8 Years
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Last Post by hielo

When a function executes Synchronously, the following statement is NOT executed until the previous one is done executing - ex:

/* here, the statements are executed one after the other. In other words, if the web server is experiencing delays and it takes 5 seconds to respond then you will see the second alert 5 seconds after the message "Checking Username".
alert("Checking Username");
var username = makeSJAXCall();

By contrast, Asynchronous calls do NOT wait for completion before moving on to the next statement - ex:

here you see the "Checking Username" message and almost immediately after that you will see the second alert. Since the Asynchronous function has not completed, username does not have a value and you should see undefined as the second message.

So how do you "campture/receive" the result. This is where the callback function comes into place, which is what is typically executed onreadystatechange.
alert("Checking Username");
var username = makeAJAXCall();
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