console 命令行工具 X clear
console
<!DOCTYPE html>
<html>

    <head>
        <meta charset="utf-8"/>
        </head>

        <body>
            <script >
            var $a = 10;
            var a = 3.14;
            var string = "I am a string";
            var notDefined;
            var noSpace = null;
            document.write($a + "<br/>");
            document.write(a+"<br/>");
            document.write(string + "<br/>");
            document.write(notDefined+"<br/>");
            document.write(noSpace+"<br/>");
            </script>
            </body>
</html>