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When the next version of Firefox is released later this year, it will be sporting
several updates to its JavaScript engine. Since JavaScript has mostly remained
stagnant since the ECMA-262, 3rd edition was released several years ago, the
introduction of new features is very exciting. Part of this JavaScript upgrade
directly affects the Array object, which gains several new methods.
The Array object has had a rather sordid and controversial past.
In Netscape's original JavaScript implementation, there was no Array
object, and developers were forced to create their own. When the first edition
of ECMA-262 was released, it formally described an Array object
with some basic functionality. Both Netscape and Microsoft implemented the specification,
but shortly thereafter, the third edition of ECMA-262 was finalized and introduced
even more Array object methods. While Netscape rushed to implement
these new methods, Microsoft lagged behind. It wasn't until Internet Explorer
reached version 5.5 that the rest of the methods were added. Since that time,
there have been no changes to the Array object in any browser.
There are seven new Array methods that can be separated into two
categories, item location methods and iterative methods. The item location methods
are:
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The item location methods, indexOf() and lastIndexOf(),
should be familiar to Java and .NET programmers. These methods return the index
of an item's first and last occurrence an array, respectively. For example:
var aColors = ["red", "blue", "green", "orange", "purple"];
alert(aColors.indexOf("green")); //outputs "2"
alert(aColors.lastIndexOf("green")); //outputs "2"
In this example, the indexOf() method looks for the string "green"
in the array aColors. Since it is the third item in the array,
the method returns 2. The lastIndexOf() method also looks for "green"
and returns the same value because there's only one instance of "green"
in the array. If there were two or more, the results would be different:
var aColors = ["red", "blue", "green",
"orange", "purple", "green", "brown"];
alert(aColors.indexOf("green")); //outputs "2"
alert(aColors.lastIndexOf("green")); //outputs "5"
Here, the lastIndexOf() method returns 5 while indexOf()
returns 2. This is because the lastIndexOf() method starts searching
from the end of the array while indexOf() starts from the beginning.
If "green" didn't exist in the array, both methods would return -1.
Using indexOf() or lastIndexOf() in this way, you
can determine if the given item is in an array, such as:
if (aColors.indexOf("brown") > -1) {
//brown is there, act accordingly
} else {
//brown isn't there
}
There is also an optional second argument to the indexOf() and
lastIndexOf() methods, which is the index at which to begin searching
the array. This is useful since an array can have more than one instance of
a given item. For example:
var aColors = ["red", "blue", "green",
"orange", "purple", "green", "brown"];
alert(aColors.indexOf("green")); //outputs "2"
alert(aColors.indexOf("green", 3)); //outputs "5"
The first indexOf() call returns 2 as usual, while the second
returns 5 because it is instructed to begin searching at position 3. From that
position, the method continues to search forward and the next instance of "green"
is in position 5. The same can be done with lastIndexOf():
var aColors = ["red", "blue", "green",
"orange", "purple", "green", "brown"];
alert(aColors.lastIndexOf("green")); //outputs "5"
alert(aColors.indexOf("green", 4)); //outputs "2"
In this case, lastIndexOf() is called with a second argument,
specifying to start searching from position 4 and work back towards the beginning
of the array. Then, the next occurrence of "green" is at position
2.
Using these methods, you can easily get all occurrences of a given item in an array by doing this:
var aNumbers = [2, 5, 9, 2, 1, 9, 8, 9, 4, 9, 2, 9];
var aIndices = [];
var iPos = aNumbers.indexOf(9);
while(iPos != -1) {
aIndices.push(iPos);
iPos = aNumbers.indexOf(9, iPos+1);
}
alert("The number 9 was found in positions " + aIndices);
This example finds the first occurrence of the number 9 in an array, then continues
to look for more by using the second argument of the indexOf() method.
Each index of the number 9 is added to the aIndices array so in
the end, you have an array of positions.
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Created: March 27, 2003
Revised: August 12, 2005
URL: http://webreference.com/programming/javascript/ncz/column4/1
