async 函数

同步
console.log(1);
console.log(2);
console.log(3);
console.log(4);

		//异步 ajax 文件读取io操作
		console.log(1);
		console.log(2);
		setTimeout(function(){
			console.log(3000);
		},3000);
		console.log(3);
		console.log(4);
		//先打印1 2 3 4，隔三秒后打印3000；
		
		//async函数返回的是resolve状态的Promise对象
		async function fn(){
			return "abc";
		}
		let result=fn();
		console.log(result);//打印：Promise {[[PromiseStatus]]: "resolved", [[PromiseValue]]: "abc"}。*/

Promise 对象

		let p = new Promise(function(resolve,reject){
			resolve("abc");
		});
		
		p.then(function(data){
			console.log(data);//打印abc。
		});
		//async函数里面的返回值传递给then方法
		async function fn(){
			return "123";
		}
		let p1 = fn();
		p1.then(function(data){
			console.log(data);//打印123.
		});
		
		//async函数用来处理异步
		
		function one(){
			return new Promise(function(resolve,reject){
				setTimeout(function(){
				console.log("one_3000");
				resolve("one_3000");
				},3000);
			})
			
		}

---

		function two(){
			return new Promise(function(resolve,reject){
				setTimeout(function(){
				console.log("two_2000");
				resolve("two_2000");
				},2000);
			})
			
		}
		
		//await只能出现在异步函数里面, 
		async function shunxu(){
			console.log("start");
			let r1 = await one();
			console.log(r1);
			let r2 = await two();
			console.log(r2);
			return "end";
		}
		let p3 = shunxu();
		p3.then(r=>{
			console.log("结束");
		});
		
		//先打印start，三秒后打印两次one_3000，打印完one_3000然后隔两秒打印两次two_2000和结束；