Head Rush Ajax

106

Chapter 2

Man, Break Neck pizza rocks. There’s

nothing like a breakfast of cold pizza, a

whipped latte from Starbuzz, and the

Webville Times to read.

<script>

var request...

function foo()

{

...

}

</script>

JavaScript

<script>

var request...

function foo()

{

...

}

</script>

Customer’s Phone Number

Customer’s Address

Customer’s Order

As long as you remember the name of the

property you use to set the callback function,

you’ll remember that the callback is run more

than one time. Do you remember what the

property was called?

It’s onreadystatechange. So when the

ready state changes from 1 to 2, for example,

your callback function, updatePage(), gets

run. In other words, updatePage() will run

several times: when the ready state changes from

1 to 2, when it changes again from 2 to 3, and

one more time, when the ready state changes

from 3 to 4.

But the server only guarantees that it’s got data

you can use when the ready state is 4. So you

need to check the current ready state before

trying to update the order form, or the page

might end up with bad or missing data.

Right—because updatePage() runs

every time the ready state changes.

So that’s why we check to make sure the ready state

is 4 before doing anything in our callback function,

right? Otherwise, our JavaScript might try and

update the page before the server is nished.

ready states and callback functions