recursion.js

var N = 50

function asyncFunc (cb) {
  setTimeout(() => cb(Math.random()), 100)
}

function loop (N, results, done) {
  // Recursion base-case
  if (results.length >= N) return done(results)

  asyncFunc((res) => {
    results.push(res)
    loop(max, results, done)
  })
}

let randomNumbers = []
loop(N, randomNumbers, function (results) {
  console.log(results)
})

https://medium.com/@emgeee/writing-an-asynchronous-for-loop-in-javascript-fe5be0c01e2#.vztmwmfva