No web developers want their
Ajax applications to hit the network with requests if the users leave necessary
text fields blank. Thus, checking that input elements of type text and the
large boxes called textareas in HTML
contain values is one of the most common forms of validation.
This hack shows the code for checking if a text control is
blank. The inline way of doing this is by assigning a
check for the field's value in the text field's event handler:
<input type="text" name="firstname" id="tfield" onblur=
"if (this.value) {doSomething( );}" />
or in the textarea's event handler:
<textarea name="tarea" rows="20" id="question" cols="20" onblur=
"if (this.value) {doSomething( );}">
The JavaScript phrase if (this.value) {...} returns
false if the user leaves a field blank, so the function call
doSomething( ) will never occur. JavaScript evaluates a blank web-form
text field as the empty string or "", which evaluates to
false when it's used in the context of a programming test. The
this keyword is a nice generic way of referring to the form field that
contains the event handler attribute.
Probably a better way of going about your
event-handling tasks is to separate the logic of your code from the HTML or
template text that comprises the application's visual aspects. The JavaScript
goes into an external file that the HTML page imports with a script
tag. Inside the external file, the code binds a field's various event handlers
to a function or the code that represents your application's behavior.
Let's take the following web page, myapp.html, which
includes the following HTML in its header:
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
<script type="text/javascript" src="js/hacks_method.js"></script>
<title>Cool Ajax application</title>
</head>
The file hacks_method.js is located in a directory
js, which is in the same directory as the HTML file. The HTML file
contains the same textarea and text field as mentioned earlier, except
these fields no longer have an onblur attribute. The JavaScript file
includes this code:
window.onload=function( ){
var txtA = document.getElementById("tarea");
if(txtA != null){
txtA.onblur=function( ){
if (this.value) { doSomething( );}
};
}
var tfd = document.getElementById("tfield");
/* An alternative:
if(tfd != null && txtA != null){tfd.onblur = txtA.onblur; }
*/
if(tfd != null){
tfd.onblur=function( ){
if (this.value) { doSomething( );}
};
}
}
window.onload involves the binding of the load event to
your blank-field checks. load occurs when the browser has completed
loading the web page, so when that happens, all the stuff after
window.onload= follows.
