关于前端:Promise-方法的实现

Promise finally

这是一个 promise 实例原型链上的办法

Promise.prototype.finally = function(func) {return this.then((value) => {Promise.resolve(func()).then(() => value)
  },
  (reason) => {Promise.resolve(func()).then(() => {throw reason})
  })
}

Promise.All

  if (!arr[Symbol.iterator]) throw 'param is not iterable';
  const promiseArr = Array.from(arr).map(a => {if (a instanceof Promise) {return a;} else {return Promise.resolve(a);
    }
  })
  if (promiseArr.length === 0) return Promise.resolve([]);
  return new Promise((resolve, reject) => {let result = [];
    let count = promiseArr.length;
    promiseArr.forEach((p, i) => {
      p.then((res => {result[i] = res;
        count--;
        if (count === 0) resolve(result);
      }, (reason) => reject(reason)))
    })
  })

留神点

1. 入参必须是可迭代的
2. 参数转化为数组，且所有的项必须转化为 Promise
3. 返回值永远是一个 Promise
4. 入参如果长度是 0 那间接返回 Promise.resolve([]) 不然 Promise.all([])返回的 promise 永远处于 pending 状态

Promise.race

const PromiseRace = (arr) => {if (arr[Symbol.iterator] === undefined) return Promise.reject('param is not iterator')
  const promiseArr = Array.from(arr).map(a => {if (a instanceof Promise) {return a;} else {return Promise.resolve(a);
    }
  })
  return new Promise((resolve, reject) => {promiseArr.forEach((p, i) => {
      p.then((res => {resolve(res);
      }, (reason) => reject(reason)))
    })
  })
}

留神 Promise.race([])返回的应该是一个 pending 状态的 promise

Promise.any

const PromiseAny = (arr) => {if (arr[Symbol.iterator] === undefined) return Promise.reject('param is not iterator')
  const promiseArr = Array.from(arr).map(a => {if (a instanceof Promise) {return a;} else {return Promise.resolve(a);
    }
  })
  if (promiseArr.length === 0) return Promise.reject(new AggregateError([],'All promises were rejected'))
  return new Promise((resolve, reject) => {const errors = [];
    const count = promiseArr.length;
    promiseArr.forEach((p, i) => {
      p.then((res => {resolve(res);
      }, (reason) => {errors[i] = reason;
        count--;
        if (count === 0) {reject(new AggregateError(errors,'All promises were rejected'))
        }
      }))
    })
  })
}

留神

1. Promise.any 返回的谬误肯定是 AggregateError 类型的

Promise.allSettled

const PromiseAllSettled = (arr) => {if (arr[Symbol.iterator] === undefined) return Promise.reject('param is not iterator')
  const promiseArr = Array.from(arr).map(a => {if (a instanceof Promise) {return a;} else {return Promise.resolve(a);
    }
  })
  if (promiseArr.length === 0) return Promise.resolve([])
  return new Promise((resolve, reject) => {const result = [];
    const count = promiseArr.length;
    promiseArr.forEach((p, i) => {
      p.then((res => {result[i] = {status: 'fulfilled', value: res};
        count--;
        if (count === 0) {resolve(result)
        }
      }, (reason) => {result[i] = {status: 'rejected', reason: reason};
        count--;
        if (count === 0) {resolve(result)
        }
      }))
    })
  })
}

返回内容的格局肯定是 {status: ‘fulfilled’, value: res} 或者{status: ‘rejected’, reason: reason}