Does an asynchronous call always create/call a new thread?

Question

Does asynchronous call always create a new thread?

Example:

If JavaScript is single threaded then how can it do an async postback? Is it actually blocking u

Answer 1

A couple notes about JavaScript in particular:

XMLHttpRequests are non-blocking by default. The send() method returns immediately after the request has been relayed to the underlying network stack. A response from the server will schedule an invocation of your callback on the event loop as discussed by the other excellent answers.

This does not require a new thread. The underlying socket API is selectable, similar to java.nio.channels in Java.

It's possible to construct synchronous XMLHttpRequest objects by passing false as the third parameter to open(). This will cause the send() method to block until a response has been received from the server, thus placing the event loop at the mercy of network latency and potentially hanging the browser until network timeout. This is a Bad Thing™.

Firefox 3.5 will introduce honest-to-god multithreaded JavaScript with the Worker class. The background code runs in a completely separate environment and communicates with the browser window by scheduling callbacks on the event loop.