Why is AJAX called asynchronous?

Question

Why is AJAX called asynchronous? How does it accomplish communication asynchronously with the server?

Answer 1

It's asynchronous in that it doesn't lock up the browser. If you fire an Ajax request, the user can still work while the request is waiting for a response. When the server returns the response, a callback runs to handle it.

You can make the XMLHttpRequest synchronous if you want, and if you do, the browser locks up while the request is outstanding (so most of the time this is inappropriate)

Answer 2

It's asynchronous because the client and the server run independently of each other for the duration of the function call.

During a normal function call, you make the call, and the calling function doesn't get to execute again until the function call finishes and returns. The caller and the callee are always synchronized.

During an asynchronous function call, you make the call, and then control returns immediately to the caller. The callee then returns a value some indeterminate amount of time later. That "indeterminate amount of time" means the caller and callee are no longer synchronized, so it's asynchronous.