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>

<!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/>");

        var number1=10;
        var number2=20;
        var sum=number1+number2;
        var string1="Hi";
        var string2="Heather";
        var string1Number1=string1+string2;
        var number2String2=number2+string2;
        var stringNumber1=number1+"";
        var stringNumber1Number2=stringNumber1+number2;

        document.write(sum+"<br/>");
        document.write(string1Number1+"<br/>");
        document.write(number2String2+"<br/>");
        document.write(stringNumber1+"<br/>");
        document.write(stringNumber1Number2+"<br/>");

    </script>
</body>
</html>