Capture an event (such as when a user changes an edit field or presses a button).
Total: <input type="text" id="total" />
document.getElementById('total').value = <some value>;
Listing 1. index.html
Listing 2. getShipping.php
<?php echo "$5.00"; ?>
This first example avoids XML entirely. The following line of code grabs the result as plain text:
results = http.responseText;
Keep in mind that the above example takes as many shortcuts as possible to keep it simple. There is no error checking or error handling whatsoever. There aren't even any HTML tag names, only ids. For example, it would be more common to create an input field that reads <input type="text" name="totalshipping" id="totalshipping" />. You probably wouldn't place the shipping cost in a field that a person could edit (although your form could re-validate the shipping when the person clicked “purchase” to correct any user changes). In addition, the example doesn't actually calculate a shipping cost. The URL in the above code points to a simple PHP script that returns the text value “$5.00” (Listing 2). A real application would take the zip code and use it to calculate the shipping cost and return that value. In short, the example cuts every possible corner to isolate how Ajax works rather than how one should code an Ajax application.
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