Tutorial: Simulating Errors

Except for user management methods like createUser that validate their arguments, MockFirebase calls will never results in asynchronous errors since all data is maintained locally. Instead, MockFirebase gives you two options for testing error handling behavior for both data and authentication methods:

1. failNext(method, err): specifies that the next invocation of method should call its completion callback with err
2. forceCancel(err [, event] [, callback] [, context]): cancels all data event listeners registered with on that match the provided arguments

While failNext is limited to specifying a single error per method, forceCancel can simulate the cancellation of any number of event listeners.

failNext

Using failNext is a simple way to test behavior that handles write errors or read errors that occur immediately (e.g. an attempt to read a path a user is not authorized to view).

Source

var log = {
  error: function (err) {
    console.error(err);
  }
};
var people = {
  ref: function () {
    return firebase.database().ref()
  },
  create: function (person) {
    people.ref().push(person, function (err) {
      if (err) log.error(err);
    });
  }
};

In our tests, we’ll override log.error to ensure that it’s properly called.

Test

MockFirebase.override();
var ref = people.ref();
var errors = [];
log.error = function (err) {
  errors.push(err);
};
people.failNext('push');
people.create({
  first: 'Ben'
});
people.flush();
console.assert(errors.length === 1, 'people.create error logged');

forceCancel

forceCancel simulates more complex errors that involve a set of event listeners on a path. forceCancel allows you to simulate Firebase API behavior that would normally occur in rare cases when a user lost access to a particular reference. For a simple read error, you could use failNext('on', err) instead.

In this example, we’ll also record an error when we lose authentication on a path.

Source

people.ref().on('child_added', function onChildAdded (snapshot) {
  console.log(snapshot.val().first);
}, function onCancel () {
  log.error(err);
});

Test

var errors = [];
log.error = function (err) {
  errors.push(err);
};
var err = new Error();
people.forceCancel(err, 'child_added');
console.assert(errors.length === 1, 'child_added was cancelled');