Uncaught TypeError: mydiv.forEach is not a function


<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title></title>
    </head>
    <body>
        <div>页面1</div>
        <div>页面2</div>
        <div>页面3</div>
    </body>
    <script>
        let mydiv = document.getElementsByTagName("div");
        mydiv.forEach(function(data){
            console.log(data);
        });
    </script>
</html>

为什么无法遍历输出？