promise & async/await

promise & async/await

promise

1. 为什么出现promise？

   回调地狱

2. promise的三个状态

   1. pending
   2. fullfilled
   3. rejected

3. 真正解决回调地狱

   promise里面不嵌套then

   每一个都返回一个promise再then

   const readFilePro = (file) => {
     return new Promise((resolve, reject) => {
       fs.readFile(file, (err, data) => {
         if (err) reject('I could not find that file');
         resolve(data);
       });
     });
   };
   
   readFilePro(`${__dirname}/dog.txt`)
     .then((data) => {
       console.log(`Breed: ${data}`);
       return (
         superagent
           .get(`https://dog.ceo/api/breeds/image/random`)
       );
     })
     .then((res) => {
       console.log(res.body.message);
   
       return writeFilePro('dog-img.txt', res.body.message);
     })
     .then(() => {
       console.log('Random dog image saved to file!');
     })
     .catch((err) => {
       console.log(err.message);
     });
   

4. promise的优雅

   promise链式执行，只需要在最后catch一个error

async/await

1. async/await返回的是promise函数
2. 对于一个async函数，在函数前面加上await代表的是停在这里等promise为resolve状态的时候再执行下面的代码，相当于下面的代码在promise的then里面
3. async/await捕获错误用的是try/catch

const getDogPic = async () => {
  try {
    const data = await readFilePro(`${__dirname}/dog.txt`);
    console.log(`Breed: ${data}`);

    const res = await superagent.get(
      `https://dog.ceo/api/breed/${data}/images/random`
    );
    // .get(`https://dog.ceo/api/breeds/image/random`);
    console.log(res.body.message);

    await writeFilePro('dog-img.txt', res.body.message);
    console.log('Random dog image saved to file!');
  } catch (err) {
    console.log(err.message);
    throw err;
  }
  return '2: READY';
};

promise.all

1. promise.all返回的是多个promise执行之后的resolve值，是一个数组

const getDogPic = async () => {
  try {
    const data = await readFilePro(`${__dirname}/dog.txt`);
    console.log(`Breed: ${data}`);

    const res1Pro = await superagent.get(
      `https://dog.ceo/api/breed/${data}/images/random`
    );

    const res2Pro = await superagent.get(
      `https://dog.ceo/api/breed/${data}/images/random`
    );

    const res3Pro = await superagent.get(
      `https://dog.ceo/api/breed/${data}/images/random`
    );

    const all = await Promise.all([res1Pro, res2Pro, res3Pro]);
    const imgs = all.map((el) => el.body.message);
    console.log(imgs);

    await writeFilePro('dog-img.txt', imgs.join('\n'));
    console.log('Random dog image saved to file!');
  } catch (err) {
    console.log(err.message);
    throw err;
  }
  return '2: READY';
};