index.html

<title>Crop Image Pure JS</title>

<link rel="stylesheet" type="text/css" href="style.css">

<style>
    html,
    body {
        width: 100%;
        height: 100%;
        margin: 0;
        padding: 0;
    }    
</style>

<main class="center">
    <img src="image.jpg" class="crop-image" alt="crop image"/>
</main>

<button class="btn-crop js-crop">Crop</button>

<script src="crop-image-pure.js"></script>