[Rygian]

If the array is implemented internally as a map, then the forEach method on t1 already knows it has only two items to process.

You can show that as follows:

  t0 = new Date();
  a = Array(100000000).fill(0);
  a[100000] = 1;
  a[10000000] = 1;
  counter = 0;
  a.forEach(e => e == 1 ? console.log(e) : counter++);
  console.log("elapsed: ", new Date() - t0); // 19415
  console.log("visited but not logged: ", counter); // 99999998

  t1 = new Date();
  a = Array(100000000);
  a[100000] = 1;
  a[10000000] = 1;
  counter = 0;
  a.forEach(e => e == 1 ? console.log(e) : counter++);
  console.log("elapsed: ", new Date() - t1); // 1936
  console.log("visited but not logged: ", counter); // 0