The `list` function is looping through a list of asynchronous operations. The `main` function is notified about the state of the `list` function on each step.

function-events.js

function list (step, end) {
    function loop (index) {
        if (index == 10) return end();
        step(index, function next () {
            loop(++index);
        });
    }
    loop(0);
}

function main () {
    list(function step (index, next) { // event on each step
        console.log(index);
        next();
    }, function end () {
        console.log('end');
    });
}

main();