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1.2 Accessing Substrings
NN 2, IE 3
1.2.1 Problem
You want to obtain a copy of a
portion of a string.
1.2.2 Solution
Use the substring( ) method (in all scriptable browsers) to
copy a segment starting at a particular location and ending either at
the end of the string (omitting the second parameter does that) or at
a fixed position within the string, counting from the start of the
string:
var myString = "Every good boy does fine.";
var section = myString.substring(0, 10); // section is now "Every good"
Use the slice( ) method (in NN 4 or later and IE 4 or
later) to set the end position at a point measured from the end of
the string, using a negative value as the second parameter:
var myString = "Every good boy does fine.";
var section = myString.slice(11, -6); // section is now "boy does"
Use the nonstandard, but widely supported, variant called
substr( ) to copy a segment starting at a
particular location for a string length (the second parameter is an
integer representing the length of the substring):
var myString = "Every good boy does fine.";
var section = myString.substr(6, 4); // section is now "good"
If the sum of the two arguments exceeds the length of the string, the
method returns a string from the start point to the end of the
string.
1.2.3 Discussion
Parameters for the ECMA-compatible slice( ) and
substring( ) methods are numbers that indicate the
zero-based start and end positions within the string from which the
extract comes. The first parameter, indicating the start position, is
required. When you use two positive integer values for the
slice( ) method arguments (and the first argument
is smaller than the second), you receive the same string value as the
substring( ) method with the same arguments.
Note that the integer values for substring( ) and
splice( ) act as though they point to spaces
between characters. Therefore, when a substring( )
method's arguments are set to 0
and 4, it means that the substring starts to the
right of the "zeroeth" position and
ends to the left of the fourth position; the length of the string
value returned is four characters, as shown in Figure 1-1.
Figure 1-1. How substrings end points are calculated
If you should supply argument values for the substring(
) or substr( ) methods in an order that
causes the first argument to be larger than the second, the
JavaScript interpreter automatically reverses the order of arguments
so that the end pointer value is always larger than the start
pointer. The slice( ) method
isn't as forgiving and returns an empty string.
None of the substring methods modifies the original string object or
value in any way. This is why you must capture the returned value in
a variable, or apply the returned value as an argument to some other
function or method.
1.2.4 See Also
Recipe 1.5 for testing whether a string contains a substring.
