doThings.js

function doThings(todo, whenDone, thereWereErrors) {
    var tasks = todo.length, errors = 0;
  
    function finish() {
        task --;
        if (task === 0) {
            // everything is done
            if (errors) {
                // but there were errors
                thereWereErrors();
            } else {
                // do the thing that depends on everything being done
                whenDone();
            }
        }
    }
    
    todo.forEach(function(thing) {
        // start in this example returns a promise
        thing.start()
            .then(function() {
                // thing has finished
                finish();
            },
            function(reason) {
                // thing errored
                errors ++;
                finish();
            });
    });
}

Works for sync or async tasks, or even a mix of both