关于前端:手写promise之分步解析

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promise 是 es6 推出实用于异步申请的构造函数，帮忙解决回调天堂的问题，以下内容将自定义实现 promise，只包含根本应用，所以一些边界状况思考没有在内。
如果对 promise 用法还不相熟的敌人可移步
Promise 的了解与应用(一)
Promise 的了解和应用(二)

首先建设 promise 的根本构造

定义构造函数

promise 的 executor 局部是放到主线程中间接执行的

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、通过构造函数创立实例时，就应该须要状态，所以在类中定义

this.status = STATUS_PENDING

3、resolve 和 reject 办法里通过状态判断
当为 pending 时，能力执行 resolve 或者 reject 办法，执行前先批改状态

then 办法

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

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

2、在 resolve 和 reject 中别离执行 this.onFulfilled 和 this.onRejected 中
此时会报错，因为 executor 会立即执行，而 then 外面的函数是微工作，
会在主线程执行实现之后执行

3、resolve 和 reject 中退出 queueMicrotask (微工作)

整体架构的初版就实现了

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 办法中对应执行的胜利或者失败的函数也正确

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

解决屡次调用 then 办法的问题

1、constructor 中定义变量用于收集所有的胜利 / 失败的回调函数

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

2、在 then 办法中通过 push 增加到数组中

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

3、在 resolve 和 reject 中遍历
this.onFulfilledCallbacks 和 this.onRejectedCallbacks 中的办法

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 的屡次调用的问题，但仍有其它的问题，一个是 resolve 和 reject 办法同时执行，二是通过定时器提早执行的 promise 的 then 办法没有输入响应后果

解决提早调用的问题

1、保留 value 和 reason

this.value = undefined
this.reason = undefined

resolve 和 reject 办法别离给 this.value 和 this.reason 赋值

2、then 办法中进行状态的判断
当状态为 pending 时，持续向 onFulfilledCallbacks、onRejectedCallbacks 数组中增加函数；当状态不为 pending 时，间接执行 onFulfilled 或 onRejected 办法

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 状态的变动
① queueMicrotask 中判断不为 pending 则 return
② 批改 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 屡次调用及计时器提早调用的问题，但此时发现 then 无奈进行链式调用

1、then 办法里返回一个 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 函数的执行后果
3、封装工具函数，用于解决 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 函数就曾经能够链式调用啦，基本功能曾经实现~

catch 函数接管一个失败的回调
1、调用 then 办法，将 onRejected 办法加到第二个 promise 的回调中

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

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

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);
  });

执行后果如下

finally 办法无论执行 resolve 或者 reject 的办法后都会执行

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");
  });

resolve 和 reject 是 Promise 的类办法，也能够通过调用 then 办法来实现

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

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

残缺实现如下

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);
    }
  )

执行后果如下

all 和 allSettled 办法都是 promise 的类办法
1、all 办法
只有有一个 promise 执行 reject 的办法就会执行 reject，当所有 promise 都返回 resolve 时，才执行 resolve 办法。
2、allSettled 办法
当所有 promise 都执行实现时，才执行 resolve 办法，返回所有 promise 的状态和后果。

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)
            }
        })
    })
}

残缺实现如下

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);
  }
);

执行后果如下

race 和 any 都是 promise 的类办法。
1、race 办法
只有有一个 promise 执行实现，就会返回它所执行的后果

2、any 办法
① 有 fulfilled 状态，会等到这个 fullfilled 执行实现，执行 resolve，后果为 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 的谬误。

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);
  }
);

以上就是自定义 promise 的所有代码啦，对于 js 高级，还有很多须要开发者把握的中央，能够看看我写的其余博文，继续更新中~

正文完

前端

发表至：前端

2021-12-19

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executor

定义构造函数

定义状态

then 办法

then

解决屡次调用 then 办法的问题

解决提早调用的问题

解决链式调用的问题

catch

finally

resolve/reject

all/allSettled

race/any

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