Simple tests of the Promise array execution order.

promise_array_test.js

'use strict';

var p = console.log;
var FS = require('fs');

// fs appendFile Promise wrapper
function write(path, data){
  return new Promise((res, rej) => FS.appendFile(path, data, err => 
    err ? rej(err) : res(p(data, 'is done!'))
  ));
}

// forEach...
exports.forEachTest = () => {
  var arr = [...Array(10).keys()].map(i => i.toString());
  try {
    p('just forEach ===========');
    arr.forEach(i => write(__dirname.concat('/', i, '.txt'), i))
    p('done?');
  } catch (e) {
    p(e);
  }
}

// Promise.all and map()
exports.promiseAllTest = () => {
  var arr = [...Array(10).keys()].map(i => i.toString());
  p('promise all ============');
  Promise.all(arr.map(i => write(__dirname.concat('/', i, '.txt'), i)))
    .then(_ => p('done'))
    .catch(e => p(e));
}

// Promise and reduce
exports.promiseReduceTest = () => {
  var arr = [...Array(10).keys()].map(i => i.toString());
  p('promise reduce ===========');
  arr.reduce((acc, cur) => acc.then(_ => write(__dirname.concat('/', cur, '.txt'), cur)), Promise.resolve())
    .then(_ => p('done'))
    .catch(e => p(e));

  /*
    this is the equivalent of...
    Promise.resolve()
      .then(_ => write(PATH, 0))
      .then(_ => write(PATH, 1))
      .then(_ => write(PATH, 2))
      ...
   */
}