V8 Release 5.1(v8project.blogspot.com)

  prev = [1,2,3,4]
  next = [x * 2 for x in old]

  prev = [1,2,3,4];
  next = prev.map(x => x * 2);

  next = [x for x in prev if x % 2 == 0]

  next = prev.filter(x => x % 2 === 0);

  next = [x * 2 for x in prev if x % 2 == 0]

  next = prev.filter(x => x % 2 == 0).map(x => x * 2);

  next = prev.mapIf(x => x * 2, x => x % 2 == 0);