Under the Microscope: HTTP Ready States






Under the Microscope: HTTP Ready States

So when exactly does the ready state of a request change? Here's a close-up look at the ready state of a request, and how it changes as a request is processed by a web server.

    request.open("GET", url, true);getCustomerInfo() calls open()
on the request object, and initializes the connection.
    getCustomerInfo()
    Javascript
    0 Connection uninitializedHere's the request object's ready state,
stored in the readyState property.
    var request = ...When the web page is loaded, a new request object is created.
    Web PageThis JavaScript function can update the HTML page using the response data 

from the server.
    updatePage()The JavaScript function you assigned to the onreadystatechange
property is run, and can use the server's data.This is the callback
function that you just learned about...
    Javascript

    request.onreadystatechange = updatePage;This property tells the browser
what function to run when the request's ready state changes.
    1 Connection initializedAt this stage, the request knows how to connect,
and what to connect to.
    request.send(null) 
;With the connection initialized, the request is sent to the server.
    Web ServerThe server
gets the request to the right script or program,
and that program responds to the request.
    2 Response in progressAt this stage, the browser makes the status and
response headers from the server available.
    PHP 
 Script
    3 Getting server responseData is downloading into the request object,
but the data's not ready to be used by your code yet.
    4 Server response readyThe server's finished with your request, and any
response data is available in the request object's responseText 
 property.

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 finished.

2.29.1. Rightbecause updatePage() runs every time the ready state changes.

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.



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