Ensure multiple asynchronous call finishes before executing the callback

wait.js

function wait(n, cb){
  if(n <= 0) cb();
  let aggregated_result = {};
  let count = 0;
  return function done(res){
    Object.assign(aggregated_result, res);
    if(++count === n) cb(aggregated_result);
  }
}

// Usage:
var done = wait(3, function(){
  console.log("Here the three async functions are done");
});

doAsync(done);
doAsync2(done);
doAsync3(done);