JavaScript to achieve floor effect

* {
            margin: 0;
            padding: 0;
        }
        
        html,
        body {
            width: 100%;
            height: 100%;
        }
        
        ul {
            width: 100%;
            height: 100%;
        }
        
        ul>li {
            list-style: none;
            width: 100%;
            height: 100%;
            font-size: 100px;
            text-align: center;
        }
        
        ol {
            position: fixed;
            left: 10px;
            top: 50%;
            transform: translateY(-50%);
        }
        
        ol>li {
            list-style: none;
            width: 100px;
            line-height: 40px;
            text-align: center;
            border: 1px solid #000;
        }
        
        .selected {
            background: skyblue;
        }

 <ul>
        <li>I am level 1</li>
        <li>I am level 2</li>
        <li>I am level 3</li>
        <li>I am level 4</li>
        <li>I am level 5</li>
    </ul>
 
    <ol>
        <li class="selected">Layer 1</li>
        <li>Layer 2</li>
        <li>Layer 3</li>
        <li>Layer 4</li>
        <li>Layer 5</li>
</ol>

// 1. Initialize the color of the floor let oPages = document.querySelectorAll("ul>li");
let colorArr = ['green', 'blue', 'purple', 'red', 'yellow'];
        for (let i = 0; i < oPages.length; i++) {
            let page = oPages[i];
            page.style.background = colorArr[i];
        }
 
        // 2. Select whoever is clicked let oItems = document.querySelectorAll("ol>li");
        let currentItem = oItems[0];
 
        // Get the height of the visible area let screenHeight = getScreen().height;
 
        let timerId = null;
        for (let i = 0; i < oItems.length; i++) {
            let item = oItems[i];
            item.onclick = function() {
                currentItem.className = "";
                this.className = "selected";
                currentItem = this;
                // Implement scrolling // window.scrollTo(0, i * screenHeight);
                // Note: Use documentElement.scrollTop to scroll the web page. Do not add units when setting the value. // document.documentElement.scrollTop = i * screenHeight + "px";
                // document.documentElement.scrollTop = i * screenHeight;
                clearInterval(timerId);
                timerId = setInterval(function() {
                    let begin = document.documentElement.scrollTop;
                    let target = i * screenHeight;
                    let step = (target - begin) * 0.2;
                    begin += step;
                    if (Math.abs(Math.floor(step)) <= 1) {
                        clearInterval(timerId);
                        document.documentElement.scrollTop = i * screenHeight;
                        return;
                    }
                    document.documentElement.scrollTop = begin;
                }, 50);
            }
        }
 
        //Get the 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
            }
        }