index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>

    <style>
        #bg-div {
            height: 500px;
            width: 1000px;
            background: url("image/car_3.jpg");
            margin: 0 auto;
            background-size: contain; /*What the meaning?*/

            /*center this div*/
        }
    </style>

    <script>
        for (var i = 1; i < 5; ++i) {
            setTimeout(function timer() {
                console.log(i);
                console.log(new Date());
            }, i * 1000);
        }

        var obj =  {
            name: "n",
            value: "val",
            addr: "city"
        }

        //how to traversal the "obj"


        //how to to object to json string
        console.log(toString(obj));


        //how to json string to object
        var strObj = '{name: "n",value: "val",addr: "city"}';


    </script>
</head>
<body>
<div id="bg-div">bg-div</div>
</body>
</html>