javascript异步编程

function longTimeOperation(){
	console.log("this is long time operation");

	setTimeout(function(){
		console.log("the long time operation cost 5000 ms");
		f2();
	}, 5000);

	var first = function first(){                                   //此处 函数名first可以去掉。
		console.log("i am doing first");
	}();

	console.log(first);

	function f2(){
		console.log("this is f2, i am callback");
	}

}

longTimeOperation();

　　输出：

    

this is long time operation
i am doing first
undefined                         //间隔5秒后执行
the long time operation cost 5000 ms
this is f2, i am callback

var LongTimeOperation = function (taskID){
	var id  = taskID;

	this.go = function(test){
		console.log("this is long time operation" + id);

		var delay = parseInt((Math.random()*10000000)%5000);

		setTimeout(function(){
			console.log("the taskid "+ id +" operation cost "+ delay +" ms");
			test();
		}, delay);

		var first = function (){
			console.log("i am doing first");
		}();

		console.log(first);

		function f2(){
			console.log("this is f2");
		}
	};
}

function second(){
	console.log("second");
}


for(var i = 0; i < 5; i++){
	var task = new LongTimeOperation(i);
	task.go(second);
}

　　输出：

this is long time operation0
i am doing first
undefined
this is long time operation1
i am doing first
undefined
this is long time operation2
i am doing first
undefined
this is long time operation3
i am doing first
undefined
this is long time operation4
i am doing first
undefined
the taskid 1 operation cost 670 ms
second
the taskid 3 operation cost 1157 ms
second
the taskid 4 operation cost 2373 ms
second
the taskid 2 operation cost 2939 ms
second
the taskid 0 operation cost 4929 ms
second
[Finished in 5.1s]