状态

pending 等待
resolved 成功
rejected 失败

        var p = new Promise((resolve,reject)=>{
            //
            setTimeout(()=>{
                resolve()
            })
        })
        setTimeout(()=>{
            console.log(p)
        })
        console.log(p)

then和catch的返回值

then和catch返回值
1.都会返回promise
2.正常情况下状态为resolve，遇到错误状态为rejected

        var p = Promise.resolve(100).then(res=>{
            console.log(res)
            throw "err"
        })
        p.then(()=>{
            console.log(200)
        }).catch(err=>{
            console.log(err)
        }).then(()=>{
            console.log(300)
        })
    </script>

异常处理

         (async () => {
             var p = Promise.reject("err");
             try {
                 let res = await p;
                 console.log(res)
             }catch(err){
                 console.log(err)
             }
         })()

        Promise.reject("err").catch((err)=>{
            console.log(err)
        })