为什么Express的默认错误处理程序从Readable流中发出错误时会捕获错误？

Question

I installed a boilerplate Express app using the command line express stream_test . I replaced the default /routes/index.js with:

var express = require('express');
var router = express.Router();
var ReadFile = require('./readFile.js');

/* GET home page. */
router.get('/', function(req, res, next) {
  var readFile = ReadFile();
  readFile
  .on('data', function(data) {
    res.write(data);
  })
  .on('end', function() {
    res.end();
  })
  .on('error', function(err) {
    console.log(err);
  });
});

module.exports = router;

readFile.js is just a simple wrapper for fs.createReadStream :

var Readable = require('stream').Readable;
var util = require('util');
var fs = require('fs');

function ReadFile(options) {
  if (!(this instanceof ReadFile)) {
    return new ReadFile(options);
  }
  Readable.call(this);

  options = options || {};

  var self = this;

  fs.createReadStream('pride_and_prejudice.txt')
  .on('data', function(data) {
    self.push(data);
  })
  .on('end', function(end) {
    self.push(null);
  });

}

util.inherits(ReadFile, Readable);
ReadFile.prototype._read = function _readGetDeals() {};

module.exports = ReadFile;

That works totally fine. It outputs the contents of pride_and_prejudice.txt to the screen when calling the route.

But let's say some requirements are not met and I want to throw an error before streaming the data:

var Readable = require('stream').Readable;
var util = require('util');
var fs = require('fs');

function ReadFile(options) {
  if (!(this instanceof ReadFile)) {
    return new ReadFile(options);
  }
  Readable.call(this);

  options = options || {};

  var self = this;

  if (!options.okay) {
    return self.emit('error', new Error('Forced crash'));
  }

  fs.createReadStream('pride_and_prejudice.txt')
  .on('data', function(data) {
    self.push(data);
  })
  .on('end', function(end) {
    self.push(null);
  });

}

That throws an error Forced crash. when options.okay is false . I expect to catch the error in my index.js route while listening on the error . But the handler never executes. To my very surprise the default error handler in app.js is catching the error:

if (app.get('env') === 'development') {
  app.use(function(err, req, res, next) {
    res.status(err.status || 500);
    res.render('error', {
      message: err.message,
      error: err
    });
  });
}

This is driving me nuts. How does the error end up there? Why isn't the event listener catching it?

Answer 1

1) When you emit error the ReadFile returns nothing, as well as the nearest trap for errors outside of the router module, then after the call ReadFile she called.

2) You can use try-catch :

try {
  var readFile = ReadFile();
} catch(e) {
  console.log(e);
}

3) Or use callback for error:

function ReadFile(options, errorCallback) {
  /**...**/
  if (!options.okay) {
    errorCallback( new Error('Forced crash') );
    return;
  }
  /**...**/ 
}

Answer 2

My thought is the 'error' event is emitted even before the stream object finished being created. As a result node doesn't find the 'error' listener that you attached since it's not yet created and throws it as Unhandled 'error' event .

One solution would be delay the event emit using setImmediate . Then it'll create the stream object and bind the error listeners first before emitting the error :

 if (!options.okay) {
    setImmediate(function(){ 
      self.emit('error', new Error('Forced crash')); 
    });
    return;
  }