Question

Promises in JS allow you to do async programming, as follows:

DoSomething().then(success, failure);

DoSomethingElse();

whenever i write the previous code it reaches DoSomethingElse() before it reaches success.
How is that possible? Isn’t JS a single threaded environment (not including web-workers)? is it done with setTimeout?

Answer 1

Yes, JavaScript is single-threaded, which means you should never block this single thread. Any long-running, waiting operation (typically AJAX calls or sleeps/pauses) are implemented using callbacks.

Without looking at the implementation here is what happens:

1. DoSomething is called and it receives success and failure functions as arguments.

2. It does what it needs to do (probably initiating long-running AJAX call) and returns

3. DoSomethingElse() is called

4. …

5. Some time later AJAX response arrives. It calls previously defined success and failure function

See also (similar problems)

• JavaScript equivalent of SwingUtilities.invokeLater()
• Are there any atomic javascript operations to deal with Ajax's asynchronous nature?
• jqGrid custom edit rule function using Ajax displays "Custom Function should return array!"