手写promise

手写 promise 实现then，catch，finally等

const promiseStatus = {
  PENDING: 'pending',
  FULFILLED: 'fulfilled',
  REJECTED: 'rejected',
}

class myPromise {
  constructor(executor) {
    this.status = promiseStatus.PENDING
    this.value = undefined
    this.error = undefined
    this.resfns = []
    this.errfns = []
    const resolve = (value) => {
      if (this.status === promiseStatus.PENDING) {
        queueMicrotask(() => {
          if (this.status !== promiseStatus.PENDING) return 
          this.status = promiseStatus.FULFILLED
          this.value = value
          this.resfns.forEach((fn) => {
            fn(this.value)
          })
        })
      }
    }

    const reject = (error) => {
      if (this.status === promiseStatus.PENDING) {
        queueMicrotask(() => {
          if (this.status !== promiseStatus.PENDING) return
          this.status = promiseStatus.REJECTED
          this.error = error
          this.errfns.forEach((fn) => {
            fn(this.error)
          })
        })
      }
    }
    executor(resolve, reject)
  }

  // then 方法
  then(resfn, errfn) {
    return new myPromise((resolve, reject) => {
      if (this.status === promiseStatus.FULFILLED && resfn) {
        try {
          const value = resfn(this.value)
          resolve(value)
        } catch (err) {
          reject(err)
        }
      }

      if (this.status === promiseStatus.REJECTED && errfn) {
        try {
          const value = errfn(this.error)
          resolve(value)
        } catch (err) {
          reject(err)
        }
      }

      if (this.status === promiseStatus.PENDING) {
        this.resfns.push(() => {
          try {
            const value = resfn(this.value)
            resolve(value)
          } catch (err) {
            reject(err)
          }
        })
        this.errfns.push(() => {
          try {
            const value = errfn(this.error)
            resolve(value)
          } catch (err) {
            reject(err)
          }
        })
      }
    })
  }

  // catch 方法
  catch(errfn) {
    return this.then(undefined, errfn)
  }

  // finally 方法
  finally(onFinally) {
    return this.then(() => onFinally(),() => onFinally());
  }
  
  // 其余方法就省略了
}

测试：

const p1 = new MyPromise((resolve, reject) => {
  setTimeout(() => {
    console.log("--- 1 ---");
    resolve(111);
  });
}).then(res => {
  console.log("p1 res :>> ", res);
});

const p2 = new MyPromise((resolve, reject) => {
  console.log("--- 2 ---");
  resolve(222);
});
const p3 = new MyPromise((resolve, reject) => {
  console.log("--- 3 ---");
  resolve(333);
});
const p3 = new MyPromise((resolve, reject) => {
  console.log("--- 3 ---");
  resolve(333);
});
const p4 = new MyPromise((resolve, reject) => {
  console.log("--- 4 ---");
  reject(444);
});
MyPromise.all([p2, p3]).then(res => {
  console.log("p2&p3 res :>> ", res);
});MyPromise.all([p2, p4]).then(res => {
  console.log("p2&p4 res :>> ", res);
}).catch(err => {
  console.log("err :>> ", err);
});

//输出结果

// --- 2 ---
// --- 3 ---
// --- 4 ---
// p2&p3 res :>> [ 222, 333 ]
// err :>> 444
// --- 1 ---
// p1 res :>> 111

参考文章：

手写Promise的基本实现 （超详细）-CSDN博客