Mongoose 4.5.0 introduces the ability to handle errors in middleware. This lets you write middleware and plugins to transform errors into something useful for your application. In other words, no more cryptic "E11000 duplicate key" errors will leak to your users if you set up the correct middleware. If you've used Express' error handling middleware, mongoose's will look familiar.

An Aside On Mongoose Post Middleware

Mongoose has 2 types of middleware, "pre" middleware and "post" middleware. Pre middleware executes before the wrapped function, and post middleware executes after the wrapped function.

schema.pre('save', function(next) {
  console.log('before save');
  next();
});

schema.post('save', function(doc) {
  console.log('after save');
});

doc.save(); // Prints 'before save' followed by 'after save'

However, since mongoose 3.x post middleware has been interchangeable with on() calls. In other words, schema.post('save') is the same thing as schema.on('save'). Therefore, by design, mongoose document post middleware does not get flow control (query post middleware does, and in 5.0 document post middleware will too).

schema.post('save', function() {
  console.log('after save');
});

doc.save(function() {
  // May print "after save" after "save callback" or vice versa,
  // the order is undefined.
  console.log('save callback');
});

Mongoose 4.5 introduces a special type of document post middleware that does get flow control: error handlers.

Introducing Mongoose Error Handling Middleware

You define an error handler the same way you define a normal post middleware. To mark a post middleware as an error handler, you need to make it take 3 parameters:

var schema = new Schema({
  name: {
    type: String,
    unique: true
  }
});

// Handler **must** take 3 parameters: the error that occurred, the document
// in question, and the `next()` function
schema.post('save', function(error, doc, next) {
  if (error.name === 'MongoError' && error.code === 11000) {
    next(new Error('There was a duplicate key error'));
  } else {
    next(error);
  }
});

var Person = mongoose.model('Person', schema);

var person = { name: 'Val' };

Person.create([person, person], function(error) {
  // Will print out "There was a duplicate key error"
  console.log(error);
});

An error handler middleware only gets called when an error occurred. In other words, the above error handler middleware will only execute if a pre save hook called next() with an error, if save() calls back with an error, or if a previous post hook called next() with an error.

schema.pre('save', function(next) {
  // This middleware will prevent `save()` from executing and go straight
  // to executing the error handling middleware
  next(new Error('pre save error'));
});

schema.post('save', function(doc, next) {
  // If this hook is defined _before_ an error handler middleware, this will
  // skip all other non-error-handler post save hooks and execute the next
  // error handler middleware
  next(new Error('post save error'));
});

As an aside, there are 4 functions in mongoose that can trigger a duplicate key error: save(), insertMany(), update(), and fineOneAndUpdate(). The create() function can also trigger a duplicate key error, but create() is just a thin wrapper around save(). Here's how you can handle duplicate key errors for all these functions:

var schema = new Schema({
  name: {
    type: String,
    unique: true
  }
}, { emitIndexErrors: true });

var handleE11000 = function(error, res, next) {
  if (error.name === 'MongoError' && error.code === 11000) {
    next(new Error('There was a duplicate key error'));
  } else {
    next();
  }
};

schema.post('save', handleE11000);
schema.post('update', handleE11000);
schema.post('findOneAndUpdate', handleE11000);
schema.post('insertMany', handleE11000);

var Person = mongoose.model('Person', schema);

With this new feature, you can write plugins that can convert MongoDB-specific errors (like the duplicate key error above) as well as mongoose-specific errors (like validation errors) into something that makes sense for your application.

Moving On

Mongoose error handling middleware gives you a centralized mechanism for handling errors in your application. You can use error handlers for logging and even transforming errors, so you can standardize error formatting for your API. I'm especially excited to see what plugins you can write using error handling middleware.

Found a typo or error? Open up a pull request! This post is available as markdown on Github
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