const { performance } = require("perf_hooks"); // if you're using node js

const arr = [];
for (i = 0; i < 10000000; ++i) {
  let random = Math.floor(Math.random() * (20 - 0 + 1)) + 0;
  arr.push(random);
}

function timer(lap) {
  if (lap)
    console.log(lap, "in:", (performance.now() - timer.prev).toFixed(3) + "ms");
  timer.prev = performance.now();
}

timer();

const arr1 = [];
for (i = 0; i < arr.length; i++) {
  arr1.push(arr[i] * 2);
}
timer("For Loop");

const arr2 = [];
arr.forEach(a => {
  arr2.push(a * 2);
});

timer("For each");

const arr3 = arr.map(a => a * 2);

timer("Map");