Detailed explanation of the process of creating floor navigation effects with JavaScript

<div id="box" class="box">
    <ul class="list">
        <li class="content-part" data-n="Column 1">Column 1</li>
        <li class="content-part" data-n="Column 2">Column 2</li>
        <li class="content-part" data-n="Column Three">Column Three</li>
        <li class="content-part" data-n="Column 4">Column 4</li>
        <li class="content-part" data-n="Column Five">Column Five</li>
    </ul>
</div>
<div class="left">
    <ul id="left-list">
        <li data-n="Column 1">Column 1</li>
        <li data-n="Column 2">Column 2</li>
        <li data-n="Column Three">Column Three</li>
        <li data-n="Column 4">Column 4</li>
        <li data-n="Column Five">Column Five</li>
    </ul>
</div>

<style>
    * {
        margin: 0;
        padding: 0;
    }
    body .box {
        width: 1200px;
    }
    body .box {
        margin: 0 auto;
    }
    ul {
        list-style: none;
    }
    body .box ul li {
        height: 800px;
        background-color: silver;
        margin-bottom: 20px;
        font-size: 30px;
        font-weight: bold;
    }
    body .left {
        position: fixed;
        left: 20px;
        bottom: 100px;
        width: 100px;
        height: 250px;
        top: 50%;
        margin-top: -125px;
        background-color: silver;
    }
    body .left ul li {
        height: 50px;
        line-height: 50px;
        text-align: center;
        cursor: pointer;
    }
    body .current {
        color: #fff;
        background-color: tomato;
    }
</style>

var oList = document.getElementById('left-list');
// Click event delegate oList.onclick = function (e) {
    if (e.target.tagName.toLowerCase() == 'li') {
        // Get the value of data-n var n = e.target.getAttribute('data-n');
        // [] attribute selector var contentPart = document.querySelector('.content-part[data-n=' + n + ']');
        //Set scroll document.documentElement.scrollTop = contentPart.offsetTop;
    }
}

//When the page scrolls, the background of the box column on the left changes accordingly var contents = document.querySelectorAll('.content-part');
var lis = document.querySelectorAll('#left-list li');
var offsetTopArr = [];
for (var i = 0; i < contents.length; i++) {
    offsetTopArr.push(contents[i].offsetTop);
}
// For the convenience of comparison, add infinite offsetTopArr.push(Infinity);
// Monitor scrolling var nowFloor = -1;
window.onscroll = function (e) {
    var nowScrollTop = document.documentElement.scrollTop;
    // The i value of break is the subscript of the box array for (var i = 0; i < offsetTopArr.length; i++) {
        if (nowScrollTop >= offsetTopArr[i] && nowScrollTop < offsetTopArr[i + 1]) {
            break;
        }
    }
    // Floors are not equal, change the style if (nowFloor != i) {
        nowFloor = i;
        for (var j = 0; j < lis.length; j++) {
            if (j == i) {
                // Add style to the current floor lis[j].className = 'current';
            } else {
                // Remove the styles of other floors lis[j].className = '';
            }
        }
    }
}