Sams Teach Yourself JavaScript in 24 Hours

Working with Substrings

So far, you've worked with entire strings. JavaScript also enables you to work with substrings, or portions of a string. You can use the substring method to retrieve a portion of a string, or the charAt method to get a single character. These are explained in the following sections.

Using Part of a String

The substring method returns a string consisting of a portion of the original string between two index values, which you must specify in parentheses. For example, the following statement displays the fourth through sixth characters of the text string:

document.write(text.substring(3,6));

At this point, you're probably wondering where the 3 and the 6 come from. There are three things you need to understand about the index parameters:

• Indexing starts with 0 for the first character of the string, so the fourth character is actually index 3.

• The second index is noninclusive. A second index of 6 includes up to index 5 (the sixth character).

• You can specify the two indexes in either order. The smaller one will be assumed to be the first index. In the previous example, (6,3) would have produced the same result. Of course, there is rarely a reason to use the reverse order.

As another example, suppose you defined a string called alpha to hold the alphabet:

alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

The following are examples of the substring() method using this string:

• alpha.substring(0,4) returns ABCD.

• alpha.substring(10,12) returns KL.

• alpha.substring(12,10) also returns KL. Because it's smaller, 10 is used as the first index.

• alpha.substring(6,7) returns G.

• alpha.substring(24,26) returns YZ.

• alpha.substring(0,26) returns the entire alphabet.

• alpha.substring(6,6) returns the null value, an empty string. This is true whenever the two index values are the same.

Getting a Single Character

The charAt method is a simple way to grab a single character from a string. You specify the character's index, or position, in parentheses. The indexes begin at 0 for the first character. Here are a few examples using the alpha string:

• alpha.charAt(0) returns A.

• alpha.charAt(12) returns M.

• alpha.charAt(25) returns Z.

• alpha.charAt(27) returns an empty string because there is no character at that position.

Finding a Substring

Another use for substrings is to find a string within another string. One way to do this is with the indexOf method. To use this method, add indexOf to the string you want to search, and specify the string to search for in the parentheses. This example searches for "this" in the test string:

loc = test.indexOf("this");

The value returned in the loc variable is an index into the string, similar to the first index in the substring method. The first character of the string is index 0.

You can specify an optional second parameter to indicate the index value to begin the search. For example, this statement searches for the word fish in the temp string, starting with the 20th character:

location = temp.indexOf("fish",19);

A second method, lastIndexOf(), works the same way, but finds the last occurrence of the string. It searches the string backwards, starting with the last character. For example, this statement finds the last occurrence of Fred in the names string:

location = names.lastIndexOf("Fred");

As with indexOf(), you can specify a location to search from as the second parameter. In this case, the string will be searched backward starting at that location.