Learning Node.js Development

The callback queue in async programming

At this point let's assume that setTimeout, the one that has a zero second delay, finishes. When it finishes, it's not going to get executed right away; it's going to take that callback and move it down into the Callback Queue, as shown here:

The Callback Queue is all the callback functions that are ready to get fired. In the previous screenshot, we move the function from Node API into the Callback Queue. Now the Callback Queue is where our callback functions will wait; they need to wait for the Call Stack to be empty.

When the Call Stack is empty we can run the first function. There's another function ...