Simple Validation

Check the length of an input string

Uses the "onchange" event handler: JavaScript is activated after the field is deselected, that is, when you move the focus from the input field to somewhere else.

Here's the code:

<form action="javascript:;" name="form1" method="post">

Enter some text here:<br />

<input type="text" length="30" name="textInput" maxlength="25" 
onchange="window.alert('This text string is ' + textInput.value.length + ' characters long.');
textInput.value=''; /* resets the input field value */
return true;" />    /* signals the browser that the execution of the block has ended */

</form>

Note:

• The event handlers, like onchange, can execute more than one statement. If so, each statement must be separated by a semicolon.
• Statements passed on to the event handlers must be enclosed in quotation marks. JavaScript recognizes both single and double quotation marks. If you have to nest quotations (see the onchange event handler above), you can use single quatation marks within double quotes, or vice versa. You cannot nest double quotes within double quotes, etc.
• The period between the words in window.alert() and textInput.value.length is an access operator, meaning "something of...".
  window.alert() calls the alert method of the window object.
• textInput.value.length asks for the length of the value of the input field named textInput. Note that the form field is addressed by its name, "textInput".