手写promise之分步解析

promise是es6推出适用于异步请求的构造函数，帮助解决回调地狱的问题，以下内容将自定义实现promise，只包括基本使用，所以一些边界情况考虑没有在内。
如果对promise用法还不熟悉的朋友可移步
Promise的理解与使用(一)
Promise的理解和使用(二)

executor

首先建立promise的基本结构

定义构造函数

promise的executor部分是放到主线程中直接执行的

class icePromise {
    constructor(executor){
        const resolve = () => {
            console.log('resolve被调用')
        }
        const reject = () => {
            console.log('reject被调用')
        }
    }
    executor(resolve, reject)
}

const promise = new icePromise((resolve, reject)=>{
    resolve()
})

定义状态

1、定义常量

const STATUS_PENDING = 'pending'
const STATUS_FULFILLED = 'fulfilled'
const STATUS_REJECTED = 'rejected'

2、通过构造函数创建实例时，就应该需要状态，所以在类中定义

this.status = STATUS_PENDING

3、resolve和reject方法里通过状态判断
当为pending时，才能执行resolve或者reject方法，执行前先修改状态

then方法

onFulfilled/onRejected中要执行then里面的回调函数，将两个函数绑定到实例的属性上
1、在类中定义一个then方法

then(onFulfilled, onRejected){
    this.onFulfilled = onFulfilled
    this.onRejected = onRejected
}

2、在resolve和reject中分别执行this.onFulfilled和this.onRejected中
此时会报错，因为executor会立刻执行，而then里面的函数是微任务，
会在主线程执行完成之后执行

3、resolve和reject中加入 queueMicrotask (微任务)

整体架构的初版就完成了

const STATUS_PENDING = "pending";
const STATUS_FULFILLED = "fulfilled";
const STATUS_REJECTED = "rejected";

class IcePromise {
  constructor(executor) {
    this.status = STATUS_PENDING;
    
    const resolve = (value) => {
      if (this.status === STATUS_PENDING) {
        this.status = STATUS_FULFILLED;
        queueMicrotask(() => {
          this.onFulfilled(value);
        });
      }
    };
    
    const reject = (reason) => {
      if (this.status === STATUS_PENDING) {
        this.status = STATUS_REJECTED;
        queueMicrotask(() => {
          this.onRejected(reason);
        });
      }
    };
    
    executor(resolve, reject);
  }
  
  then(onFulfilled, onRejected) {
    this.onFulfilled = onFulfilled;
    this.onRejected = onRejected;
  }
}

const promise = new IcePromise((resolve, reject) => {
  resolve("resolve");
  reject("reject");
});

promise.then(
  (value) => {
    console.log("success1", value);
  },
  (reason) => {
    console.log("fail1", reason);
  }
);

promise.then(
  (value) => {
    console.log("success2", value);
  },
  (reason) => {
    console.log("fail2", reason);
  }
);

返回两次promise的状态，只执行第一个resolve，then方法中对应执行的成功或者失败的函数也正确

1_promise的基本设计.png

但存在的问题是，执行两次then方法，只执行了第二个，以下对then方法进行优化。

then

解决多次调用then方法的问题

1、constructor中定义变量用于收集所有的成功/失败的回调函数

this.onFulfilledCallbacks = []
this.onRejectedCallbacks = []

2、在then方法中通过push添加到数组中

this.onFulfilledCallbacks.push()
this.onRejectedCallbacks.push()

3、在resolve和reject中遍历
this.onFulfilledCallbacks和this.onRejectedCallbacks中的方法

此时代码如下

const STATUS_PENDING = "pending";
const STATUS_FULFILLED = "fulfilled";
const STATUS_REJECTED = "rejected";

class IcePromise {
  constructor(executor) {
    this.status = STATUS_PENDING;
    this.onResolvedCallbacks = [];
    this.onRejectedCallbacks = [];
    
    const resolve = (value) => {
      if (this.status === STATUS_PENDING) {
        queueMicrotask(() => {
          this.onResolvedCallbacks.forEach((fn) => {
            fn(value);
          });
        });
      }
    };
    
    const reject = (reason) => {
      if (this.status === STATUS_PENDING) {
        queueMicrotask(() => {
          this.onRejectedCallbacks.forEach((fn) => {
            fn(reason);
          });
        });
      }
    };
    executor(resolve, reject);
  }
  
  then(onFulfilled, onRejected) {
      this.onResolvedCallbacks.push(onFulfilled);
      this.onRejectedCallbacks.push(onRejected);
  }
}

// 测试代码
const promise = new IcePromise((resolve, reject) => {
  resolve("resolve---");
  reject("----reject");
});
promise.then(
  (value) => {
    console.log("res1", value);
  },
  (reason) => {
    console.log("err1", reason);
  }
)
promise.then(
  (value) => {
    console.log("res2", value);
  },
  (reason) => {
    console.log("err2", reason);
  }
);
// 确定状态后再调用
setTimeout(() => {
  promise.then(
    (res) => {
      console.log("res3", res);
    },
    (err) => {
      console.log("err3", err);
    }
  );
}, 1000);

解决then的多次调用的问题，但仍有其它的问题，一个是resolve和reject方法同时执行，二是通过定时器延迟执行的promise的then方法没有输出响应结果

2_then多次调用.png

解决延迟调用的问题

1、保存value和reason

this.value = undefined
this.reason = undefined

resolve和reject方法分别给this.value和this.reason赋值

2、then方法中进行状态的判断
当状态为pending时，继续向onFulfilledCallbacks、onRejectedCallbacks数组中添加函数；当状态不为pending时，直接执行onFulfilled或onRejected方法

if (this.status === STATUS_FULFILLED && onFulfilled) {
  onFulfilled(this.value);
}
if (this.status === STATUS_REJECTED && onRejected) {
  onRejected(this.reason);
}

if (this.status === STATUS_PENDING) {
  this.onResolvedCallbacks.push(onFulfilled);
  this.onRejectedCallbacks.push(onRejected);
}

3、pending状态的变化
① queueMicrotask中判断不为pending则return
② 修改pending状态

const STATUS_PENDING = "pending";
const STATUS_FULFILLED = "fulfilled";
const STATUS_REJECTED = "rejected";

class IcePromise {
  constructor(executor) {
    this.status = STATUS_PENDING;
    this.onResolvedCallbacks = [];
    this.onRejectedCallbacks = [];
    this.value = undefined;
    this.reason = undefined;
    
    const resolve = (value) => {
      if (this.status === STATUS_PENDING) {
        queueMicrotask(() => {
          if (this.status !== STATUS_PENDING) return;
          this.status = STATUS_FULFILLED;
          this.value = value;
          this.onResolvedCallbacks.forEach((fn) => {
            fn(this.value);
          });
        });
      }
    };
    
    const reject = (reason) => {
      if (this.status === STATUS_PENDING) {
        queueMicrotask(() => {
          if (this.status !== STATUS_PENDING) return;
          this.status = STATUS_REJECTED;
          this.reason = reason;
          this.onRejectedCallbacks.forEach((fn) => {
            fn(this.reason);
          });
        });
      }
    };
    executor(resolve, reject);
  }
  
  then(onFulfilled, onRejected) {
    if (this.status === STATUS_FULFILLED && onFulfilled) {
      onFulfilled(this.value);
    }
    if (this.status === STATUS_REJECTED && onRejected) {
      onRejected(this.reason);
    }
    if (this.status === STATUS_PENDING) {
      this.onResolvedCallbacks.push(onFulfilled);
      this.onRejectedCallbacks.push(onRejected);
    }
  }
}

// 测试代码
const promise = new IcePromise((resolve, reject) => {
  resolve("resolve---");
  reject("----reject");
});

promise.then(
  (value) => {
    console.log("res1", value);
  },
  (reason) => {
    console.log("err1", reason);
  }
)

promise.then(
  (value) => {
    console.log("res2", value);
  },
  (reason) => {
    console.log("err2", reason);
  }
);

// 确定状态后再调用
setTimeout(() => {
  promise.then(
    (res) => {
      console.log("res3", res);
    },
    (err) => {
      console.log("err3", err);
    }
  );
}, 1000);

promise.then(
  (value) => {
    console.log("res4", value);
  },
  (reason) => {
    console.log("err4", reason);
  }
).then(
  (value) => {
    console.log("res5", value);
  },
  (reason) => {
    console.log("err5", reason);
  }
)

解决了resolve和reject多次调用及计时器延迟调用的问题，但此时发现then无法进行链式调用

3_then延迟调用.png

解决链式调用的问题

1、then方法里返回一个 new icePromise，将判断逻辑放进去
2、this.onFulfilledCallbacks 和 this.onRejectedCallbacks 传入回调函数，
回调函数返回resolve或者reject函数的执行结果
3、封装工具函数，用于处理try catch

const STATUS_PENDING = "pending";
const STATUS_FULFILLED = "fulfilled";
const STATUS_REJECTED = "rejected";
const respondWithCatchError = (fn, value, resolve, reject) => {
  try {
    const result = fn(value);
    resolve(result);
  } catch (error) {
    reject(error);
  }
};

class IcePromise {
  constructor(executor) {
    this.status = STATUS_PENDING;
    this.onResolvedCallbacks = [];
    this.onRejectedCallbacks = [];
    this.value = undefined;
    this.reason = undefined;
    
    const resolve = (value) => {
      if (this.status === STATUS_PENDING) {
        queueMicrotask(() => {
          if (this.status !== STATUS_PENDING) return;
          this.status = STATUS_FULFILLED;
          this.value = value;
          this.onResolvedCallbacks.forEach((fn) => {
            fn(this.value);
          });
        });
      }
    };
    
    const reject = (reason) => {
      if (this.status === STATUS_PENDING) {
        queueMicrotask(() => {
          if (this.status !== STATUS_PENDING) return;
          this.status = STATUS_REJECTED;
          this.reason = reason;
          this.onRejectedCallbacks.forEach((fn) => {
            fn(this.reason);
          });
        });
      }
    };
    executor(resolve, reject);
  }
  
  then(onFulfilled, onRejected) {
    return new Promise((resolve, reject) => {
      if (this.status === STATUS_FULFILLED && onFulfilled) {
        respondWithCatchError(onFulfilled, this.value, resolve, reject);
      }
      if (this.status === STATUS_REJECTED && onRejected) {
        respondWithCatchError(onRejected, this.reason, resolve, reject);
      }
      if (this.status === STATUS_PENDING) {
        this.onResolvedCallbacks.push(() => {
          respondWithCatchError(onFulfilled, this.value, resolve, reject);
        });
        this.onRejectedCallbacks.push(() => {
          respondWithCatchError(onRejected, this.reason, resolve, reject);
        });
      }
    });
  }
}
// 测试代码
const promise = new IcePromise((resolve, reject) => {
  resolve("resolve---");
  reject("----reject");
});
promise
  .then(
    (value) => {
      console.log("res1", value);
    },
    (reason) => {
      console.log("err1", reason);
    }
  )
  .then(
    (value) => {
      console.log("res2", value);
    },
    (reason) => {
      console.log("err2", reason);
    }
  )
  .then(
    (res) => {
      console.log("res3", res);
    },
    (err) => {
      console.log("err3", err);
    }
  );

此时then函数就已经可以链式调用啦，基本功能已经实现~

4_then链式调用.png

catch

catch函数接收一个失败的回调
1、调用then方法，将onRejected方法加到第二个promise的回调中

catch(onRejected){
    this.then(null, onRejected)
}

2、then方法中对传入的 onRejected进行判断，当没有传递时，就抛出异常

const defaultOnRejected = (reason) => {
  throw reason;
};
onRejected = onRejected || defaultOnRejected;

整体实现如下

const STATUS_PENDING = "pending";
const STATUS_FULFILLED = "fulfilled";
const STATUS_REJECTED = "rejected";

const respondWithCatchError = (fn, value, resolve, reject) => {
  try {
    const result = fn(value);
    resolve(result);
  } catch (error) {
    reject(error);
  }
};

class IcePromise {
  constructor(executor) {
    this.status = STATUS_PENDING;
    this.onResolvedCallbacks = [];
    this.onRejectedCallbacks = [];
    this.value = undefined;
    this.reason = undefined;

    const resolve = (value) => {
      if (this.status === STATUS_PENDING) {
        queueMicrotask(() => {
          if (this.status !== STATUS_PENDING) return;
          this.status = STATUS_FULFILLED;
          this.value = value;
          this.onResolvedCallbacks.forEach((fn) => {
            fn(this.value);
          });
        });
      }
    };

    const reject = (reason) => {
      if (this.status === STATUS_PENDING) {
        queueMicrotask(() => {
          if (this.status !== STATUS_PENDING) return;
          this.status = STATUS_REJECTED;
          this.reason = reason;
          this.onRejectedCallbacks.forEach((fn) => {
            fn(this.reason);
          });
        });
      }
    };

    executor(resolve, reject);
  }

  then(onFulfilled, onRejected) {
    const defaultOnFulfilled = (value) => {
      return value;
    };
    const defaultOnRejected = (reason) => {
      throw reason;
    };
    onFulfilled = onFulfilled || defaultOnFulfilled;
    onRejected = onRejected || defaultOnRejected;

    return new Promise((resolve, reject) => {
      if (this.status === STATUS_FULFILLED && onFulfilled) {
        respondWithCatchError(onFulfilled, this.value, resolve, reject);
      }
      if (this.status === STATUS_REJECTED && onRejected) {
        respondWithCatchError(onRejected, this.reason, resolve, reject);
      }

      if (this.status === STATUS_PENDING) {
        this.onResolvedCallbacks.push(() => {
          respondWithCatchError(onFulfilled, this.value, resolve, reject);
        });
        this.onRejectedCallbacks.push(() => {
          respondWithCatchError(onRejected, this.reason, resolve, reject);
        });
      }
    });
  }

  catch(onRejected) {
    this.then(null, onRejected);
  }
}

const promise = new IcePromise((resolve, reject) => {
  reject("----reject");
  resolve("resolve---");
});

// 测试代码
promise
  .then((value) => {
    console.log("res1", value);
  })
  .then((value) => {
    console.log("res2", value);
  })
  .catch((error) => {
    console.log("catch", error);
  });

执行结果如下

5_catch方法.png

finally

finally方法无论执行resolve或者reject的方法后都会执行

finally(onFinally){
    this.then(()=>{
        onFinally()
    }, ()=>{
        onFinally()
    })
}

整体实现如下

const STATUS_PENDING = "pending";
const STATUS_FULFILLED = "fulfilled";
const STATUS_REJECTED = "rejected";

const respondWithCatchError = (fn, value, resolve, reject) => {
  try {
    const result = fn(value);
    resolve(result);
  } catch (error) {
    reject(error);
  }
};

class IcePromise {
  constructor(executor) {
    this.status = STATUS_PENDING;
    this.onResolvedCallbacks = [];
    this.onRejectedCallbacks = [];
    this.value = undefined;
    this.reason = undefined;
    
    const resolve = (value) => {
      if (this.status === STATUS_PENDING) {
        queueMicrotask(() => {
          if (this.status !== STATUS_PENDING) return;
          this.status = STATUS_FULFILLED;
          this.value = value;
          this.onResolvedCallbacks.forEach((fn) => {
            fn(this.value);
          });
        });
      }
    };
    
    const reject = (reason) => {
      if (this.status === STATUS_PENDING) {
        queueMicrotask(() => {
          if (this.status !== STATUS_PENDING) return;
          this.status = STATUS_REJECTED;
          this.reason = reason;
          this.onRejectedCallbacks.forEach((fn) => {
            fn(this.reason);
          });
        });
      }
    };
    executor(resolve, reject);
  }
  
  then(onFulfilled, onRejected) {
    const defaultOnFulfilled = (value) => {
      return value;
    };
    const defaultOnRejected = (reason) => {
      throw reason;
    };
    onFulfilled = onFulfilled || defaultOnFulfilled;
    onRejected = onRejected || defaultOnRejected;
    return new Promise((resolve, reject) => {
      if (this.status === STATUS_FULFILLED && onFulfilled) {
        respondWithCatchError(onFulfilled, this.value, resolve, reject);
      }
      if (this.status === STATUS_REJECTED && onRejected) {
        respondWithCatchError(onRejected, this.reason, resolve, reject);
      }
      if (this.status === STATUS_PENDING) {
        this.onResolvedCallbacks.push(() => {
          respondWithCatchError(onFulfilled, this.value, resolve, reject);
        });
        this.onRejectedCallbacks.push(() => {
          respondWithCatchError(onRejected, this.reason, resolve, reject);
        });
      }
    });
  }
  
  catch(onRejected) {
    this.then(null, onRejected);
  }
  
  finally(onFinally) {
    this.then(
      () => {
        onFinally();
      },
      () => {
        onFinally();
      }
    );
  }
}

// 测试代码
const promise = new IcePromise((resolve, reject) => {
  reject("----reject");
});
promise
  .then(
    (value) => {
      console.log("res1", value);
    },
    (reason) => {
      console.log("err1", reason);
    }
  )
  .finally(() => {
    console.log("finally");
  });

6_finally方法.png

resolve/reject

resolve和reject是Promise的类方法，也可以通过调用then方法来实现

static resolve(value){
    return new icePromise((resolve)=>resolve(value))
}

static reject(reason){
    return new icePromise((resolve, reject)=>reject(reason))
}

完整实现如下

const STATUS_PENDING = "pending";
const STATUS_FULFILLED = "fulfilled";
const STATUS_REJECTED = "rejected";

const respondWithCatchError = (fn, value, resolve, reject) => {
  try {
    const result = fn(value);
    resolve(result);
  } catch (error) {
    reject(error);
  }
};

class IcePromise {
  constructor(executor) {
    this.status = STATUS_PENDING;
    this.onResolvedCallbacks = [];
    this.onRejectedCallbacks = [];
    this.value = undefined;
    this.reason = undefined;
    
    const resolve = (value) => {
      if (this.status === STATUS_PENDING) {
        queueMicrotask(() => {
          if (this.status !== STATUS_PENDING) return;
          this.status = STATUS_FULFILLED;
          this.value = value;
          this.onResolvedCallbacks.forEach((fn) => {
            fn(this.value);
          });
        });
      }
    };
    
    const reject = (reason) => {
      if (this.status === STATUS_PENDING) {
        queueMicrotask(() => {
          if (this.status !== STATUS_PENDING) return;
          this.status = STATUS_REJECTED;
          this.reason = reason;
          this.onRejectedCallbacks.forEach((fn) => {
            fn(this.reason);
          });
        });
      }
    };
    executor(resolve, reject);
  }
  
  then(onFulfilled, onRejected) {
    const defaultOnFulfilled = (value) => {
      return value;
    };
    const defaultOnRejected = (reason) => {
      throw reason;
    };
    onFulfilled = onFulfilled || defaultOnFulfilled;
    onRejected = onRejected || defaultOnRejected;
    return new Promise((resolve, reject) => {
      if (this.status === STATUS_FULFILLED && onFulfilled) {
        respondWithCatchError(onFulfilled, this.value, resolve, reject);
      }
      if (this.status === STATUS_REJECTED && onRejected) {
        respondWithCatchError(onRejected, this.reason, resolve, reject);
      }
      if (this.status === STATUS_PENDING) {
        this.onResolvedCallbacks.push(() => {
          respondWithCatchError(onFulfilled, this.value, resolve, reject);
        });
        this.onRejectedCallbacks.push(() => {
          respondWithCatchError(onRejected, this.reason, resolve, reject);
        });
      }
    });
  }
  
  catch(onRejected) {
    this.then(null, onRejected);
  }
  
  finally(onFinally) {
    this.then(
      () => {
        onFinally();
      },
      () => {
        onFinally();
      }
    );
  }
  
  static resolve(value) {
    return new Promise((onResolve) => {
      onResolve(value);
    });
  }
  
  static reject(reason) {
    return new Promise((onResolve, onRejected) => {
      onRejected(reason);
    });
  }
}

// 测试代码
const promise = Promise.reject(1);
promise
  .then(
    (value) => {
      console.log("res1", value);
    },
    (reason) => {
      console.log("err1", reason);
    }
  )

执行结果如下

7_resolve方法.png

all/allSettled

all和allSettled方法都是promise的类方法
1、all方法
只要有一个promise执行reject的方法就会执行reject，当所有promise都返回resolve时，才执行resolve方法。
2、allSettled方法
当所有promise都执行完成时，才执行resolve方法，返回所有promise的状态和结果。

static all(promise){
    return new icePromise((resolve, reject)=>{
      const values = []
      promises.forEach(promise => {
        promise.then(res => {
          values.push(res)
          if (values.length === promises.length) {
            resolve(values)
          }
        }, err => {
          reject(err)
        })
      })
    })
    })
}

static allSettled(promise){
    return new icePromise((resolve, reject)=>{
        const values = []
        promise.then(res=>{
            values.push({ status: '', value: '' })
            if(values.length === promise.length){
                resolve(values)
            }
        }, err=>{
            values.push({ status: '', value: '' })
            if(values.length === promise.length){
                resolve(values)
            }
        })
    })
}

完整实现如下

const STATUS_PENDING = "pending";
const STATUS_FULFILLED = "fulfilled";
const STATUS_REJECTED = "rejected";
const respondWithCatchError = (fn, value, resolve, reject) => {
  try {
    const result = fn(value);
    resolve(result);
  } catch (error) {
    reject(error);
  }
};
class IcePromise {
  constructor(executor) {
    this.status = STATUS_PENDING;
    this.onResolvedCallbacks = [];
    this.onRejectedCallbacks = [];
    this.value = undefined;
    this.reason = undefined;
    const resolve = (value) => {
      if (this.status === STATUS_PENDING) {
        queueMicrotask(() => {
          if (this.status !== STATUS_PENDING) return;
          this.status = STATUS_FULFILLED;
          this.value = value;
          this.onResolvedCallbacks.forEach((fn) => {
            fn(this.value);
          });
        });
      }
    };
    const reject = (reason) => {
      if (this.status === STATUS_PENDING) {
        queueMicrotask(() => {
          if (this.status !== STATUS_PENDING) return;
          this.status = STATUS_REJECTED;
          this.reason = reason;
          this.onRejectedCallbacks.forEach((fn) => {
            fn(this.reason);
          });
        });
      }
    };
    executor(resolve, reject);
  }
  then(onFulfilled, onRejected) {
    const defaultOnFulfilled = (value) => {
      return value;
    };
    const defaultOnRejected = (reason) => {
      throw reason;
    };
    onFulfilled = onFulfilled || defaultOnFulfilled;
    onRejected = onRejected || defaultOnRejected;
    return new Promise((resolve, reject) => {
      if (this.status === STATUS_FULFILLED && onFulfilled) {
        respondWithCatchError(onFulfilled, this.value, resolve, reject);
      }
      if (this.status === STATUS_REJECTED && onRejected) {
        respondWithCatchError(onRejected, this.reason, resolve, reject);
      }
      if (this.status === STATUS_PENDING) {
        this.onResolvedCallbacks.push(() => {
          respondWithCatchError(onFulfilled, this.value, resolve, reject);
        });
        this.onRejectedCallbacks.push(() => {
          respondWithCatchError(onRejected, this.reason, resolve, reject);
        });
      }
    });
  }
  catch(onRejected) {
    this.then(null, onRejected);
  }
  finally(onFinally) {
    this.then(
      () => {
        onFinally();
      },
      () => {
        onFinally();
      }
    );
  }
  static resolve(value) {
    return new Promise((onResolve) => {
      onResolve(value);
    });
  }
  static reject(reason) {
    return new Promise((onResolve, onRejected) => {
      onRejected(reason);
    });
  }
  static all(promises) {
    return new Promise((resolve, reject) => {
      const result = [];
      promises.forEach((promise) => {
        promise.then(
          (value) => {
            result.push(value);
            if (result.length === promises.length) {
              resolve(result);
            }
          },
          (reason) => {
            reject(reason);
          }
        );
      });
    });
  }
  static allSettled(promises) {
    return new Promise((resolve, reject) => {
      const result = [];
      promises.forEach((promise) => {
        promise.then(
          (value) => {
            result.push({
              status: STATUS_FULFILLED,
              value,
            });
            if (result.length === promises.length) {
              resolve(result);
            }
          },
          (reason) => {
            result.push({
              status: STATUS_REJECTED,
              reason,
            });
            if (result.length === promises.length) {
              resolve(result);
            }
          }
        );
      });
    });
  }
}

// 测试代码
const promise1 = IcePromise.resolve(1);
const promise2 = new IcePromise((resolve, reject) => {
  setTimeout(() => {
    reject(2);
  });
});
const promise3 = IcePromise.resolve(3);

IcePromise.all([promise1, promise2, promise3]).then(
  (value) => {
    console.log("res1", value);
  },
  (reason) => {
    console.log("err1", reason);
  }
);
IcePromise.allSettled([promise1, promise2, promise3]).then(
  (value) => {
    console.log("res2", value);
  },
  (reason) => {
    console.log("err2", reason);
  }
);

执行结果如下

8_all方法.png

race/any

race和any都是promise的类方法。
1、race方法
只要有一个promise执行完成，就会返回它所执行的结果

2、any方法
① 有fulfilled状态，会等到这个fullfilled执行完成，执行resolve，结果为value
② 如果所有的Promise都是reject的，那么也会等到所有的Promise都变成rejected状态后报一个AggregateError的错误。

static race(promises){
    return new icePromise((resolve, reject)=>{
        promises.forEach(promise=>{
            promise.then(resolve, reject)        
        })
    })
}

static any(promises){
    const reasons = []
    return new icePromise((resolve, reject)=>{
        promises.forEach(promise=>{
            promise.then(resolve, err=>{
                reasons.push(err)
                if(reasons.length === promises.length){
                    reject(new AggregateError(reasons))
                }
            })
        })
    })
}

整体实现如下

const STATUS_PENDING = "pending";
const STATUS_FULFILLED = "fulfilled";
const STATUS_REJECTED = "rejected";
const respondWithCatchError = (fn, value, resolve, reject) => {
  try {
    const result = fn(value);
    resolve(result);
  } catch (error) {
    reject(error);
  }
};
class IcePromise {
  constructor(executor) {
    this.status = STATUS_PENDING;
    this.onResolvedCallbacks = [];
    this.onRejectedCallbacks = [];
    this.value = undefined;
    this.reason = undefined;
    const resolve = (value) => {
      if (this.status === STATUS_PENDING) {
        queueMicrotask(() => {
          if (this.status !== STATUS_PENDING) return;
          this.status = STATUS_FULFILLED;
          this.value = value;
          this.onResolvedCallbacks.forEach((fn) => {
            fn(this.value);
          });
        });
      }
    };
    const reject = (reason) => {
      if (this.status === STATUS_PENDING) {
        queueMicrotask(() => {
          if (this.status !== STATUS_PENDING) return;
          this.status = STATUS_REJECTED;
          this.reason = reason;
          this.onRejectedCallbacks.forEach((fn) => {
            fn(this.reason);
          });
        });
      }
    };
    executor(resolve, reject);
  }
  then(onFulfilled, onRejected) {
    const defaultOnFulfilled = (value) => {
      return value;
    };
    const defaultOnRejected = (reason) => {
      throw reason;
    };
    onFulfilled = onFulfilled || defaultOnFulfilled;
    onRejected = onRejected || defaultOnRejected;
    return new Promise((resolve, reject) => {
      if (this.status === STATUS_FULFILLED && onFulfilled) {
        respondWithCatchError(onFulfilled, this.value, resolve, reject);
      }
      if (this.status === STATUS_REJECTED && onRejected) {
        respondWithCatchError(onRejected, this.reason, resolve, reject);
      }
      if (this.status === STATUS_PENDING) {
        this.onResolvedCallbacks.push(() => {
          respondWithCatchError(onFulfilled, this.value, resolve, reject);
        });
        this.onRejectedCallbacks.push(() => {
          respondWithCatchError(onRejected, this.reason, resolve, reject);
        });
      }
    });
  }
  catch(onRejected) {
    this.then(null, onRejected);
  }
  finally(onFinally) {
    this.then(
      () => {
        onFinally();
      },
      () => {
        onFinally();
      }
    );
  }
  static resolve(value) {
    return new Promise((onResolve) => {
      onResolve(value);
    });
  }
  static reject(reason) {
    return new Promise((onResolve, onRejected) => {
      onRejected(reason);
    });
  }
  static all(promises) {
    return new Promise((resolve, reject) => {
      const result = [];
      promises.forEach((promise) => {
        promise.then(
          (value) => {
            result.push(value);
            if (result.length === promises.length) {
              resolve(result);
            }
          },
          (reason) => {
            reject(reason);
          }
        );
      });
    });
  }
  static allSettled(promises) {
    return new Promise((resolve, reject) => {
      const result = [];
      promises.forEach((promise) => {
        promise.then(
          (value) => {
            result.push({
              status: STATUS_FULFILLED,
              value,
            });
            if (result.length === promises.length) {
              resolve(result);
            }
          },
          (reason) => {
            result.push({
              status: STATUS_REJECTED,
              reason,
            });
            if (result.length === promises.length) {
              resolve(result);
            }
          }
        );
      });
    });
  }
  static race(promises) {
    return new Promise((resolve, reject) => {
      promises.forEach((promise) => {
        promise.then(resolve, reject);
      });
    });
  }
  static any(promises) {
    return new Promise((resolve, reject) => {
      const reasons = [];
      promises.forEach((promise) => {
        promise.then(resolve, (reason) => {
          reasons.push(reason);
          if (reasons.length === promises.length) {
            reject(new AggregateError(reasons));
          }
        });
      });
    });
  }
}

// 测试代码
const promise1 = new IcePromise((resolve, reject) => {
  setTimeout(() => {
    reject(1);
  });
});
const promise2 = IcePromise.reject(2);
const promise3 = IcePromise.reject(3);
IcePromise.race([promise1, promise2, promise3]).then(
  (value) => {
    console.log("res1", value);
  },
  (reason) => {
    console.log("err1", reason);
  }
);
IcePromise.any([promise1, promise2, promise3]).then(
  (value) => {
    console.log("res1", value);
  },
  (reason) => {
    console.log("err1", reason);
  }
);

9_race方法.png

以上就是自定义promise的所有代码啦，关于js高级，还有很多需要开发者掌握的地方，可以看看我写的其他博文，持续更新中~

手写promise之分步解析

executor

定义构造函数

定义状态

then方法

then

解决多次调用then方法的问题

解决延迟调用的问题

解决链式调用的问题

catch

finally

resolve/reject

all/allSettled

race/any

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