asd.js

var dir = "down";

var babyScroll;

var stepTime = 1;

var downLimit = document.documentElement.scrollHeight - document.documentElement.clientHeight;
var upLimit = 0;

function babyScroll() {
  if(dir == "down") {
    window.scrollTo(0, window.scrollY + 10);
    if (window.scrollY >= downLimit - 1) {
      clearInterval(babyScrollId);
      dir = "up";
      babyScrollId = setInterval(babyScroll, stepTime);
    }
  } else {
    window.scrollTo(0, window.scrollY - 10);
    if (window.scrollY <= upLimit) {
      clearInterval(babyScrollId);
      dir = "down";
      babyScrollId = setInterval(babyScroll, stepTime);
    }
  }
};
var babyScrollId = setInterval(babyScroll, stepTime);