Promise

三种状态

pending 初始状态
fulfilled 成功状态
rejected 拒绝状态

初始状态

let p = new Promise(() => {})
console.log(p);

成功状态

let p = new Promise((resolve) => {
    resolve()
})
console.log(p);

拒绝状态

let p = new Promise((resolve, reject) => {
    reject()
})
console.log(p);

单一状态与状态中转

单一状态：Promise的状态变为fulfilled或resolved不会再改变

let p = new Promise((resolve, reject) => {
    resolve()
    reject()
})
console.log(p);

状态中转：resolve()接收一个Promise时会按照接收的状态传递

let rej = new Promise((resolve, reject) => {
    reject()
})
let p = new Promise((resolve, reject) => {
    resolve(rej)
})
console.log(rej);
console.log(p);

方法

then
catch
finally
done
resolve
reject
all
allSettled
race
any
try

1.then(callback1,callback2)

接收resolve或reject的返回值，根据不同状态调用回调函数
then返回的也是Promise，处理后fulfilled，报错为rejected

let p = new Promise((resolve, reject) => {
    reject("拒绝")
})
let p1 = p.then(null, res => {
    console.log(res);
})
console.log(p1);

then返回一个Promise可以继续处理

new Promise((resolve, reject) => {
    reject("拒绝")
}).then(null, err => {
    console.log(err);
    return err
}).then(err => {
    console.log(err);
})
//拒绝
//拒绝

2.catch

等价于 .then(null,rejection) 用于指定发生错误时的回调函数

new Promise((resolve, reject) => {
    try {
        throw new Error('错误');
    }
    catch (e) {
        reject(e)
    }//等价于 reject(new Error("错误"))
}).then(val => {
    console.log(val);
}).catch(res => {
    console.log(res);
})

catch捕获try中抛出的错误，然后由reject改变状态并传递
错误具有"冒泡性质"，会向下传递，直到被catch捕获
支持then处理resolve,catch处理reject，catch可以去捕获then中的错误，类似于try/catch的写法
resolve后再抛出错误不会被捕获，也不会传递到外层代码

new Promise((resolve, reject) => {
    resolve("接受")
    throw new Error('错误');
}).then(val => {
    console.log(val); //接受
}).catch(res => {
    console.log(res);
})

3.finally

无论Promise对象最后状态是fulfilled还是rejected，都会执行其中的回调函数
finally的实现：

Promise.prototype.finally = function (callback) {
    let P = this.constructor
    return this.then(
        value => P.resolve(callback()).then(()=>value),
        reason => P.resolve(callback()).then(()=>{throw reason})
    )
}

new Promise((resolve, reject) => {
    reject("错误")
}).finally(() => {
    console.log(1); //1
}).catch(res => {
    console.log(res);//错误
})

可以看出finally执行一个普通的回调函数，状态会向下传递

4.done

最后的错误无法捕获，可以用done抛出一个全局错误
done的实现

Promise.prototype.done = function (onFulfilled,onRejected) {
    this.then(onFulfilled,onRejected)
    .catch(function(reason){
        setTimeout(()=>{throw reason},0)
    })
}

new Promise((resolve, reject) => {
    reject(0)
}).done(val => {
    console.log(val);
}, res => {
    console.log(res);
})

done也可以像then一样使用

5.Promise.resolve() 与 Promise.reject()

将一个对象转为Promise对象
以Promise.resolve(参数)为例

Promise.resolve("foo")
//等价于
new Promise(resolve => resolve("foo"))

参数为Promise的实例

Promise.resolve将不做任何修改

let p = new Promise((resolve, reject) => {
  reject("错误")
})
let p1 = Promise.resolve(p)
console.log(p1);

参数是一个thenable对象，有then方法的对象
Promise.resolve()会将这个对象转成Promise对象

然后立即执行thenable中的then方法

let thenable = {
  then: function (resolve, reject) {
      resolve("接受")
  }
}
Promise.resolve(thenable)
  .then(val => {
      console.log(val);//接受
  })

参数没有then方法或不是对象

Promise.resolve()返回一个新的Promise对象，状态为fulfilled

let obj = {
  a: 0
}
Promise.resolve(obj)
  .then(val => {
      console.log(val.a);//a
  })

不带任何参数
直接返回一个fulfilled状态的Promise对象
```
Promise.resolve()
  .then(() => {
      console.log("接受状态");//接受转态
  })
```
Promise.reject()也会返回一个新的Promise实例，状态为rejected
与Promise.resolve()不同的是，Promise.reject()的参数会原封不动的传递

6.Promise.all()与Promise.allSettled()

Promise.all可以将多个Promise实例包装成一个新的Promise实例
如果不是则会调用Promise.resolve方法，转为Promise实例
参数为iterable类型

Promise.all返回的Promise的状态取决于参数的状态，全部fulfilled为fulfilled，有一个rejected返回第一个rejected

let p1 = Promise.resolve("成功")
let p2 = "foo"
let p3 = Promise.reject("失败1")
let p4 = Promise.reject("失败2")
  .catch(res => {
      console.log("自己的" + res);//自己的失败2
      return res
  })

Promise.all([p1, p2, p4])
  .then(val => {
      console.log(val);//["成功","foo","失败2"]
  })

Promise.all([p2, p4, p3])
  .then(val => {
      console.log(val);
  })
  .catch(res => {
      console.log("all的" + res);//all的失败1
  })

参数有自己的catch时，状态为fulfilled
参数中有一个rejected则会调用all上的catch
Promise.allSettled会返回所有参数的状态与返回值

let p1 = Promise.resolve("成功")
let p2 = "foo"
let p3 = Promise.reject("失败1")
let p4 = Promise.reject("失败2")
    .catch(res => {
        console.log("自己的" + res);
        return res
    })
Promise.allSettled([p1, p2, p4])
    .then(val => {
        console.log(val);
    })
Promise.allSettled([p2, p4, p3])
    .then(val => {
        console.log(val);
    })

7.Promise.race()

相较于Promise.all Promise.race会返回一个fulfilled或rejected的参数
Promise.race返回的Promise的状态由跑的最快的Promise决定的

let p1 = new Promise((resolve, reject) => {
    setTimeout(resolve, 1000, "p1接受")
})
let p2 = new Promise((resolve, reject) => {
    setTimeout(reject, 1500, "p2拒绝")
})

Promise.race([p1, p2])
    .then(val => {
        console.log(val);//p1接受
    })
    .catch(res => {
        console.log(res);
    })

8.Promise.any()

Promise.any正好与Promise.all相反，有一个fulfilled状态就为fulfilled

let p1 = new Promise((resolve, reject) => {
  setTimeout(resolve, 2000, "p1接受")
})
let p2 = new Promise((resolve, reject) => {
  setTimeout(reject, 1500, "p2拒绝")
})
let p3 = new Promise((resolve, reject) => {
  setTimeout(resolve, 1000, "p3接受")
})
Promise.any([p1, p2, p3])
  .then(val => {
      console.log(val);//p3接受
  })
  .catch(res => {
      console.log(res);
  })

如果全部为rejected则会抛出一个全部拒绝的错误

Promise.any([p2])
    .then(val => {
        console.log(val);
    })
    .catch(res => {
        console.log(res);//AggregateError: All promises were rejected
    })

9.Promise.try()

让同步函数同步执行
两种实现方式

//方式一
Promise.try = function (func) {
    return (async () => func())()
}
//方式二
Promise.try = function (func) {
    return new Promise((resolve, reject) => {
        resolve(func())
    })
}

func()如果是同步函数会立即执行，如果是异步函数会将返回值或错误交给then处理

let f = () => console.log("now");
Promise.try(f)
console.log("next");
//now
//next

【js】Promise上的一些方法