| Main Site Topics Ken Ward's JavaScript Tutorial 1 |
In the previous tutorial we made a new method for the Array
object in javascript because it hasn't got one. We used the "new" operator. Here
we will create new methods for the Array object using the keyword function.
In this case we have invented two new methods for the array: indexof, which finds the
first occurrence of a string in an array, and lastindexof, which finds the last occurrence
of a string in an array. Where the string occurs only once in the array, both give the
same answer. Both methods are case sensitive, and find whole words only.
The following example demonstrates finding the first and the last occurrence of a word
in an array. This time, you can make your own array, if you like, separating the items by
commas (don't put any spaces in though).
The Example
Keeping the values the same as the demonstration above, clicking on the "First
Occurrence" button gives an alert, "Present at 1", meaning the word
"Dick" is the second item.
The code is:
// the
function takes two parameters: the value of the text box containing the string with words
separated by commas, and the one containing the single name.
function findFirst(Indx,name)
{
var myText=Indx.value;
var myWord=name.value;
//the word and the text are the arguments of
this function
//we make a regular array using
split(",") which makes its items from those separated by commas
myArray=myText.split( ",");
//if the array contains a position
greater than -1 (the items start at 0), then we have found a match
if
(myArray.indexof(myWord)>-1)
{
alert( 'Present at:'+myArray.indexof(myWord))
}
else
alert( 'Absent');
}
So we use indexof to find the position of an item in an array. This finds the first
occurrence.
The next code shows the function to find the last occurrence of an item in an array
(should their be more than one, otherwise it finds the only occurrence and gives the same
answer as the previous).
function findLast(Indx,name)
{
var myText=Indx.value;
var myWord=name.value;
myArray=myText.split( ",");
if
(myArray.lastindexof(myWord)>-1)
{
alert('Present at:'+myArray.lastindexof(myWord))
}
else
alert( 'Absent');
}
As before, we use a custom made method, lastindexof, to find the last occurrence of a
string in an array.
Creating the Methods
As in the previous lesson, we create the new methods, but this time use the function
object to create them rather than the new constructor.
Array.prototype.indexof = function(myItem) {
//we make a variable, result, equal to minus
one, so if nothing changes it, we will get -1 from this function
var result=-1;
//check whether the array has any items
or the function has the wrong number of arguments
if ((this.length<1)||(arguments.length!=1))
{
return -1;
exit;
}
//having checked whether we have any
items in the array and whether we have the right number of arguments, we can procede.
for (i=0;i<this.length;i++) {
//check each item beginning at the beginning
if (this[i]==myItem) {
// We could do various things here, such as
using the indexOf and lastIndexOf (methods for a string) to find out if the string was
contained in the item, and convert to the same case to be case insentive... here we used
equality
result=i;
break ;
} //end of if statement
} //end of for statement
return result;
//Attach custom method to array object
if (!Array.prototype.indexof)
{
Array.prototype.indexof=indexof;
}
}
The final bit of code of relevance here is creating the lastindexof method for the
array, which differs from the indexof method only in the for statement:
Array.prototype.lastindexof = function(myItem) {
result=-1;
if ((this.length<1)||(arguments.length!=1))
{
return -1;
exit;
}
for (i=this.length-1;i>-1;i--) {
//in the above, we start at the end and check
the items towards the beginning. The rest of the code is the same as before
if (this[i]==myItem) {
result=i;
break ;
} //end of if statement
} //end of for statement
return result;
//Attach custom method to array object
if
(!Array.prototype.lastindexof)
{
Array.prototype.lastindexof=lastindexof;
}
}
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