Promise

• 链式操作减低了编码难度
• 代码可读性明显增强

状态

• pending
• resolve
• reject

原型方法

• then
• catch
• finally

静态方法

• all: 所有都 resolve 才 resolve，有一个 reject 就 reject
• race: 有一个 resolve 或 reject 就 resolve 或 reject
• resolve: 返回一个 resolve 的 Promise
• reject: 返回一个 reject 的 Promise
• allSettled: 返回一个包含所有结果的数组

Promise 实现

简单实现

const STATUS = {
  PENDING: 'PENDING',
  FULFILLED: 'FULFILLED',
  REJECTED: 'REJECTED',
}

class Promise {
  constructor(executor) {
    this.status = STATUS.PENDING
    this.value = undefined // 成功的值
    this.reason = undefined // 失败原因
    this.onResolvedCallbacks = [] // 存放成功的回调
    this.onRejectedCallbacks = [] // 存放失败的回调

    const resolve = val => {
      if (this.status === STATUS.PENDING) {
        this.status = STATUS.FULFILLED
        this.value = val
        this.onResolvedCallbacks.forEach(fn => fn())
      }
    }

    const reject = reason => {
      if (this.status === STATUS.PENDING) {
        this.status = STATUS.REJECTED
        this.reason = reason
        this.onRejectedCallbacks.forEach(fn => fn())
      }
    }

    try {
      executor(resolve, reject)
    } catch (e) {
      console.log(e)
    }
  }

  then(onFulfilled, onRejected) {
    if (this.status === STATUS.FULFILLED) {
      onFulfilled(this.value)
    }
    if (this.status === STATUS.REJECTED) {
      onRejected(this.reason)
    }
    if (this.status === STATUS.PENDING) {
      this.onResolvedCallbacks.push(() => {
        onFulfilled(this.value)
      })
      this.onRejectedCallbacks.push(() => {
        onRejected(this.reason)
      })
    }
  }
}

Promise.all() 实现

Promise.all = function (promises) {
  if (!Array.isArray(promises)) {
    throw new Error("arguments must be an array");
  }

  return new Promise((resolve, reject) => {
    let result = [];
    let count = 0;
    for (let i = 0; i < promises.length; i++) {
      promises[i].then(
        (data) => {
          result[i] = data;
          if (++count === promises.length) {
            resolve(result);
          }
        },
        (err) => {
          reject(err);
        }
      );
    }
  });
};

Promise.race() 实现

Promise.race = function (promises) {
  if (!Array.isArray(promises)) {
    throw new Error("arguments must be an array");
  }

  return new Promise((resolve, reject) => {
    for (let i = 0; i < promises.length; i++) {
      promises[i].then(
        (data) => {
          resolve(data);
        },
        (err) => {
          reject(err);
        }
      );
    }
  });
};