Asynchronous forEach

AsyncFor.js

// Callback-style (recursion explicit)
function forAsync(items, fn, next) {
  var max = items.length;
  function rec(i) {
    if (i === max) {
      next();
    }
    fn(items[i], i);
    rec(i + 1);
  }
  rec(0);
}

// Promise-style (recursion in reduce)
function forAsync(items, fn) {
  return items.reduce(function(promised, item, i) {
    return promised.then(function() {
      return fn(item, i);
    });
  }, Promise.resolve());
}

// Usage
forAsync([1,2,3,4], function(item, i) {
  setTimeout(function(){
    console.log(item);
  }, item * 500);
});