Working with AJAX and the Prototype JavaScript Library - Using the Try.these() function and the each loop
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Among the wealth of functions that come packaged with Prototype, there's one that can be particularly useful in those cases where you need to execute different JavaScript routines, depending on the browser in which they're running.
In this case, I'm referencing to the "Try.these()" function, which will test different code blocks specified as parameters, and only execute the one that works. Also, these code blocks, which are wrapped up in functions, are evaluated in the order in which they're specified, as the below example clearly shows. Take a look at it, please:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-
8859-1" />
<title>Example Try.these() function</title>
<script language="javascript" src="prototype-1.4.0.js"></script>
<script language="javascript">
// example using Try.these() function - returns a cross-browser
request object
function getHttpRequestObject(){
return Try.these(
function(){return new XMLHttpRequest();},
function(){return new ActiveXObject("Microsoft.XMLHTTP");}
)
}
window.onload=function(){
if(document.getElementById && document.createElement &&
document.getElementsByTagName){
var xmlobj=getHttpRequestObject();
alert(xmlobj);
}
}
</script>
</head>
<body>
<h1>Example using Try.these() function</h1>
</body>
</html>
As you can see, the above example demonstrates in a friendly fashion how the "Try.these()" function can be used to create a cross-browser HTTP requester object. Here, and depending on the browser where the previous script will be executed, one of the two functions specified as arguments will work successfully, in this way returning the type of object fully supported by the client. Pretty simple, isn't it?
Of course, I'm sure that you'll find more useful examples where the "Try.these()" function can be utilized, therefore let me leap forward and show you another handy feature included with the Prototype library for working easily with iterators.
The "each()" function allows you to quickly traverse any enumerable object, like array structures, among others. Logically, the implementation of this function can be grasped more easily if you take a look at the following example. It uses a conventional "for" loop to traverse an array of DIVs.
That said, the corresponding code sample is the following:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-
8859-1" />
<title>Example of conventional loop</title>
<script language="javascript" src="prototype-1.4.0.js"></script>
<script language="javascript">
// example using conventional loop
function displayHTML(){
var divs=document.getElementsByTagName('div');
for(var i=0;i<divs.length;i++){
var htmlcont=document.getElementById('htmlcontainer');
htmlcont.innerHTML+=divs[i].innerHTML;
}
}
window.onload=function(){
if(document.getElementById && document.createElement &&
document.getElementsByTagName){
var btn=$('testbutton');
if(!btn){return};
btn.onclick=displayHTML;
}
}
</script>
</head>
<body>
<div>
<p>This is a sample paragraph, which is wrapped by the first
DIV</p>
</div>
<div>
<p>This is a sample paragraph, which is wrapped by the
second DIV</p>
</div>
<div>
<p>This is a sample paragraph, which is wrapped by the third
DIV</p>
</div>
<div id="htmlcontainer"></div>
<input type="button" value="Show function result"
id="testbutton" />
</body>
</html>
All right, now that you hopefully recalled how to use a regular "for" loop, pay attention to the example below, which demonstrates the use of the "each()" function that I mentioned previously:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-
8859-1" />
<title>Example of each loop</title>
<script language="javascript" src="prototype-1.4.0.js"></script>
<script language="javascript">
// example using each loop
function displayHTML(){
var divs=$A(document.getElementsByTagName('div'));
divs.each(function(div){alert(div.innerHTML);});
}
window.onload=function(){
if(document.getElementById && document.createElement &&
document.getElementsByTagName){
var btn=$('testbutton');
if(!btn){return};
btn.onclick=displayHTML;
}
}
</script>
</head>
<body>
<div>
<p>This is a sample paragraph, which is wrapped by the first
DIV</p>
</div>
<div>
<p>This is a sample paragraph, which is wrapped by the
second DIV</p>
</div>
<div>
<p>This is a sample paragraph, which is wrapped by the third
DIV</p>
</div>
<input type="button" value="Show function result"
id="testbutton" />
</body>
</html>
Wasn't that great? As you can see, the above example first fetches an array of DIVs, and then uses the "each()" function to iterate over them. Of course, at first glance, the syntax that corresponds to this function may look a bit strange, but those developers that work frequently with Ruby will find it very familiar.
True to form, the previous example is rather primitive, but you should notice one important thing concerning the use of the "each()" function. As you saw, I specified a function to be called inside the loop, a fact demonstrated by the following line of code:
divs.each(function(div){alert(div.innerHTML);});
This function is called an "iterator function," and it can be used with any enumerable object to perform a specific task on the object in question. Nevertheless, this certainly will be one of the topics that will be discussed in the last tutorial of the series. For the moment, have some fun using the recently-discovered "each()" loop. Happy coding!
Final thoughts
In this second installment of the series, I took a quick look at the AJAX module that comes integrated with Prototype, undoubtedly one of its strongest points. Besides, I offered a simple introduction to the "Try.these()" and "each()" functions, something that should give you a clearer idea of the capacity of this library for working with iterators.
Nonetheless, iterators and other useful features will be properly covered in the last part of the series, so I hope to see you there!
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