普歌-手写系列01—实现promise(简易)、promise.all、promise.race、promise.any

(面试重点)手写实现promise、promise.all、promise.race、promise.any

大三的我找暑期实习，有幸也面过腾讯、阿里、快手、字节、作业帮等等公司，也算是一个行走的面经…现在把一些真实遇到的手写系列分享给大家

如果有错误，及时提出来，谢谢哈

手写实现一个promise

 class MyPromise2 {
   constructor(executor) {
     // 规定状态
     this.state = "pending";
     // 保存 `resolve(res)` 的res值
     this.value = undefined;
     // 保存 `reject(err)` 的err值
     this.reason = undefined;
     // 成功存放的数组
     this.successCB = [];
     // 失败存放的数组
     this.failCB = [];
     let resolve = (value) => {
       if (this.state === "pending") {
         this.state = "fulfilled";
         this.value = value;
         this.successCB.forEach((f) => f());
       }
     };
     let reject = (reason) => {
       if (this.state === "pending") {
         this.state = "rejected";
         this.reason = reason;
         this.failCB.forEach((f) => f());
       }
     };
     try {
       // 执行
       executor(resolve, reject);
     } catch (error) {
       // 若出错，直接调用reject
       reject(error);
     }
   }
   then(onFulfilled, onRejected) {
     if (this.state === "fulfilled") {
       onFulfilled(this.value);
     }
     if (this.state === "rejected") {
       onRejected(this.reason);
     }
     if (this.state === "pending") {
       this.successCB.push(() => {
         onFulfilled(this.value);
       });
       this.failCB.push(() => {
         onRejected(this.reason);
       });
     }
   }
 }

手写实现promise.all()

• 简单说只有都成功才返回成功的数组，只要有一个失败就报错

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

手写实现promise.race()

• Promise.race() 同样返回一个合成的 Promise 实例，其会返回这一组中最先解决(fulfilled)或拒绝(rejected)的 Promise 实例的返回值

Promise.MyRace = function (promises) {
  return new Promise((resolve, reject) => {
    //遍历promises，获取每个promise的结果
    promises.forEach((item, index) => {
      Promise.resolve(item).then(
        (res) => {
          resolve(res);
        },
        (err) => {
          reject(err);
        }
      );
    });
  });
};

手写实现promise.any()

• Promise.any() 相当于 Promise.all() 的反向操作，同样返回一个合成的 Promise 实例，都报错了返回都报错的集合。

Promise.any = function (promises) {
  return new Promise((resolve, reject) => {
    let count = 0;
    let errResult = [];
    if (promises.length == 0) {
      resolve([]);
    }
    for (let item of promises) {
      Promise.resolve(item).then(
        (res) => {
          resolve(res);
        },
        (err) => {
          errResult[count++] = err;
          if (count == promises.length) {
            reject(errResult);
          }
        }
      );
    }
  });
};