How does a node js process know when to stop

Question

Since many node.js scripts follow a pattern of doing something asynchronously (example below), how do they know when to stop?

In the following code, how does node determine after processing the writeFile, and registering the callback appropriately, that the process should be kept alive until the callback(s) run?

fs = require('fs');

fs.writeFile('foo', 'cat', function() {
  console.log('wrote to foo!'); 
  fs.readFile('foo', 'utf8', function(err, data) {
    console.log(data);
  });
}); 

Answer 1

Hello,

node keeps track of all outstanding work requests. Your fs.writefile() call creates a work request for I/O and adds your callback to that request. node saves the work request into its tables at the same time it is starting the I/O activity. Your code execution exits when you reach the end of your function. (But your memory/variables/etc. remain)

Later the I/O finishes and node takes the work request out of its tables. It sees the callback attached to the request and so calls that function with the results of the I/O request. Your global data is still around and any variables in closures still exist, so it seems to your code like it never stopped.

If you do nothing more, don't make any more requests, then when you return from your functions node will stop, because then there won't be any remaining requests in the queues.