console.log("script start");    
    setTimeout(function () {
             console.log("setTimeout");
         }, 0);   
     //具体数字不定，这取决于你的硬件配置和浏览器
     for(var i = 0; i < 1000000; i ++){
         //do something        
     }   
     console.log("script end");

  var arr = new Array(10000);
    arr.fill(1);
    function asyncForEach(array, handler){
        var t = setInterval(function () {
            if(array.length === 0){
                clearInterval(t);
            }else {
                handler(arr.shift());
            }
        }, 0);
    }

    //异步遍历
    asyncForEach(arr, function (value) {
        console.log(value);
    });

    //同步遍历
    arr.forEach(function (value, index, arr) {
        console.log(value);
    });