/*
 * promiseSerial resolves Promises sequentially.
 * Puts results into an array and get it from then()
 *
 * @example
 * const urls = ['/url1', '/url2', '/url3']
 * const funcs = urls.map(url => () => $.ajax(url))
 *
 * promiseSerial(funcs)
 *   .then(console.log.bind(console))
 *   .catch(console.error.bind(console))
 */
const chainPromise = funcs =>
  funcs.reduce((promise, func) =>
    // can pass result to func if prev result is needed
    promise.then(result => func().then(Array.prototype.concat.bind(result))),
    Promise.resolve([]));