快速错误处理模式

Question

I'm looking into Express to create a simple JSON API, and I'm not sure how to organize my input parameter validation and error handling. Errors can originate from the validation step but also from the database access step. This is what I have so far:

router.use(function(req, res, next) {
    validate(req.query).then(function() {
        next()
    }).catch(function(e) {
        next(e)
    })
})

router.get("/", function(req, res, next) {
    someDatabaseAccess(req.query).then(function(results) {
        res.json(results)
    }).catch(function(e) {
        next(e)
    })
})

router.use(function(e, req, res, next) {

    // ... (handling specific errors)

    res.status(400)
    res.json(someDummyResponse(e))
})

The validation looks like this:

const validate = function(q) {
    return new Promise(function(resolve, reject) {
        if (q.someParameter) {
            if (somethingWrong(q.someParameter)) {
                reject(new Error("Something wrong!"))
            }
        }
        resolve()
    })
}

Does this make sense? Is there anything I should do differently/in a less convoluted way?

Answer 1

To validate, I suggest to have a look at JSONSchema tools. For example, I use package tv4 as a validator, but there are lots of similar. First you create a schema of the object:

const tv4 = require('tv4');

const schema = {
  type: object,
  properties: {
    name: string,
    phone: string
  },
  required: ['name']
};

Then in you route, you do:

app.post('/someroute', (req, res, next) => {
  if (!tv4.validate(req.body, schema) ) 
    return next( new Error('not valid input'));
  // do some cool stuff here
  res.end();
});

The error handling in express is basically about adding a middleware function as a last one, and with additional params:

// add middleware
app.use(bodyParser.json())

// add custom middleware
app.use(function(req, res, next) {
  // some cool stuff
  next();
});    

// add routes
app.get('/', () => {})

// error handler always last
// Pay attention to additional `err` param
app.use(function (err, req, res, next) {
  if (err) {
    // log error, whatever handling logic
  }
  else next();
});

Answer 2

A more structured way would be to have a separate error config file and throw that error using a middleWare that way your app is better structured

error.json

"err":[
   "errorLog501":{
     "status":501,
     "message":"request is invalid"
    }
]

`

var errCodes = require('./error.json')
var errMiddleware = function(req, res, next) {
   if (req.body.hello) {
   // some cool stuff
     res.json(errCodes.errorLog501)
   } else {
     next();
   }
}

app.use(errMiddleware); //everytime a request happens the middleware called

It is very Important to pass status code in JSON response so that the Frontend can display appropriate error messages and user can know the state of application