JS中判断undefined、null、与NaN的方法

1.判断undefined:

if ( typeof AJAX_MONITOR != "undefined") {
    AJAX_MONITOR.PDC.init({
        product_id : 234, //产品线id
        sample : 1,//采样率，范围0-1。计算方法：100万/页面PV
        max : 2//一次最大上传个数

    });
}

var platform = platform_setting_json[data.PLATFORM_TYPE];
if (typeof(platform) == "undefined") {
    return "未定义";
} else {
    return platform;
}

2.判断null:

var platform = platform_setting_json[data.PLATFORM_TYPE];
if (typeof(platform) != "undefined" && !platform && tplatformmp != 0) {
    return "null";
} else {
    return platform;
}

3.判断NaN:

var platform = platform_setting_json[data.PLATFORM_TYPE];
if (isNaN(parseInt(platform)) {
    return "NaN";
} else {
    return platform;
}