CountdownLatch

latch.js

var CountdownLatch = function (limit){
  this.limit = limit;
  this.count = 0;
  this.waitBlock = function (){};
};
CountdownLatch.prototype.countDown = 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);

setTimeout(function (){
  console.log('work A');
  barrier.countDown();
}, 100);
setTimeout(function (){
  console.log('work B');
  barrier.countDown();
}, 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

@herrgrossmann this is great, thanks! Maybe publish to NPM?