

function doReplace(form) {

  form.result.value =
Âreplace(form.initial.value,form.oldterm.value,form.newterm.value,
Âform.casesens.checked,form.word.checked);

}

// STOP HIDING -->
</SCRIPT>

<TEXTAREA NAME="initial" ROWS=2 COLS=30>Search Text</TEXTAREA><BR>
<TEXTAREA NAME="result" ROWS=2 COLS=30>Result Text</TEXTAREA><BR>
Search For: <INPUT TYPE="text" NAME="oldterm"><BR>
Replace With: <INPUT TYPE="text" NAME="newterm"><BR>
<INPUT TYPE="checkbox" NAME="casesens"> Case Sensitive
<INPUT TYPE="checkbox" NAME="word"> Whole Word Search<BR>
<INPUT TYPE="button" VALUE="REPLACE" onClick="doReplace(this.form);">
</FORM>
</DIV>

</TD>

</TR>

</TABLE>

</BODY>

</HTML>

This script produces results like those in Figure 10.3.

Figure 10.3 : The search and replace functions can be used in any JavaScript application.

You use three functions to implement the search and replace system: search(), replace(), and space().

search() and replace() use a similar approach to handling their tasks, but differ in the specific actions they take when they find the term they are looking for.

The replace() Function

The replace() function takes five arguments: the string to work on, the term to search for, the term to replace it with, and two Boolean values. The two Boolean values indicate whether to pay attention to the case of letters in searching and whether to search only for whole words (if not, substrings will be matched and replaced).

  var work = target;
  var ind = 0;
  var next = 0;

As would be expected, you start by setting up the work variables.

  if (!caseSens) {
    oldTerm = oldTerm.toLowerCase();
    work = target.toLowerCase();
  }

Then you check whether you are paying attention to case. If not, you change the search term and the string to search to lowercase, using the toLowerCase() method. This means that case is ignored in the searches because any variation in case in either string has been removed.

  while ((ind = work.indexOf(oldTerm,next)) >= 0) {
    if (wordOnly) {
      var before = ind - 1;
      var after = ind + oldTerm.length;
      if (!(space(work.charAt(before)) && space(work.charAt(after)))) {
        next = ind + oldTerm.length;
        continue;
      }
    }

All the work is done in the preceding while loop. In the condition of the while loop, you search the target string for the next occurrence of the search term, store the index in ind, and see if it is greater than zero.

If you have found an occurrence of the term, you next check if you are searching for whole words or substrings. If you are searching for whole words, you use the space() function to check if the characters before and after the word boundary are word breaks. If they aren't, you update next to the index of the character after the term you just found and start the loop again.

    target = target.substring(0,ind) + newTerm +
    target.substring(ind+oldTerm.length,target.length);
    work = work.substring(0,ind) + newTerm +
    work.substring(ind+oldTerm.length,work.length);
    next = ind + newTerm.length;
    if (next >= work.length) { break; }
  }

If you reach this point in the loop, then you have found a term you want to replace. You use the substring method to update both the string itself and change the variable next to the index of the character after the term you have just added.

Finally, you check if you have reached the end of the target string, and if not, you run the loop again to look for another occurrence.

The search() Function

This function is similar in structure to the replace() function. The differences lie in the while loop:

  while ((ind = target.indexOf(term,next)) >= 0) {
    if (wordOnly) {
      var before = ind - 1;
      var after = ind + term.length;
      if (!(space(target.charAt(before)) && space(target.charAt(after)))) {
        next = ind + term.length;
        continue;
      }
    }
    return true;
  }

return false;

As before, you perform the search for the search term using the indexOf() method in the condition of the while loop. Again, you check if you are searching for whole words, and if you are, you check for word boundaries. If you haven't found a complete word, you prepare to search again and return to the top of the loop with the continue statement.

If you get beyond the if statements, you have found an occurrence of the term and return a true value from the function. If you finish the while loop without returning true, then you haven't found a match and you return a false value.

The space() Function

The space() function plays a support role for search() and replace().

Given a character as an argument, the function checks whether it is one of a series of characters considered word breaks or delimiters. If it is, the function returns true-otherwise, it returns a false value.

The way this is done is simple. All the possible word breaks are stored in the string space. A for loop goes through the string, character by character and compares each character to the argument. If there is a match, a true result is returned.

After the loop, a comparison is made between the argument and either the empty string or the null value. If these are true, the function returns a true value as well.

If you have failed all these conditions, then a false value is returned because the argument character is not a word break.

If the programmer wants to change the definition of a word break, he or she simply has to change the declaration of the variable space.

The Math Object

Where the string object enables you to work with text literals, the Math object provides methods and properties to move beyond the simple arithmetic manipulations offered by the arithmetic operators.

Among the features offered by the Math object are several special values such as PI, natural logarithms, and common square roots, trigonometric methods, rounding methods, an absolute value method, and more.

Table 10.2 outlines all the properties and methods of the Math object.

Some of these functions require further discussion.

The Trigonometric Methods

You will notice that the trigonometric methods, such as acos() and sin() use radians to measure the size of angles instead of the more familiar degrees.

This isn't too difficult to handle. Where you have 360 degrees in a circle, there are 2¥PI (or roughly 6.283) radians in a circle.

So, where the arc tangent of 1 is 45 degrees, in radians, the result is roughly 0.785398.

The log() and exp() Methods

The log() and exp() functions are related in that they use e, Euler's constant, as their base.

The relationship between logarithms and exponential expressions is that if log(a) = b, then exp(b) = a.

The abs() Method

The absolute value method returns the positive value of a number. That is, it removes a negative sign from a number so that abs(4) and abs(-4) both have a value of 4.

Calculating Geometric Measurements

To highlight some of these math functions, you are going to build a simple calculator that calculates the angles and the lengths of the sides of a right angle triangle and calculates the area, diameter, and circumference of a circle.

In order to do this, you will use the trigonometric functions and PI.

As a reminder, with a right angle triangle, if you want to calculate the sine, cosine, or tangent of any of the other two angles, you can use the following formulas:

sine = opposite side / hypotenuse
cosine = adjacent side / hypotenuse
tangent = opposite side / adjacent side

The script should be able to fill in all the information about the shapes when there is sufficient information in the relevant form. The results of Listing 10.7 look like Figure 10.4.

Figure 10.4 : Using the Math object to perform more complex mathematical calculations.

Listing 10.7. Using the trigonometric functions.

<HTML>

<HEAD>
<TITLE>Example 10.7</TITLE>

<SCRIPT LANGUAGE="JavaScript">
<!-- HIDE FROM OTHER BROWSERS

function circle(form,changed) {

  with (Math) {
    var area = form.area.value;
    var diameter = form.diameter.value;
    var circumference = form.circumference.value;

    if (changed == "area") {
      var radius = sqrt(area / PI);
      diameter = 2 * radius;
      circumference = PI * diameter;
    }

    if (changed == "diameter") {
      area = PI * (diameter / 2) * (diameter / 2);
      circumference = PI * diameter;
    }

    if (changed == "circumference") {
      diameter = circumference / PI;
      area = PI * (diameter / 2) * (diameter / 2);
    }

    form.area.value = area;
    form.diameter.value = diameter;
    form.circumference.value = circumference;

  }

}

var toDegrees = 360 / (Math.PI * 2);
var toRadians = (Math.PI * 2) / 360;

function angle(form,changed) {

  with (Math) {

    var angle = (changed == "angleA") ? form.angleA.value *
    toRadians : form.angleB.value;
    var otherAngle = (90 * toRadians) - angle;
    var hypotenuse = form.hypotenuse.value;
    var sine = sin(angle);
    var opposite = sine * hypotenuse;
    var cosine = cos(angle);
    var adjacent = cosine * hypotenuse;

    if (changed == "angleA") {
      form.angleB.value = otherAngle * toDegrees;
      form.sideA.value = adjacent;
      form.sideB.value = opposite;
    } else {
      form.angleA.value = otherAngle * toDegrees;
      form.sideB.value = adjacent;
      form.sideC.value = opposite;
    }

  }

}

function side(form,changed) {

  with (Math) {

    var side = (changed == "sideA") ? form.sideA.value : form.sideB.value;
    var hypotenuse = form.hypotenuse.value;
    var otherSide = sqrt(pow(hypotenuse,2) - pow(side,2));
    var angle = acos(side/hypotenuse);
    var otherAngle = acos(otherSide/hypotenuse);

    if (changed == "sideA") {
      form.sideB.value = otherSide;
      form.angleA.value = angle * toDegrees;
      form.angleB.value = otherAngle * toDegrees;
    } else {
      form.sideA.value = otherSide;
      form.angleB.value = angle * toDegrees;
      form.angleA.value = otherAngle * toDegrees;
    }

  }

}

function hyp(form) {

  angle(form,"angleA");

}

// STOP HIDING FROM OTHER BROWSERS -->
</SCRIPT>

</HEAD>

<BODY>

<TABLE WIDTH="100%">

<TR>

<TD>
<H1>Circle</H1>
<FORM METHOD=POST>
Area: <INPUT TYPE="text" NAME="area" VALUE=0
onChange="circle(this.form,this.name);"><BR>
Diameter: <INPUT TYPE="text" NAME="diameter" VALUE=0
onChange="circle(this.form,this.name);"><BR>
Circumference: <INPUT TYPE="text" NAME="circumference" VALUE=0
onChange="circle(this.form,this.name);">
</FORM>
</TD>

<TD>
<H1>Triangle</H1>
<FORM METHOD=POST>
Angle A: <INPUT TYPE="text" NAME="angleA" VALUE=45
onChange="angle(this.form,this.name);"><BR>
Angle B: <INPUT TYPE="text" NAME="angleB" VALUE=45
onChange="angle(this.form,this.name);"><BR>
Side A: <INPUT TYPE="text" NAME="sideA" VALUE=1
onChange="side(this.form,this.name);"><BR>
Side B: <INPUT TYPE="text" NAME="sideB" VALUE=1
onChange="side(this.form,this.name);"><BR>
Hypotenuse: <INPUT TYPE="text" NAME="hypotenuse" VALUE=1.414
onChange="hyp(this.form);">
</FORM>
</TD>

</TR>

</TABLE>

</BODY>

</HTML>

Figure 10.4 shows the result.

Figure 10.4 : Using the Math object to perform more complex mathematical calculations.

This script is rather simple, but it shows how to use the methods and properties available in the Math object.

Of the two forms, the circle form is the simpler because it has less information to deal with. The following two sections analyze the two functions.

The circle() Function

The circle() function takes two arguments: the form object and the name of the field that was just changed. The calculations are based on the name of this field.

  with (Math) {
    var area = form.area.value;
    var diameter = form.diameter.value;
    var circumference = form.circumference.value;

You start by extracting whatever information is in the form and storing it in local variables. Notice the use of the with (Math) command. This enables all of the Math properties and methods in the function to be used without the Math prefix.

    if (changed == "area") {
      var radius = sqrt(area / PI);
      diameter = 2 * radius;
      circumference = PI * diameter;
    }

    if (changed == "diameter") {
      area = PI * (diameter / 2) * (diameter / 2);
      circumference = PI * diameter;
    }

    if (changed == "circumference") {
      diameter = circumference / PI;
      area = PI * (diameter / 2) * (diameter / 2);
    }

The three if statement blocks simply calculate the other two fields based on the value of the changed field. All of these use two basic formulas:

Area of Circle = PI¥radius¥radius

Circumference of Circle = PI¥diameter

Notice the use of the sqrt() method without the preceding Math prefix, which is made possible with the earlier with (Math) command.

    form.area.value = area;
    form.diameter.value = diameter;
    form.circumference.value = circumference;

Once the calculation is done, you can reassign the results to the form.

Working with the Triangle

The triangle function assumes that you are working with a right triangle. This means the angle across from the hypotenuse is always 90 degrees.

The relationship between the remaining angles and sides is that sideA is adjacent to angleA and sideB is adjacent to angleB.

Before you can proceed to calculate the information in this form, you need to be able to convert between degrees and radians. All the trigonometric functions either take a radian value as a parameter or return a radian value. Users, on the other hand, are likely to prefer working in degrees.

You get around this by using the variables toDegrees and toRadians which represent the number of degrees per radian and the number of radians per degree:

var toDegrees = 360 / (Math.PI * 2);
var toRadians = (Math.PI * 2) / 360;

The angle() function is called whenever you change one of the two angle values. It uses the fact that all the angles of a triangle add up to 180 degrees to calculate the remaining angle. Then, using the sin() and cos() methods and the formulas for sine and cosine, the program calculates the length of the opposite and adjacent sides for the changed angle.

Finally, based on which angle was changed, the results are assigned to the correct form field.

The side() function plays a similar role when either sideA or sideB is changed. Using the value of the changed side and the value of the hypotenuse, you can calculate the value of the third side using the formula:

sideA¥sideA + sideB¥sideB = hypotenuse¥hypotenuse

After you have the value for the three sides, you can use the acos() method and the formula for cosine to calculate the value of the two angles.

The hypotenuse function simply calculates what the value of sideA and sideB should be based on the current angle settings by calling angle(form,"angleA"). You could just as easily have made the call angle(form,"angleB").

The forms in the body of the HTML file are fairly simple. They call the appropriate function in the onChange event handler for each text field.

Working with the History List

When you use the Navigator browser, you will notice the history list, which is accessible under the Go menu.

The history object makes this list accessible in JavaScript. Early versions of JavaScript made the actual URLs in the list available to the script, but this was too large a security hole because it could be used by malicious scripts to steal information to access some secure Web sites. In addition, it could be used to breach privacy by supplying a page author with information about what sites a visitor had previously visited.

The history object provides methods for working with the list without actually reflecting the value of URLs and entries into a script.

Properties and Methods of the history Object

Table 10.3 outlines the properties and methods available in the history object.

For instance, history.back() goes to the previous page while history.go(-3) goes back to the page visited three pages ago (like clicking the Back button three times on the Navigator toolbar) and history.go(2) goes two URLs forward in the list.

The history.go() method can also take a string instead of an integer as an argument. When a string is used, the method loads the nearest entry in the history that contains the string as part of its URL. The matching of the string against the URL is case insensitive.

One of the more popular uses of the history object is to provide back and forward buttons in individual frames or dynamic back buttons, which take users back to the last page they were on.

Summary

Manipulating data has been the focus of much of this chapter.

Using the string object, you now know how to add HTML tags using methods, how to change the case of a string, and how to search for the string and perform basic search and replace functions.

The Math object enables you to extend the type of mathematical calculations you can perform to include trigonometry, logarithms, and square roots and also provides several values as properties, including PI, E, and LN2.

The history object is a little different. By providing the ability to jump to URLs in the history list (without breaching security by providing the actual URL information), it is possible to build dynamic back and forward buttons into documents.

In the next chapter you will put everything you have learned together into producing a fun cartoon face drawing program.

Commands and Extensions Review

Exercises

1. What would the output of the following code segment look like assuming there were no HTML tags elsewhere in the file affecting the output?
   
   var sample = "test.";
   sample.big();
   sample.blink();
   sample.bold();
   sample.strike();
   sample.fontsize(7);
   document.write(sample.italics());
2. In the text searching and replacing functions (refer to Listings 10.5 and 10.6), we have left out a critical feature: a wildcard. Extend the search and replace script to add a simple wildcard capability to the search() function. Use the following criteria:
   • Use the asterisk character for your wildcard.
   • The wildcard represents zero or more of any letter.
   • Only one wildcard is enabled in a search term.
   • Wildcards are valid only in the middle of a search term: "text*" and "*text" are not valid-catch this and inform the user.
     If you search the string "Hello there" for "lo*e" you should get a match as you would with "H*lo", but "the*h" would not succeed.
3. What are the lines of code necessary to implement a dynamic forward and back button in an HTML page. The buttons should work just like the ones in the Navigator toolbar. (Hint: you need only one form with two buttons to do this.)

Answers

1. The phrase "test." would print in italics. All the other method() calls are useless. Remember that these methods return the new value. They do not directly alter the string. So, the results of all the other method calls went unassigned and unused. If we change the code to read
   
   var sample = "test.";
   sample = sample.big();
   sample = sample.blink();
   sample = sample.bold();
   sample = sample.strike();
   sample = sample.fontsize(7);
   document.write(sample.italics());
   then all the attributes will be applied to the displayed text.
2. The new search function would look like this:
   
   function search(target,term,caseSens,wordOnly) {
   
     var ind = 0;
     var ind2 = 0;
     var next = 0;
     var wildcard = -1;
     var firstTerm = "";
     var secondTerm = "";
   
     if (!caseSens) {
       term = term.toLowerCase();
       target = target.toLowerCase();
     }
   
     if ((wildcard = term.indexOf("*",0)) >= 0) {
   
       if (!checkWildCards(term)) {
         alert("Improper use of the wildcard character.");
         return false;
       }
   
       firstTerm = term.substring(0,wildcard);
       secondTerm = term.substring(wildcard+1,term.length);
   
       while ((ind = target.indexOf(firstTerm,next)) >= 0) {
         var afterFirst = ind + firstTerm.length;
   
         ind2 = target.indexOf(secondTerm,afterFirst);
         if (ind2 < 0) { break; }
   
         if (wordOnly) {
           for (var i = ind+firstTerm.length; i <= ind2 - 1; i++)
             if (space(target.charAt(i))) {
               next = i + 1;
               continue;
             }
   
           var before = ind - 1;
           var after = ind2 + secondTerm.length;
           if (!(space(target.charAt(before)) && space(target.charAt(after))))
                     Â{
   next = ind2 + secondTerm.length;
             if (next >= target.length) { break; }
             continue;
           }
         }
         return true;
       }
   
       return false;
   
     }
   
     while ((ind = target.indexOf(term,next)) >= 0) {
       if (wordOnly) {
         var before = ind - 1;
         var after = ind + term.length;
         if (!(space(target.charAt(before)) && space(target.charAt(after))))
                Â {
   next = ind + term.length;
           continue;
         }
       }
       return true;
     }
   
     return false;
   }
   You would also need to add the checkWildCards() function:
   function checkWildCards(term) {
   
     if (term.charAt(0) == "*") { return false; }
     if (term.charAt(term.length-1) == "*") { return false; }
   
     var first = term.indexOf("*",0);
     if (term.indexOf("*",first+1) >= 0) { return false; }
   
     return true;
   
   }
   You have not changed the basic functionality of the search() function. Rather, you have added a component in the middle which handles the wildcard searches. If there is no wildcard in the search term, the search functions operate in the same way as they did before, simply bypassing the wildcard section.
   Assuming you have found a wildcard character, you perform an altered search, which is based on the simple search performed when there is no wildcard.
   You start by checking that the use of the wildcard character is valid by calling checkWildCards(). If everything is correct, then you split the search term into two terms: the portion before the wildcard and the portion after it.
   You then search for the first term in the condition of the while loop. If the first term occurs, you look for the second term. If the second term isn't there, you have no match and break out of the loop. Otherwise, you check if you are doing a whole word search. If you are, the function performs a check on the character before the occurrence of the first term and after the occurrence of the second term for word delimiter characters using the space() function. You also check that there are no word delimiters between the two terms. If the search fails either of these tests, you jump back to the top of the loop to continue searching the target string.
   Otherwise, if you get past the wordOnly if statement, you know you have found a match and return a true value from the function.
3. The code
   <FORM METHOD=POST>
   <INPUT TYPE=button VALUE="BACK" onClick="history.back();">
   <INPUT TYPE=button VALUE="FORWARD" onClick="history.forward();">
   </FORM>
   will implement dynamic forward and back buttons.