为什么定义了全局变量title.play,stop在onload的匿名函数里不能用呢

var data=['Phone5','Ipad','三星笔记本','佳能相机','惠普打印机','谢谢参与','50元充值卡','1000元超市购物券'],
    timer=null,
    flag=0,
    title=document.getElementById('title'),
    play=document.getElementById('play'),
    stop=document.getElementById('stop');
window.onload=function () {
    // vartitle=document.getElementById('title'),
    // play=document.getElementById('play'),
    // stop=document.getElementById('stop');
    //鼠标点击开始抽奖
    play.onclick=playFun;
    stop.onclick=stopFun;
    //回车键开始抽奖
    document.onkeyup=function(event){
        event=event||window.event;
        if (event.keyCode==13) {
            if (flag==0) { playFun();} 
            else { stopFun(); }
        }
    }
    function playFun() {
        //setInterval计时器，每50毫秒执行一次函数
        clearInterval(timer);  //停止计时器
        timer=setInterval(function(){
            var random=Math.random(),  //0-1随机数
            num=Math.floor(random*data.length);  //给范围向下取整
            title.innerHTML=data[num];  //给标题赋值
        },50);
        play.style.backgroundColor="#eee";
        play.style.color="#aaa";
        play.style.cursor="default";
        stop.style.backgroundColor="#19448e";
        stop.style.color="#fff";
        stop.style.cursor="pointer";
        flag=1;
    }
    function stopFun() {
        clearInterval(timer);
        play.style.backgroundColor="#19448e";
        play.style.color="#fff";
        play.style.cursor="pointer";
        stop.style.backgroundColor="#eee";
        stop.style.color="#aaa";
        stop.style.cursor="default";
        flag=0;
    }
}