ES7's async/await syntax.

1-sleep-es7.js

// ES7, async/await
function sleep(ms = 0) {
  return new Promise(r => setTimeout(r, ms));
}

(async () => {
  console.log('a');
  await sleep(1000);
  console.log('b');
})()

2-sleep-es6-promise.js

// ES6, native Promises, arrow functions, default arguments
function sleep(ms = 0) {
  return new Promise(r => setTimeout(r, ms));
}

console.log('a');
sleep(1000).then(() => {
  console.log('b');
});

// or if we were to be strictly identical, an "async" function returns a
// Promise, so this is more accurate, but the fluff isn't needed for in this case

(function() {
  return new Promise((resolve, reject) => {
    sleep(1000).then(() => {
      console.log('b');
      resolve();
    });
  });
})();

3-sleep-es6-just-promise.js

// ES6, native Promises ONLY
function sleep(ms) {
  ms = ms || 0;
  return new Promise(function(resolve) {
    setTimeout(resolve, ms);
  });
}

(function() {
  console.log('a');
  sleep(1000).then(function() {
    console.log('b');
  });
})();

4-sleep-es5.js

// ES5, nothing special
function sleep(ms, callback) {
  ms = ms || 0;
  setTimeout(callback, ms);
}

(function() {
  console.log('a');
  sleep(1000, function() {
    console.log('b');
  });
})();