Express async/await error handling

Question

class XYZ {

  constructor(app) {

    // app is express object
    this.app = app;

    this.app.route('/api/url')
      .get(this.wrap(this.urlhandler.bind(this)));

    // to send error as json -> this never gets invoked ??
    this.app.use((err, req, res, next) => {
      res.status(400).json({error: err});
      next(err);
    });

  }

  // wraps each async route function to handle rejection and throw errors
  wrap(fn) {
    return function (req, res, next) {
      Promise.resolve(fn(req, res, next)).catch(function (err) {
        console.log('ERROR OCCURED: > > > > > > >  ', err.code);
        next(err)
      });
    }
  }

}

Each express ASYNC route is wrapped to catch any rejection or throw error within route handler functions. Whenever there is such rejection or errors - wrap gets invoked fine and I am able to see print of "ERROR OCCURED > > > .."

However I am unable to channel that error to error handler middleware where I intend to send 400 with JSON error.

How can I fix this in above scenario ??

Using your code, with a dummied "urlhandler" that always rejects, I can't replicate your described issue, the `res.status(400).json({error: err});` does get invoked for me — Jaromanda X, Oct 25 '17 at 21:34

score 1 · Answer 1 · answered Oct 26 '17 at 18:17

Sorry, I couldn't isolate a sample piece of code to demonstrate the bug easily. I am using mysql, express - mixed with promises and ES6.

Below is a sample piece of problem I had and its solution.

The problem : I was adding error handling middleware before I initialised routes. Adding error middleware at the end solves it!!! (Thanks to this stackoverflow question).

var mysql = require('mysql');
var express = require('express');
var bodyParser = require('body-parser')
var morgan = require('morgan');

//define class
class Sql {

  constructor(pool) {
    this.pool = pool;
  }

  exec(query, params) {
    let _this = this;
    return new Promise(function (resolve, reject) {
      _this.pool.query(query, params, function (error, rows, _fields) {
        if (error) {
          return reject(error);
        }
        return resolve(rows);
      });
    });
  }

}


//define class
class Api {

  constructor(mysqlPool, app) {
    this.mysql = new Sql(mysqlPool)
    this.app = app;

    // this.app.use(this.errMw)
    //console.log('IF ERROR MIDDLEWARE IS ADDED HERE - it was NEVER CALLED');

    /**************** add middleware and routes ****************/
    this.app.get('/', this.asyncWrap(this.root.bind(this)))
    this.app.use(this.errMw) // << USE ERROR MIDDLEWARE HERE
  }

  errMw(err, req, res, next) {

    res.status(400).json({error: err});
    next(err);
  }

  asyncWrap(fn) {
    return (req, res, next) => {
      Promise.resolve(fn(req, res, next))
        .catch((err) => {
          console.log('err > > > > >', err.message);
          next(err);
        });
    }
  }


  run(cbk) {
    this.app.listen(3000)
  }

  async root(req, res) {
    res.json(await this.mysql.exec('select * from customer', []))
  }

}


/**************** START : mysql,express,api -> run ****************/

let args = {}
args['host'] = 'localhost'
args['user'] = 'root'
args['password'] = 'secret'
args['database'] = 'models'

let mysqlPool = mysql.createPool(args)
let app = express()
//app.use(morgan('tiny'))
app.use(bodyParser.json())
app.use(bodyParser.urlencoded({
  extended: true
}))


let api = new Api(mysqlPool, app);
api.run()


/**************** END : mysql,express,api -> run ****************/

Express async/await error handling

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