Dependency Injection (DI) is a core feature of AngularJS that helps manage dependencies and services efficiently. However, unit tests may fail due to issues in how dependencies are injected. The most common errors include:
- Unknown provider errors (
$injector:unpr) - Module or service not found
- Incorrect mocking of dependencies
- Improper use of AngularJS modules in test cases
In this guide, we’ll explore why these errors occur, how to debug them, and how to fix them with step-by-step solutions.
1. Common Dependency Injection Errors in Unit Tests
1️⃣ Error: “[$injector:unpr] Unknown provider”
Cause
This error occurs when AngularJS cannot find a required dependency.
For example, if a service is missing in the test setup:
describe('MyController', function () {
beforeEach(module('myApp')); // Forgot to include required modules
it('should work', inject(function (myService) {
expect(myService.someMethod()).toBeDefined();
}));
});
Error:
[$injector:unpr] Unknown provider: myServiceProvider <- myService
Fix
Ensure that the required module and dependencies are injected correctly:
beforeEach(module('myApp')); // Include the module
beforeEach(inject(function (_myService_) {
myService = _myService_; // Inject dependency properly
}));
2️⃣ Error: “Module Not Available”
Cause
If your test does not load the correct module, services and components won’t be found.
Incorrect test setup (missing module):
describe('MyService Tests', function () {
beforeEach(module('someNonExistentModule')); // Wrong module name
});
Error:
Error: Module 'someNonExistentModule' is not available!
Fix
Ensure that the correct module name is loaded:
beforeEach(module('myApp')); // Ensure module is correctly included
3️⃣ Error: “[$injector:modulerr] Failed to instantiate module”
Cause
This usually happens if a required dependency is missing in the module definition.
Example: A module depends on ngRoute, but it is not loaded
angular.module('myApp', ['ngRoute', 'myService']); // ngRoute dependency missing
Error:
[$injector:modulerr] Failed to instantiate module myApp due to:
[$injector:modulerr] Failed to instantiate module ngRoute
Fix
Make sure the module is loaded before running tests:
beforeEach(module('ngRoute')); // Ensure required dependencies are loaded
beforeEach(module('myApp'));
2. How to Properly Inject Dependencies in Unit Tests
1. Injecting Services in Unit Tests
If a service is used in your application, inject it properly in your test case:
describe('MyService Tests', function () {
var myService;
beforeEach(module('myApp')); // Load the module
beforeEach(inject(function (_myService_) {
myService = _myService_; // Inject service
}));
it('should have a defined service method', function () {
expect(myService.someMethod).toBeDefined();
});
});
2. Mocking Dependencies Properly
Some services depend on APIs, which may cause issues in unit tests. Mock these dependencies to isolate tests.
Example: A service depends on $http
app.service('myService', function ($http) {
this.getData = function () {
return $http.get('/api/data');
};
});
If you do not mock $http, tests may fail due to real API calls.
Fix: Mock $http using $httpBackend
describe('MyService Tests', function () {
var myService, $httpBackend;
beforeEach(module('myApp'));
beforeEach(inject(function (_myService_, _$httpBackend_) {
myService = _myService_;
$httpBackend = _$httpBackend_;
}));
it('should return mock data', function () {
$httpBackend.whenGET('/api/data').respond(200, { result: 'success' });
myService.getData().then(function (response) {
expect(response.data.result).toBe('success');
});
$httpBackend.flush();
});
});
This ensures the service does not make real API calls, preventing failures.
3. Mocking Services Instead of Injecting Real Ones
If a controller depends on a service, mock the service instead of injecting the real one.
Example: Controller depends on myService
app.controller('MyController', function ($scope, myService) {
$scope.data = myService.getData();
});
Incorrect Test (injecting real service):
describe('MyController Tests', function () {
var $controller, $scope;
beforeEach(module('myApp'));
beforeEach(inject(function (_$controller_) {
$controller = _$controller_;
}));
it('should fail due to missing myService', function () {
var $scope = {};
var ctrl = $controller('MyController', { $scope: $scope }); // myService missing
expect($scope.data).toBeDefined();
});
});
Fix: Use a mock for myService
describe('MyController Tests', function () {
var $controller, $scope, myServiceMock;
beforeEach(module('myApp'));
beforeEach(module(function ($provide) {
myServiceMock = {
getData: function () {
return "mockData";
}
};
$provide.value('myService', myServiceMock); // Provide mock service
}));
beforeEach(inject(function (_$controller_) {
$controller = _$controller_;
}));
it('should use mock data', function () {
var $scope = {};
var ctrl = $controller('MyController', { $scope: $scope, myService: myServiceMock });
expect($scope.data).toBe("mockData");
});
});
This prevents dependency injection errors and isolates the test correctly.
3. Debugging Dependency Injection Errors
If unit tests still fail, follow these debugging steps:
Check for Missing Modules
Run this in the browser console:
angular.module('myApp'); // Check if module is defined
If the module is missing, ensure it is correctly included in your test.
Check Injected Dependencies
Log dependencies to verify they are injected correctly:
console.log(myService); // Should not be undefined
Use angular.mock.module() for Dependency Overrides
If a dependency is missing, override it in tests:
beforeEach(module(function ($provide) {
$provide.value('someService', { someMethod: function () { return "mocked"; } });
}));