async_runner.js

let fs = require("fs");

function run(taskDef) {

    // create the iterator
    let task = taskDef();

    // start the task
    let result = task.next();

    // recursive function to iterate through
    (function step() {

        // if there's more to do
        if (!result.done) {

            // resolve to a promise to make it easy
            let promise = Promise.resolve(result.value);
            promise.then(function(value) {
                result = task.next(value);
                step();
            }).catch(function(error) {
                result = task.throw(error);
                step();
            });
        }
    }());
}

// Define a function to use with the task runner

function readFile(filename) {
    return new Promise(function(resolve, reject) {
        fs.readFile(filename, function(err, contents) {
            if (err) {
                reject(err);
            } else {
                resolve(contents);
            }
        });
    });
}

// Run a task

run(function*() {
    let contents = yield readFile("config.json");
    doSomethingWith(contents);
    console.log("Done");
});