<!DOCTYPE html>
<html>
    <head>
        <meta charset=utf-8>
    </head>
    <body>
    	
        <script type="text/javascript">
            const closure = () => {
                // 补全代码
                let num = 1;
                return function() {
                    return num++;
                }
            }
        </script>
    </body>
</html>