[ES6] 異步操作 async 函數

const asyncReadFile = async function () {
  const afile = await readFile('/etc/fstab');
  const bfile = await readFile('/etc/shells');
  console.log(afile.toString());
  console.log(bfile.toString());
};

asyncReadFile();

async function () {
  //...裡面有異步操作
}

async function () {
  //123就是原始類型的值
  //也可以講解成await命令后面Promise 对象，狀態同步變成resolve
   return await 123;
}

async function f() {
  return 'hello world';
  //這邊等同事返回一個”原始类型的值“
  //又等於是一個狀態輸出resolve
}
f().then(v => console.log(v))
// 輸出"hello world"

const promise = new Promise(function(resolve, reject) {
  resolve('ok');
  throw new Error('test');
});
promise
  .then(function(value) { console.log(value) })
  .catch(function(error) { console.log(error) });
// 輸出ok
// 最後丟出的 Error 就沒被執行了！

async function f() {
  await Promise.reject('出错了');
//   throw new Error('出错了');
  return 123;
}
f().then(
  v => console.log("then",v),
//e => console.log("then",e)
).catch(
  e => console.log("catch",e)
)

f().then(
  v => console.log("then",v),
//e => console.log("then",e)
)

f().then(
  v => console.log("then",v),
//e => console.log("then",e)
).catch(
  e => console.log("catch",e)
)

async function f() {
  await Promise.reject('出错了');
  await Promise.resolve('hello world'); // 不会执行
}

async function f() {
  try {
    await Promise.reject('出错了');
  } catch(e) {
  }
  return await Promise.resolve('hello world');
}

f()
.then(v => console.log(v))
// hello world

async function f() {
  await Promise.reject('出错了')
    .catch(e => console.log(e));
  return await Promise.resolve('hello world');
}

f()
.then(v => console.log(v))
// 出错了
// hello world

async function chainAnimationsAsync(elem, animations) {
  let ret = null;
  try {
    for(let anim of animations) {
      ret = await anim(elem);
    }
  } catch(e) {
    /* 忽略错误，继续执行 */
  }
  return ret;
}

let foo = await getFoo();
let bar = await getBar();

// 寫法一
let [foo, bar] = await Promise.all([getFoo(), getBar()]);

// 寫法二
let fooPromise = getFoo();
let barPromise = getBar();
let foo = await fooPromise;
let bar = await barPromise;

async function logInOrder(urls) {
  for (const url of urls) {
    const response = await fetch(url);
    console.log(await response.text());
  }
}

async function logInOrder(urls) {
  // 同時讀取遠程URL
  const textPromises = urls.map(async url => {
    const response = await fetch(url);
    return response.text();
  });

  // 按次序输出
  for (const textPromise of textPromises) {
    console.log(await textPromise);
  }
}