dropdown-js/index.html

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>JS Plugin</title>
    <style>
        * {
            box-sizing: border-box;
        }

        body {
            font-family: sans-serif;
        }

        .app {
            width: 100%;
            height: 100vh;
            overflow: hidden;
            display: flex;
            justify-content: center;
            padding-top: 50px;
        }

        .dropdown {
            position: relative;
            width: 400px;
        }

        .dropdown.open .dropdown__menu {
            display: block;
        }

        .dropdown__label {
            padding: 10px 10px;
            border: 1px solid rgb(34, 34, 34);
            width: 100%;
            cursor: pointer;
        }

        .dropdown__menu {
            list-style: none;
            margin: 0;
            padding: 0;
            display: none;
            border-right: 1px solid;
            border-left: 1px solid;
            position: absolute;
            width: 100%;
            top: 39px;
        }

        .dropdown__menu li {
            padding: 10px 10px;      
            border-bottom: 1px solid;
        }

        .dropdown__menu li:hover {
            background-color: rgba(0, 0, 0, .2);
            cursor: pointer;
        }
    </style>
</head>

<body>

    <div class="app">
        <div class="dropdown" id="dropdown">
            <div class="dropdown__label"></div>
            <ul class="dropdown__menu"></ul>
        </div>
    </div>

    <script src="./script.js"></script>
</body>

</html>