手动实现 Promise

Promise 代表了一个异步操作的最终完成或者失败

定义状态

const PENDING = "pending";
const FULFILLED = "fulfilled";
const REJECTED = "rejected";

构造函数定义

function MyPromise(excutor){
  // 初始状态为 pending
  this.status = PENDING;
  // 成功的值
  this.value = undefined;
  // 失败的原因
  this.reason = undefined;
  // then 的回调函数集合，后面会用到
  fulfilledCallback = [];
  rejectedCallback = [];

  const resolve = (value) => {
    if (this.status === PENDING) {
      this.status = FULFILLED;
      this.value = value;
      // 状态变更后执行 then 的回调函数
      this.fulfilledCallback.forEach(fn => fn(value))
    }
  }

  const reject = (reason) => {
    if (this.status === PENDING) {
      this.status = rejected;
      this.reason = reason;
      this.rejectedCallback.forEach(fn => fn(reason))
    }
  }

  try {
    excutor(resolve, reject);
  } catch (error) {
    reject(error);
  }
}

原型方法

MyPromise.prototype.then = function(onResolve, onReject){

  onResolve = onResolve === undefined ? value => value : onResolve,
  onReject = onReject === undefined ? error => { throw error } : onReject;

  return new MyPromise((resolve, reject) => {

    function handle(func, value){
      try {
        const result = func(value);
        // 如果返回一个 Promise
        if (result instanceof MyPromise) {
            result.then(resolve, reject);
        } else {
          resolve(result);
        }
      } catch (err) {
          reject(err);
      }
    }

    if (this.status === FULFILLED) {
      queueMicrotask(() => {
        handle(onResolve, this.value);
      });
    }

    if (this.status === REJECTED) {
      queueMicrotask(() => {
        handle(onReject, this.reason);
      });
    }

     // 状态为 pending，将回调函数保存到前面定义的函数集合中
    if (this.status === PENDING) {
      // 保存到 fulfilledCallback
      this.fulfilledCallback.push(value => {
        queueMicrotask(() => {
          handle(onResolve, value);
        });
      });

      // 保存到 rejectedCallback
      this.rejectedCallback.push(reason => {
        queueMicrotask(() => {
          handle(onReject, reason)
        });
      });
    }
  })
};

  MyPromise.prototype.catch = function (onReject) {
    return this.then(undefined, onReject);
  };

  MyPromise.prototype.finally = function (cb) {
    return this.then(
      (value) => MyPromise.resolve(cb()).then(() => value),
      (reason) =>
        MyPromise.resolve(cb()).then(() => {
          throw reason;
        })
    );
  };

静态方法

MyPromise.resolve = function (value) {
  // 如果参数是MyPromise实例，直接返回这个实例
  if (value instanceof MyPromise) return value;
  return new MyPromise((resolve) => resolve(value));
};

MyPromise.reject = function (value) {
  // 如果参数是MyPromise实例，直接返回这个实例
  if (value instanceof MyPromise) return value;
  return new MyPromise((resolve, reject) => reject(value));
};

MyPromise.race = function (promises) {
  return new Promise((resolve, reject) => {
    promises.forEach((p) => {
      MyPromise.resolve(p).then(resolve, reject);
    });
  });
};

MyPromise.all = function (promises) {
  let result = [];
  const len = promises.length;
  return new MyPromise((resolve, reject) => {
    promises.forEach((p, index) => {
      MyPromise.resolve(p).then(
        (res) => {
          result[index] = res;
          if (result.length === len) {
            resolve(result);
          }
        },
        (error) => {
          reject(error);
        }
      );
    });
  });
};

MyPromise.allSettled = function (promises) {
  let result = [];
  const len = promises.length;
  return new MyPromise((resolve, reject) => {
    promises.forEach((p, index) => {
      MyPromise.resolve(p).then(
        (value) => {
          result[index] = {
            status: FULFILLED,
            value,
          };
          if (result.length === len) {
            resolve(result);
          }
        },
        (reason) => {
          result[index] = {
            status: REJECTED,
            reason,
          };
          if (result.length === len) {
            resolve(result);
          }
        }
      );
    });
  });
};

最后的最后

上述代码用到了 queueMicrotask()来执行微任务
mdn上也提供了 queueMicrotask()的polyfill实现
它通过使用立即 resolve 的 promise 创建一个微任务（microtask）
如下：

if (typeof window.queueMicrotask !== "function") {
  window.queueMicrotask = function (callback) {
    Promise.resolve()
      .then(callback)
      .catch(e => setTimeout(() => { throw e; }));
  };
}