MongoClient.connect(url + db_name).then(db=> {
  return db.collection('blogs');
}).then(coll=> {
  return coll.find().toArray();
}).then(blogs=> {
  console.log(blogs.length);
}).catch(err=> {
  console.log(err);
})

MongoClient.connect(url + db_name).then(db=> {
  let coll = db.collection('blogs');
  coll.find().toArray().then(blogs=> {
    console.log(blogs.length);
    db.close();
  }).catch(err=> {
    console.log(err);
  });
}).catch(err=> {
  console.log(err);
})

MongoClient.connect(url + db_name).then(db=> {
  return {db:db,coll:db.collection('blogs')};
}).then(result=> {
  return {db:result.db,blogs:result.coll.find().toArray()};
}).then(result=> {
  return result.blogs.then(blogs=> {  //注意这里，result.coll.find().toArray()返回的是一个Promise，因此这里需要再解析一层
    return {db:result.db,blogs:blogs}
  })
}).then(result=> {
  console.log(result.blogs.length);
  result.db.close();
}).catch(err=> {
  console.log(err);
});

const co = require('co');
co(function* (){
  let db = yield MongoClient.connect(url + db_name);
  let coll = db.collection('blogs');
  let blogs = yield coll.find().toArray();
  console.log(blogs.length);
  db.close();
}).catch(err=> {
  console.log(err);
});

(async function(){
  let db = await MongoClient.connect(url + db_name);
  let coll = db.collection('blogs');
  let blogs = await coll.find().toArray();
  console.log(blogs.length);
  db.close();
})().catch(err=> {
  console.log(err);
});

let getBlogs = async function(){
  let db = await MongoClient.connect(url + db_name);
  let coll = db.collection('blogs');
  let blogs = await coll.find().toArray();
  db.close();
  return blogs;
};

getBlogs().then(result=> {
  console.log(result.length);
}).catch(err=> {
  console.log(err);
})

//返回一个Promise
let timer = async functiontimer(){
  return new Promise((resolve,reject) => {
    setTimeout(()=> {
      resolve('500');
    },500);
  });
}

timer().then(result=> {
 console.log(result); //500
}).catch(err=> {
  console.log(err.message);
});

//返回一个同步的值
let sayHi = async functionsayHi(){
 let hi = await 'hello world';  
 return hi; //等同于return Promise.resolve(hi);
}

sayHi().then(result=> {
 console.log(result);
});

let sayHi = async functionsayHi(){
  throw new Error('出错了');
}
sayHi().then(result=> {
 console.log(result);
}).catch(err=> {
  console.log(err.message);  //出错了
});

let count = ()=>{
  return new Promise((resolve,reject) => {
    setTimeout(()=>{
      reject('故意抛出错误');
    },500);
  });
}

let list = ()=>{
  return new Promise((resolve,reject)=>{
    setTimeout(()=>{
      resolve([1,2,3]);
    },500);
  });
}

let getList = async ()=>{
  let c = await count();
  let l = await list();
  return {count:c,list:l};
}
console.time('begin');
getList().then(result=> {
  console.log(result);
}).catch(err=> {
  console.timeEnd('begin');
  console.log(err);
});
//begin: 507.490ms
//故意抛出错误

let count = ()=>{
  return new Promise((resolve,reject) => {
    setTimeout(()=>{
      resolve(100);
    },500);
  });
}

let list = ()=>{
  return new Promise((resolve,reject)=>{
    setTimeout(()=>{
      resolve([1,2,3]);
    },500);
  });
}

let getList = async ()=>{
  let result = await Promise.all([count(),list()]);
  return result;
}
console.time('begin');
getList().then(result=> {
  console.timeEnd('begin'); //begin: 505.557ms
  console.log(result);    //[ 100, [ 1, 2, 3 ] ]
}).catch(err=> {
  console.timeEnd('begin');
  console.log(err);
});

async function sleep(timeout) {
 return new Promise((resolve, reject) => {
  setTimeout(function() {
   resolve();
  }, timeout);
 });
}

(async function() {
 console.log('Do some thing, ' + new Date());
 await sleep(3000);
 console.log('Do other things, ' + new Date());
})();

var fetchDoubanApi = function() {
 return new Promise((resolve, reject) => {
  var xhr = new XMLHttpRequest();
  xhr.onreadystatechange = function() {
   if (xhr.readyState === 4) {
    if (xhr.status >= 200 && xhr.status < 300) {
     var response;
     try {
      response = JSON.parse(xhr.responseText);
     } catch (e) {
      reject(e);
     }
     if (response) {
      resolve(response, xhr.status, xhr);
     }
    } else {
     reject(xhr);
    }
   }
  };
  xhr.open('GET', 'https://api.douban.com/v2/user/aisk', true);
  xhr.setRequestHeader("Content-Type", "text/plain");
  xhr.send(data);
 });
};

(async function() {
 try {
  let result = await fetchDoubanApi();
  console.log(result);
 } catch (e) {
  console.log(e);
 }
})();

async function getStockPriceByName(name) {
 var symbol = await getStockSymbol(name);
 var stockPrice = await getStockPrice(symbol);
 return stockPrice;
}

getStockPriceByName('goog').then(function (result){
 console.log(result);
});

var sleep = function (time) {
  return new Promise(function (resolve, reject) {
    setTimeout(function () {
      resolve();
    }, time);
  })
};

var start = async function () {
  // 在这里使用起来就像同步代码那样直观
  console.log('start');
  await sleep(3000);
  console.log('end');
};

start();

var sleep = function (time) {
  return new Promise(function (resolve, reject) {
    setTimeout(function () {
      // 返回 ‘ok'
      resolve('ok');
    }, time);
  })
};

var start = async function () {
  let result = await sleep(3000);
  console.log(result); // 收到 ‘ok'
};

var sleep = function (time) {
  return new Promise(function (resolve, reject) {
    setTimeout(function () {
      // 模拟出错了，返回 ‘error'
      reject('error');
    }, time);
  })
};

var start = async function () {
  try {
    console.log('start');
    await sleep(3000); // 这里得到了一个返回错误
    
    // 所以以下代码不会被执行了
    console.log('end');
  } catch (err) {
    console.log(err); // 这里捕捉到错误 `error`
  }
};

..省略以上代码
var start = async function () {
  for (var i = 1; i <= 10; i++) {
    console.log(`当前是第${i}次等待..`);
    await sleep(1000);
  }
};

..省略以上代码

let one2ten = [1,2,3,4,5,6,7,8,9,10];

// 错误示范
one2ten.forEach(function (v) {
  console.log(`当前是第${v}次等待..`);
  await sleep(1000); // 错误!! await只能在async函数中运行
});

// 正确示范
for(var v of one2ten) {
  console.log(`当前是第${v}次等待..`);
  await sleep(1000); // 正确, for循环的上下文还在async函数中
}