将消息记录为快速app.listen回调

Question

I am introducing winston logging into my application and i would like to replace all info or error level logs with winston's .info and .error . So far everything works except when I try and log a info message from within app.listen callback.

appStartup.js file where logger is defined

const app = require('express')();
const co = require('co');
const cors = require('cors');
const winston = require('winston');

const loggerTimeFormat = () => (new Date()).toLocaleTimeString();

const winstonLogger = new winston.Logger({
  transports: [
    new winston.transports.Console({
      timestamp: loggerTimeFormat,
    }),
  ],
});

module.exports = function appStartup() {
  co(function* services() {
    app.db = mongo.connect();
    app.use(cors());
    app.set('logger', winstonLogger);
  }).catch((e) => {
    console.error(e);
  });

  return app;
};

In my server.js file I have:

const app = require('./src/appStartup');

const appPort = process.env.PORT;
app().listen(appPort, () => {
  console.log('Info message here');
});

I would like to replace the console.log in server.js with a winston info log I tried doing this:

const app = require('./src/appStartup');
const appPort = process.env.PORT;
const appObj = app();

appObj.listen(appPort, (appObj.get('logger')) => {
  logger.info('Info message here');
});

Not sure if that's the right syntax though, I get Parsing error: Assigned to rvalue for appObj in appObj.get('logger') . Any idea how can I make this work?

Answer 1

Don't you mean this?

appObj.listen(appPort, (logger = appObj.get('logger')) => {
  logger.info('Info message here');
});

logger is the name of the argument, appObj.get('logger') is its default value.

Also, you're using co() when there's no need to use it, and its likely causing the problem because it's executing the code inside the services function at the next tick of the event loop (ie it's calling app.set('logger', ...) after calling appObj.get('logger') ).

Try this:

module.exports = function appStartup() {
  app.use(cors());
  app.set('logger', winstonLogger);
  return app;
};

EDIT : if appStartup depends on something asynchronous, one solution would be to return a promise from it, and handle it "upstream":

module.exports = function appStartup() {
  app.use(cors());
  app.set('logger', winstonLogger);
  return mongo.connect().then(db => {
    app.db = db;
    return app;
  });
};

// server.js
const app = require('./src/appStartup');
const appPort = process.env.PORT;

app().then(appObj => {
  appObj.listen(appPort, (logger = appObj.get('logger')) => {
    logger.info('Info message here');
  });
}).catch(e => {
  console.error('Unable to create app', e);
  process.exit(1);
});

Or using async/await , which works on recent (8.x) Node.js versions:

module.exports = async function appStartup() {
  app.use(cors());
  app.set('logger', winstonLogger);
  app.db = await mongo.connect();
  return app;
};

// server.js
const app     = require('./src/appStartup');
const appPort = process.env.PORT;

void async function() { // async IIFE
  try {
    var appObj = await app();
  } catch(e) {
    console.error('Unable to create app', e);
    process.exit(1);
  }
  appObj.listen(appPort, (logger = appObj.get('logger')) => {
    logger.info('Info message here');
  });
}();

Answer 2

It is an known issue with babel-eslint . Try upgrading it to a latest version and it will resolve the issue.

Check this thread for more info

As a temporary solution you may try considering rewriting your arrow function in es5 way.

Hope this helps!