js Promise

Promise抽象的表示一个异步操作

三状态

pending 最初状态

fulfilled 成功状态 返回的状态码 200-299

rejected 拒绝状态 

        let p=new Promise((resolve,reject)=>resolve());
        setTimeout(console.log,0,p);

调用resolve会切换到fulfilled状态

调用reject会切换到rejected状态，并抛出错误

        //1秒后再输出a
        let p=new Promise((resolve,reject)=>setTimeout(resolve,1000))
        p.then(()=>console.log('a'))
        console.log('b')