error.js

// error in next tick
try {
  setImmediate(function () {
    throw new Error('error happened');
  });
} catch (err) {
  // 捕获不到异常
}

// get json by callback
function getJSON(file, callback) {
  fs.readFile(file, function (err, content) {
    if (err) return callback(err);
    try {
      content = JSON.parse(content);
    } catch (err) {
      return callback(err);
    }
    return callback(null, content);
  });
}

// callback hell
getUser(function (err, user) {
  if (err) return callback(err);
  getArticle(user, function (err, article) {
    if (err) return callback(err);
    getComment(article, function (err, comment)) {
      if (err) return comment;
      // res = ...
      callback(null, res);
    });
  });
});

// get json by co
co(function* getJSON(file) {
    var content = yield fs.readFile(file);
    content = JSON.parse(content);
}).catch(error);

// sync style in co
co(function* () {
  var user = yield getUser();
  var article = yield getArticle(user);
  var comment = yield getComment(article);
  // res = ...
}).catch(error);

//hello koa

var koa = require('koa');
var app = koa();

app.use(function* () {
  this.body = 'hello koa';
});

app.listen(3000);

// error handler in express

app.use(function (req, res, next) {
  fs.readFile('package.json', function (err, content) {
    if (err) return next(err);
    try {
      content = JSON.parse(content);
    } catch (err) {
      return next(err);
    }
    content.user = 'dead_horse';
    res.json(content);
  });
});

app.use(function errorHandler (err, req, res, next) {
  res.status(500).send(err.message);
});

// error handle in koa

app.use(function* (next) {
  try {
    yield next;
  } catch (err) {
    this.body = err.message;
    this.status = 500;
  }
});

app.use(function* () {
  var content = yield fs.readFile('package.json');
  var content = JSON.parse(content);
  content.user = 'dead_horse';
  this.body = content;
});

// stream in express

app.use(function (req, res) {
  var stream = fs.createReadStream('filename.txt');
  stream
  .on('error', onerror)
  .pipe(zlib.createGzip())
  .on('error', onerror)
  .pipe(res);

  res.once('close', function () {
    // 如果客户端终止了这个请求，可能导致 `fd` 泄漏
    // 需要 `unpipe` 来让上游关闭这个 `fd`
    stream.unpipe();
  });
  
  function onerror(err) {
    res.status(500).send(err.message);
  }
});

// stream in koa
app.use(function* () {
  this.body = fs.createReadStream('filename.txt');
  this.body = this.body.pipe(zlib.createGzip());
});

// middleware

app.use(function* responseTime(next) {
  var start = Date.now();
  // 在进入下一个中间件前执行
  yield next;
  // 在执行完后面的所有中间件之后执行
  var used = Date.now() - start;
  this.set('X-Response-Time', used);
});

app.use(function* respond() {
  this.body = 'hello koa';
  this.status = 200;
});

// middleware

app.use(function* () {
  var start = Date.now();
  
  // yield next() => yield respond();
  this.body = 'hello koa';
  this.status = 200;
  
  var used = Date.now() - start;
  this.set('X-Response-Time', used);
});

// delay

// 设置响应的 body
app.use(function* respond(next) {
  this.body = fs.createReadStream(this.path);
  yield next;
});
// 设置 etag
app.use(function* etag(next) {
  var stat = yield fs.stat(this.body.path);
  this.response.etag = require('etag')(stat);
  yield next;
});
// 将 body gzip
app.use(function* gzip(next) {
  this.body = this.body.pipe(zlib.createGzip());
});

app.use(function* () {
  // 设置响应 body
  this.body = fs.createReadStream(this.path);
  
  // 设置 etag
  var stat = yield fs.stat(this.body.path);
  this.response.etag = require('etag')(stat);
  
  // 修改响应 body (gzip 压缩)
  this.body = this.body.pipe(zlib.createGzip());
  
  // koa 自带的最外层的中间件，向外写响应
  this.body.pipe(this.res);
});