CountdownLatch

latch.js

var CountdownLatch = function (limit) {
  this.limit = limit;
  this.count = 0;
  this.waitBlock = function () {};
};
CountdownLatch.prototype.async = function (fn, ctx) {
  var _this = this;
  fn.call(ctx, function () {
    _this.count = _this.count + 1;
    if(_this.limit <= _this.count){
      return _this.waitBlock();
    }
  });
};
CountdownLatch.prototype.await = function (callback) {
  this.waitBlock = callback;
};

latch_barrier.js

var barrier = new CountdownLatch(2);

barrier.async(function (done){
  setTimeout(function (){
    console.log('work A');
    done();
  }, 100);
});
barrier.async(function (done){
  setTimeout(function (){
    console.log('work B');
    done();
  }, 200);
});
console.log('wait for all to finish...');
barrier.await(function(){
  console.log('done all');
});

==>
wait for all to finish...
work A
work B
done all