forEach.js

function forEach(arr, func) {
  const len = arr.length;
  for(let i = 0; i < len; ++i) func(arr[i])
}

const l = [1,2,3,4,5];

function logTimes2(x) {
  console.log(x*2);
}

forEach(l, logTimes2);
forEach(l, function(x){logTimes2(x)});
forEach(l, x => console.log(x*2));