Promise的四个静态方法

resolve

代码

// 得到的是一个成功的Promise对象
Promise.resolve = function (val) {
  return new Promise((resolve, reject) => {
    resolve(val);
  });
};

调用

Promise.resolve(200).then((res) => {
  console.log(res);
});

reject

代码

// 得到的是一个失败的Promise对象
Promise.reject = function (val) {
  return new Promise((resolve, reject) => {
    reject(val);
  });
};

调用

Promise.reject(200).then(res => {
  console.log(res，"===");
}, err => {
  console.log(err);
})

race

代码

// race：竞速原则，谁先满足条件，谁先处理，其他忽略
// Promise.race([promise1,promise2,promise3]).then(res=>{})
Promise.race = function (promises) {
  return new Promise((resolve, reject) => {
    // 遍历promises，看谁先满足条件就先处理
    // Promise的状态只能改变一次，只要第一个修改了，后续再改也没用
    promises.forEach((p) => {
      p.then(resolve, reject);
    });
  });
};

调用

let p1 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve(1);
  }, 1000);
});
let p2 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve(2);
  }, 2000);
});
let p3 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve(3);
  }, 3000);
});
Promise.race([p1, p2, p3]).then((res) => {
  console.log(res);
});

all

代码

// all：等待原则，传入多个promise，等所有的promise都满足条件，返回所有的成功结果
Promise.all = function (promises) {
  let arrResult = []; // 存放结果
  let count = 0; // 计数器

  function processData(index, val, resolve) {
    arrResult[index] = val; // 按顺序存取结果
    count++;
    if (count === promises.length) {
      resolve(arrResult);
    }
  }

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

调用

let p1 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve(1);
  }, 1000);
});
let p2 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve(2);
  }, 2000);
});
let p3 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve(3);
  }, 3000);
});
Promise.all([p1, p2, p3]).then((res) => {
  console.log(res);
});