Explore the effect of Promises on JS speed and order of execution

defer-with-promise.js

var trash = [];

function computationallyIntensiveStuff() {
  trash.push(Math.pow(Math.pow(Math.cos(Math.sin(Math.random())), Math.random()), Math.random() * 100000));
  trash.filter(function (a) { return a > Math.random(); });
}

new Promise(function (resolve, reject) {
  for (var i = 0; i < 1000; i++) {
    computationallyIntensiveStuff();
  }
  resolve();
}).then(function () {
  console.log('Promise done: ' + Date.now());
});

console.log('Next statement: ' + Date.now());

// Next statement: 1418958320178
// Promise done:   1418958320180 (2 ms later)

defer-with-settimeout.js

var trash = [];

function computationallyIntensiveStuff() {
  trash.push(Math.pow(Math.pow(Math.cos(Math.sin(Math.random())), Math.random()), Math.random() * 100000));
  trash.filter(function (a) { return a > Math.random(); });
}

setTimeout(function (resolve, reject) {
  for (var i = 0; i < 1000; i++) {
    computationallyIntensiveStuff();
  }
  console.log('Promise done: ' + Date.now());
}, 0);

console.log('Next statement: ' + Date.now());

// Next statement: 1418958579477
// Promise done:   1418958579509 (32 ms later)

no-promises.js

var trash = [];
var startTime = Date.now();
var log = [];

function computationallyIntensiveStuff(whoCalled) {
  trash.push(Math.pow(Math.pow(Math.cos(Math.sin(Math.random())), Math.random()), Math.random() * 100000));
  trash.filter(function (a) { return a > Math.random(); });
  log.push(whoCalled);
}

for (var i = 0; i < 1000; i++) {
  computationallyIntensiveStuff('Drew');
}

for (var i = 0; i < 1000; i++) {
  computationallyIntensiveStuff('Clay');
}

console.log(log);
console.log(Date.now() - startTime);

// [ 'Drew',
//   ...998x
//   'Drew',
//   'Clay',
//   ...998x
//   'Clay' ]
// 132 (ms)

promises.js

var trash = [];
var startTime = Date.now();
var log = [];

function computationallyIntensiveStuff(whoCalled) {
  trash.push(Math.pow(Math.pow(Math.cos(Math.sin(Math.random())), Math.random()), Math.random() * 100000));
  trash.filter(function (a) { return a > Math.random(); });
  log.push(whoCalled);
}

Promise.all([
  new Promise(function (resolve, reject) {
    for (var i = 0; i < 1000; i++) {
      computationallyIntensiveStuff('Drew');
    }
    resolve();
  }),

  new Promise(function (resolve, reject) {
    for (var i = 0; i < 1000; i++) {
      computationallyIntensiveStuff('Clay');
    }
    resolve();
  }),
]).then(function () {
  console.log(log);
  console.log(Date.now() - startTime);
});

// [ 'Drew',
//   ...998x
//   'Drew',
//   'Clay',
//   ...998x
//   'Clay' ]
// 138 (ms)