Are callbacks always asynchronous? [duplicate]

Answer 1

Hmmm.. something looks wrong (I have done v little JavaScript): you pass myCallBack to the function doSomething() but you dont call it back!? You would have to have a call to f() inside doSomething() or pass it to another function which will call it back once your long operation is complete.. And no callbacks are not inherently asynchronous - in your case you are running it all on the same thread (even if accessTheDatabase() is asynchronous in which case it would immediately return!) - so it will still go Step1, Step2, Step3, Step4. I beleive you want:

function main() {
  console.log("step 1");
  console.log("step 2");
  doSomething(myCallBack);
  console.log("step 4");
}

function doSomething(f) {
  accessTheDatabase(f); // assuming this is an asynchronous operation and calls the callback f once done
}

function myCallBack() {
   console.log("step 3");
}

In answer to the second part of your question: the callback would be run on which ever thread you are calling it from - to run a callback on another thread you would have to join() that thread first then invoke() or begininvoke() the callback - though there may already be some built in way to dispatch your callback onto a queue of things to be run on the thread you want to run it on (often the case with UI threads) NOTE: It may already be that accessTheDatabase() does run the callback on the thread that called it - using one of the afore mentioned methods...