async js with node

Question

What is the progression of this function using the async.js library?

var async = require('async');

var square = function (num, doneCallback) {
  console.log(num * num);
  // Nothing went wrong, so callback with a null error.
  return doneCallback(null);
};

// Square each number in the array [1, 2, 3, 4]
async.each([1, 2, 3, 4], square, function (err) {
  // Square has been called on each of the numbers
  // so we're now done!
  console.log("Finished!");
});

In the 'square' function, is the return doneCallback(null) ran every time a new number is passed, or is it ran after all the numbers are finished?

I think it is ran after all the numbers have been passed and console.log'd, IMO the return would interrupt and stop the function. Is this what is actually happening?

Answer 1

No, the doneCallback happens before the return, because the result of the doneCallback is the function's return value. doneCallback will be called once for each time that square is invoked.