then
then
函数的会接管两个回调函数,一个是 onFulfilled
函数,一个是 onRejected
函数
如果这两个回调函数没有写返回值,默认会 return undefined;
进入下一个函数的 onFulfilled
函数中
const p = new Promise((resolve, reject) => {resolve(22);
});
p.then((success1) => {console.log("success1", success1); // "success2" 22
// 没写 return 默认返回 return undefined
},
(err1) => {console.log("err1", err1);
}
).then((success2) => {console.log("success2", success2); // "success2" undefined
},
(err2) => {console.log("err2", err2);
}
);
在这两个回调中 return xxx
,相当于调用 return new Promise((resolve) => resolve(xxx));
const p = new Promise((resolve, reject) => {resolve(22);
});
p.then((success1) => {return new Promise((resolve, reject) => resolve("success"));
// 等价于
return "success";
},
(err1) => {console.log("err1", err1);
}
).then((success2) => {console.log("success2", success2); // "success2" "success"
},
(err2) => {console.log("err2", err2);
}
);
在 onFulfilled
函数和 onRejected
函数中默认返回的都是胜利,如果须要返回失败须要显示调用 reject
或者用 throw
抛出谬误能够
const p = new Promise((resolve, reject) => {resolve(22);
});
p.then((success1) => {return new Promise((resolve, reject) => reject("error"));
// 等价于
// throw "error";
},
(err1) => {console.log("err1", err1);
}
).then((success2) => {console.log("success2", success2);
},
(err2) => {console.log("err2", err2); // "err2" error
}
);
总结:在 then
回调中 return xxx
会被主动包装成 return new Promise((resolve) => resolve(xxx));
,
catch
catch
是用来解决 rejected
状态,是 then
函数的一种特例,相当于 then(null, (err) => {});
catch
为什么能捕捉后面的谬误?
在 onRejected
函数中,如果没有显示抛出谬误,默认会 return undefined;
进入一下个 onFulfilled
函数
finally
finally
不论以后 promise
是什么状态都会执行,也是 then
函数的一种特例,相当于 `then(result => result, err => new Promsie((\_, reject) => reject(err)));
Promise.resolve
Promise.resolve
是 Promise
在 fulfilled
状态时的简写,相当于 new Promise(resolve => resolve(xxx))
onFulfilled
函数接管参数
-
当参数是一般参数时,会间接传递给前面
then
函数new Promise((resolve) => resolve({name: "uccs"})).then((data) => {console.log(data); });
-
当参数是
Promise
对象时,前面的then
会依据传递的Promise
对象的状态变动执行哪一个回调const p = new Promise((resolve, reject) => {setTimeout(resolve, 1000, "我执行了"); }); new Promise((resolve) => resolve(p)).then((data) => {console.log(data); });
-
具备
then
办法的对象-
用这种这种形式,如果须要扭转
Promise
状态是,不能应用return
模式,这个then
办法也是有两个回调函数的:onFulfilled
和onRejected
const thenable = {then(resolve, reject) {console.log("thenable"); }, }; new Promise((resolve) => resolve(thenable)).then((data) => {console.log(data); });
-
Promise.reject
Promise.reject
是 Promise
在 rejected
状态是的简写,相当于 new Promise((_, reject) => reject(xxx))
onRejected
函数不论接管什么参数,都会一成不变的向后传递,作为后续办法的参数