Using generators to execute async code in a sync manner

index.js

'use strict';

const EventEmitter = require('events');

/**
 * Runs generator and puts EventEmitter context in it to let user
 * emit any events inside generator and subscribe to them from outside.
 *
 * @param  {Function}     gen Function which returns generator
 * @return {EventEmitter}     Returns emitter instance which is bound
 */
function runGenerator(gen) {
    let res;
    let emitter  = new EventEmitter();
    let iterator = gen.call(emitter);

    (function iterate(value) {
        res = iterator.next(value);
        if (!res.done) {
            return (res.value instanceof Promise)
                ? res.value.then((value) => iterate(value))
                : setImmediate(() => iterate(res.value));
        }
    })();

    return emitter;
}


// EXAMPLE

const Pool   = require('pg').Pool;
const Cursor = require('pg-cursor');
const pool   = new Pool();

let it = runGenerator(function* () {

    let chunk  = 2;
    let sql    = 'SELECT * FROM generate_series(0, 9, 1)';
    let client = yield pool.connect();
    let cursor = yield client.query(new Cursor(sql));
    let data;

    this.emit('init');

    while (data !== null) {
        data = yield new Promise((resolve) => {
            cursor.read(chunk, (err, rows) => resolve(rows.length ? rows : null));
        });

        if (data !== null) {
            this.emit('data', data);
        }
    }

    yield client.release();

    this.emit('end');
});

it.on('init', () => console.log('connection established'));
it.on('data', (data) => {
    // ... do anything with data ...
});
it.on('end', () => console.log('client released'));