script变量

<script >

    //定义变量
   /* var a  = 3;
    alert(a);

    a = "abc";
    alert(a);*/

   //定义number类型

    var num = 1;
    var num2 = 1.2;
    var num3 = NaN;

    //输出到页面上
    document.write(num+"<br>");
    document.write(num2+"<br>");
    document.write(num3+"<br>");
    //定义string类型

    var str = "abc";
    var str2 = 'edf';
    document.write(str+"<br>");
    document.write(str2+"<br>");

    //定义boolean
    var flag = true;
    document.write(flag+"<br>");

    // 定义null,undefined
    var obj = null;
    var obj2 = undefined;
    var obj3 ;
    document.write(obj+"<br>");
    document.write(obj2+"<br>");
    document.write(obj3+"<br>");

</script>