Await

gistfile1.txt

doSomeLongRunningTask(function(response, error) {
  // This function is running as a CALLBACK inside the long task 
  // So it logs on completion 
  console.log(response)
  
  // Return the response
  return response;
}

var res = doSomeLongRunningTask(function(response, error) {
  // You might expect response to get returned and assigned to `res`
  // But since the task runs long, and you haven't asked to await it, your `res` var gets `undefined`
  return response;
}

var res = await doSomeLongRunningTask(function(response, error) {
  // As long as `doSomeLongRunningTask` is an async function, you can `await` it, and `res` will get assigned at the end of the long task
  return response;
}