渡一 24 json，异步加载，时间线

Json:是一种为了传输数据的对象
JSON.parse();string-->json
JSON.stringify();json-->string

window.onload = function(){
    console.log("a");
}
//onload=等randerTree加载完成

document.onreadystatechange = function(){
    console.log(document.readState);
}

//DOMContentLoaded等domTree加载完
document.addEventListener("DOMContentLoaded",function(){
    console.log("a");
},false)

function loadScript(url,callback){
    var script = document.createElement("script");
    script.type = "text/javascript";
    if(script.readyState){//ie
        script.onreadystatechange = function(){
            if(script.readyState == "complete" || script.readState == "loaded"){
                tools[callback]();
            }
        }
    }else{
        script.onload = function(){
            tools[callback]();
        }
    }
    script.src = url;
    document.head.appendChild(script);
}