Understanding fat arrows

with fat arrows.js

// https://greenin.space/how-do-you-write-an-asynchronous-for-loop-in-javascript/
var N = 5

function asyncFunc (cb) {  
  console.log("in asyncFunc")
  setTimeout(() => cb(Math.random()), 3000)
  console.log("out asyncFunc")
}

function loop (max, results, done) {  
  // Recursion base-case
  if (results.length >= max) return done(results)
console.log("in loop")
  asyncFunc((res) => {
    results.push(res)
    loop(max, results, done)
  })
}

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

/*
VM230:13 in loop
VM230:5 in asyncFunc
VM230:7 out asyncFunc
////////// pause 3 secs
VM230:13 in loop
VM230:5 in asyncFunc
VM230:7 out asyncFunc
///////// pause 3 secs
VM230:13 in loop
VM230:5 in asyncFunc
VM230:7 out asyncFunc
////////// pause 3 secs
VM230:13 in loop
VM230:5 in asyncFunc
VM230:7 out asyncFunc
//////// pause 3 secs
VM230:13 in loop
VM230:5 in asyncFunc
VM230:7 out asyncFunc
//////// pause 3 secs
VM230:22 [0.29334537521936, 0.08791952556930482, 0.02379906317219138, 0.9299911002162844, 0.4993608531076461]
*/