Sample code for implementing follow ads with JavaScript

        * {
            margin: 0;
            padding: 0;
        }
        
        img {
            position: absolute;
            left: 0;
        }
        
        p {
            text-align: center;
            line-height: 40px;
        }

    <img src="images/left_ad.png" alt="">
    <p>I am the main text 1</p>
    <p>I am the main text 2</p>
    <p>I am the main text 3</p>
    <p>I am the main text 4</p>
    <p>I am the main text 5</p>
    <p>I am the main text 6</p>
    <p>I am the main text 7</p>
    <p>I am the main text 8</p>
    <p>I am the main text 9</p>
    <p>I am the main text 10</p>
    <p>I am the main text 11</p>
    <p>I am the main text 12</p>
    <p>I am the main text 13</p>
    <p>I am the main text 14</p>
    <p>I am the main text 15</p>
    <p>I am the main text 16</p>
    <p>I am the main text 17</p>
    <p>I am the main text 18</p>
    <p>I am the main text 19</p>
    <p>I am the main text 20</p>
    <p>I am the main text 21</p>
    <p>I am the main text 22</p>
    <p>I am the main text 23</p>
    <p>I am the main text 24</p>
    <p>I am the main text 25</p>
    <p>I am the main text 26</p>
    <p>I am the main text 27</p>
    <p>I am the main text 28</p>
    <p>I am the main text 29</p>
    <p>I am the main text 30</p>
    <p>I am text 31</p>
    <p>I am text 32</p>
    <p>I am the text 33</p>
    <p>I am the main text 34</p>
    <p>I am the main text 35</p>
    <p>I am text 36</p>
    <p>I am text 37</p>
    <p>I am text 38</p>
    <p>I am text 39</p>
    <p>I am Text 40</p>
    <p>I am text 41</p>
    <p>I am text 42</p>
    <p>I am text 43</p>
    <p>I am text 44</p>
    <p>I am the main text 45</p>
    <p>I am text 46</p>
    <p>I am Text 47</p>
    <p>I am Text 48</p>
    <p>I am text 49</p>
    <p>I am text 50</p>

        //1. Get the element to be operated const oAdImg = document.querySelector("img");
 
        //2. Calculate the top value of the ad image = (viewport height - ad height)/2
        const screenHeight = getScreen().height;
        const imgHeight = oAdImg.offsetHeight;
        const offsetY = (screenHeight - imgHeight) / 2;
        // console.log(offsetY);
 
        //3. Set the calculated top value to the ad image // oAdImg.style.top = offsetY + 'px';
        easeAnimation(oAdImg, {
            "top": offsetY
        });
 
        //4. Listen for scrolling events on the web page window.onscroll = function() {
            //Get the scrolling distance of the webpage //The value of the top of the ad image + the scrolling distance of the webpage let pageOffsetY = getPageScroll().y;
            easeAnimation(oAdImg, {
                "top": offsetY + pageOffsetY
            });
        };
 
        // Browser viewport width and height function getScreen() {
            let width, height;
            if (window.innerWidth) {
                width = window.innerWidth;
                height = window.innerHeight;
            } else if (document.compatMode === "BackCompat") {
                width = document.body.clientWidth;
                height = document.body.clientHeight;
            } else {
                width = document.documentElement.clientWidth;
                height = document.documentElement.clientHeight;
            }
            return {
                width: width,
                height: height
            }
        }
 
        // Ease animation function easeAnimation(ele, obj, fn) {
            clearInterval(ele.timerId);
            ele.timerId = setInterval(function() {
                //The flag variable is used to mark whether all properties have completed the animation. let flag = true;
 
                for (let key in obj) {
                    let target = obj[key];
 
                    // 1. Get the current position of the element let style = getComputedStyle(ele);
                    let begin = parseInt(style[key]) || 0;
 
                    // 2. Define variable record step size // Formula: (end position - start position) * easing coefficient (0 ~ 1)
                    let step = (target - begin) * 0.3;
 
                    // 3. Calculate the new position begin += step;
                    if (Math.abs(Math.floor(step)) > 1) {
                        flag = false;
                    } else {
                        begin = target;
                    }
                    // 4. Reset the element's position ele.style[key] = begin + "px";
                }
 
                //Judge whether the animation has been executed if (flag) {
                    //After the animation is finished, turn off the timer clearInterval(ele.timerId);
 
                    //Judge whether the fn function is passed in, and execute it if it is, otherwise do not execute fn && fn();
                }
            }, 100);
        }
 
        //Webpage scrolling distance function getPageScroll() {
            let x, y;
            if (window.pageXOffset) {
                x = window.pageXOffset;
                y = window.pageYOffset;
            } else if (document.compatMode === "BackCompat") {
                x = document.body.scrollLeft;
                y = document.body.scrollTop;
            } else {
                x = document.documentElement.scrollLeft;
                y = document.documentElement.scrollTop;
            }
            return {
                x: x,
                y: y
            }
        }