var fs = require('fs');
var readFile = function (fileName) {
 return new Promise(function (resolve, reject) {
 fs.readFile(fileName, function(error, data) {
  if (error) reject(error);
  resolve(data);
 });
 });
};
var gen = function* () {
 var f1 = yield readFile('/etc/fstab');
 var f2 = yield readFile('/etc/shells');
 console.log(f1.toString());
 console.log(f2.toString());
};

var asyncReadFile = async function () {
 var f1 = await readFile('/etc/fstab');
 var f2 = await readFile('/etc/shells');
 console.log(f1.toString());
 console.log(f2.toString());
};

var result = asyncReadFile();

function timeout(ms) {
 return new Promise((resolve) => {
 setTimeout(resolve, ms);
 });
}
async function asyncPrint(value, ms) {
 await timeout(ms);
 console.log(value);
}
asyncPrint('hello world', 5000);

async function timeout(ms) {
 await new Promise((resolve) => {
 setTimeout(resolve, ms);
 });
}
async function asyncPrint(value, ms) {
 await timeout(ms);
 console.log(value);
}
asyncPrint('hello world', 5000);

// 函数声明
async function foo() {}
// 函数表达式
const foo = async function () {};
// 对象的方法
let obj = { async foo() {} };
obj.foo().then(...)
// Class 的方法
class Storage {
 constructor() {
 this.cachePromise = caches.open('avatars');
 }
 async getAvatar(name) {
 const cache = await this.cachePromise;
 return cache.match(`/avatars/${name}.jpg`);
 }
}
const storage = new Storage();
storage.getAvatar('hzzly').then(…);
// 箭头函数
const foo = async () => {};

async function f() {
 return 'hello world';
}
f().then(v => console.log(v))
// "hello world"

async function getTitle(url) {
 let response = await fetch(url);
 let html = await response.text();
 return html.match(/<title>([\s\S]+)<\/title>/i)[1];
}
getTitle('https://tc39.github.io/ecma262/').then(console.log)
// "ECMAScript 2017 Language Specification"

async function myFunction() {
 try {
 await somethingThatReturnsAPromise();
 } catch (err) {
 console.log(err);
 }
}
// 另一种写法
async function myFunction() {
 await somethingThatReturnsAPromise()
 .catch(function (err) {
 console.log(err);
 };
}

//异步操作（即互不依赖），被写成继发关系。这样比较耗时，因为只有getFoo完成以后，才会执行getBar，完全可以让它们同时触发。
let foo = await getFoo();
let bar = await getBar();
// 写法一
let [foo, bar] = await Promise.all([getFoo(), getBar()]);
// 写法二
let fooPromise = getFoo();
let barPromise = getBar();
let foo = await fooPromise;
let bar = await barPromise;