如何将从控制器返回的数据传递到Express'路由器？

Question

I'm trying to make a catch-all of sorts to return data to my Author endpoint. If the url that is passed to the endpoint contains no query parameters, I want the router to return the full list of authors available. If the url contains firstName and lastName parameters, I want the controller to find the authors that match and, pass that data back to the router.

Currently if I send the urls http://localhost:3001/authors or http://localhost:3001/authors?firstName=tom&lastName=dooly , I get an error Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client .

Can anyone tell me why this is happening and how to fix it?

main:

var express = require('express');
var path = require('path');
var favicon = require('serve-favicon');
var logger = require('morgan');
var cookieParser = require('cookie-parser');
var bodyParser = require('body-parser');
var mongoose = require('mongoose');

var app = express();
var dev_db_url = 'mongodb://localhost:27017/'
var mongoDB = process.env.MONGODB_URI || dev_db_url;

mongoose.connect(dev_db_url);

mongoose.Promise = global.Promise;
var db = mongoose.connection;
db.on('error', console.error.bind(console, 'MongoDB connection error:'));



// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'jade');

// uncomment after placing your favicon in /public
//app.use(favicon(path.join(__dirname, 'public', 'favicon.ico')));
app.use(logger('dev'));
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));


var index = require('./routes/index');
var users = require('./routes/users');
var feedEntries = require('./routes/feedEntries');
var authors = require('./routes/authors');


app.use('/', index);
app.use('/users', users);
app.use('/feedEntries', feedEntries);
app.use('/authors', authors);


// catch 404 and forward to error handler
app.use(function(req, res, next) {
  var err = new Error('Not not Found');
  err.status = 404;
  next(err);
});


app.use(function(err, req, res, next) {

  res.locals.message = err.message;
  res.locals.error = req.app.get('env') === 'development' ? err : {};


  res.status(err.status || 500);
  res.render('error');
});

module.exports = app;

route:

var express = require('express');
var router = express.Router();

var authorController = require('../controllers/authorController');

authorController.findAuthorsByFirstAndLastName);


router.get('/', function (req, res) {
    if(req.query.firstName||req.query.lastName) {
        res.send(authorController.findAuthorsByFirstAndLastName(req,res));
    }else{
        res.send(authorController.author_list(req,res));
    }
  });


module.exports = router;

controller:

var Author = require('../models/author')
var async = require('async')


exports.author_list = function(req, res, next) {

    Author.find({},function(err, authors) {
        if (err){
            res.send(err);
        }
            return.json(authors);
    });

  };

  exports.findAuthorsByFirstAndLastName = function (req, res, next){
    var query = {}

    if(req.query.firstName||req.query.lastName) {

        query = {$or:[{firstName:{$regex: req.query.firstName, $options: 'i'}},
            {lastName:{$regex: req.query.lastName, $options: 'i'}}]}
    }

    else {
        return res.status(500).send({ error: 'Unable to parse data'});
    }

    var firstName =  req.body.firstName;
    var lastName = req.body.lastName;

    Author.find(query , function (err, authors) {
        if(err) {
            res.send(err);
        }
        res.json(authors);
     });
  };

Answer 1

You get cannot set headers after they are sent when you have two res.[whatever] s in your route. So you have res.send(functionCallThatAlsoDoesRes.Send) . That's what's causing the error.

If you want a route to take multiple actions between the request and the response, you can write those as separate middlewares. Middlewares always take the arguments req , res , and next (a function that says to go to the next middleware in the list).

So, you might write:

authorController.findAuthorsByFirstAndLastName = function(req, res, next) {
  if (!(req.query.firstName || req.query.lastName)) {
    res.locals.getFullAuthorList = true
    return next()
  } else {
    const query = /* whatever */
    Author.find(query, (err, authors) => {
      if (err) return next(err)
      res.locals.authors = authors
      next()
    })
  }
}

authorController.author_list = function(req, res, next) {
  if (!res.locals.getFullAuthorList) return next() // if we already have authors we don't need to do anything
  Author.find({}, (err, authors) => {
    if (err) return next(err)
    res.locals.authors = authors
    next()
  })
}

Then in your route, you'd say:

router.get('/', authorController.findAuthorsByFirstAndLastName, authorController.author_list, (req, res) => {
    res.json({ authors: res.locals.authors })
})

If you haven't seen res.locals before, it's just a property on the response object that is available for you to attach things to. It persists throughout the request/response cycle and is cleared for each new request.