Edit in JSFiddle

(function () {
    var obj = {
        city: "Beijing",
        12: 12,
        7: 7,
        0: 0,
        "-2": -2,
        "age": 15,
        "-3.5": -3.5,
        7.7: 7.7,
        _: "___",
        online: true,
        3: 3,
            "23": "23",
            "44": 44
    },
    info = document.getElementById("info"),
        key;

    for (key in obj) {
        info.innerHTML += key + " : " + obj[key] + "<br/>";
    }
}());

<div id="info"></div>