[$injector:strictdi] Missing explicit dependency injection

The error [$injector:strictdi] Missing explicit dependency injection in AngularJS occurs when AngularJS’ strict dependency injection mode (strictDI) detects that a component (such as a controller, service, or factory) does not explicitly declare its dependencies.

Strict DI mode enforces that dependencies are explicitly injected into the constructor function rather than relying on AngularJS to infer them. This is crucial because AngularJS uses minification in production to shrink the size of JavaScript files, and minification renames variables, which can break the automatic dependency injection mechanism. Therefore, strictDI ensures that the dependencies are correctly injected regardless of minification.

Step 1: Understanding the Error

AngularJS uses dependency injection (DI) to inject services, factories, and other dependencies into controllers, directives, and other components. When strict DI mode is enabled, AngularJS expects dependencies to be explicitly defined using array notation in the function signature, to prevent issues with minification.

For example, without strict DI, AngularJS might automatically inject dependencies into a function:

angular.module('app')
  .controller('MyController', function($scope, MyService) {
    // Controller logic
  });

In this case, AngularJS automatically injects $scope and MyService based on their names. However, during minification, the names of $scope and MyService might be changed, breaking the dependency injection.

Step 2: Enabling Strict DI

To enable strict DI mode in AngularJS, you need to follow the best practice of explicitly defining dependencies in the following format:

1. Array Notation (Explicit Dependency Injection)

In strict DI mode, you should use array notation to specify the dependencies explicitly:

angular.module('app')
  .controller('MyController', ['$scope', 'MyService', function($scope, MyService) {
    // Controller logic
  }]);

In this example, the first element in the array is the name of the dependency ($scope and MyService), and the last element is the function that receives these dependencies.

This way, even if minification changes the names of $scope or MyService, AngularJS can still inject the correct services because it relies on the array notation to identify the dependencies.

2. Using ng-annotate for Auto-Injection

To make this process easier, especially in larger applications, you can use a tool like ng-annotate to automatically add the necessary annotations to your code. This tool will help you add array-based dependency annotations even if you’re using the more compact syntax.

For example, you could write:

angular.module('app')
  .controller('MyController', function($scope, MyService) {
    // Controller logic
  });

After running ng-annotate, the code would be transformed into:

angular.module('app')
  .controller('MyController', ['$scope', 'MyService', function($scope, MyService) {
    // Controller logic
  }]);

Step 3: How to Fix the [$injector:strictdi] Error

To fix the [$injector:strictdi] Missing explicit dependency injection error, follow these steps:

1. Use Array Notation for Dependency Injection

Whenever you define a controller, service, directive, or factory, use array notation to explicitly specify the dependencies.

Example for a controller:

angular.module('app')
  .controller('MyController', ['$scope', 'MyService', function($scope, MyService) {
    // Controller logic
  }]);

Example for a factory:

angular.module('app')
  .factory('MyFactory', ['$http', '$q', function($http, $q) {
    // Factory logic
  }]);

2. Ensure All Components are Annotated

If you’re using AngularJS components like controllers, factories, services, or directives, ensure that all dependencies are correctly annotated in the array format.

Example for a directive:

angular.module('app')
  .directive('myDirective', ['MyService', function(MyService) {
    return {
      restrict: 'E',
      link: function(scope, element, attrs) {
        // Directive logic
      }
    };
  }]);

3. Use ng-annotate for Automatic Annotations

To avoid manually adding annotations for each component, you can use ng-annotate or a similar tool to automatically add the array syntax to your code.

• First, install ng-annotate globally: npm install -g ng-annotate
• Then, run it on your JavaScript files: ng-annotate -a <inputFile.js> -o <outputFile.js>

This will ensure that all dependencies are explicitly declared.

4. Verify Minification Configuration

If you’re using a build tool like Grunt, Gulp, or Webpack to minify your code, make sure that it is properly configured to handle strict DI annotations. Many tools have settings to integrate with ng-annotate or other similar packages to automatically add the array-based annotations during the build process.

Step 4: Common Mistakes to Avoid

1. Forgetting to Use Array Notation: If you forget to use array notation in strict DI mode, AngularJS will throw the [$injector:strictdi] error. Always ensure that you provide dependencies explicitly using the array notation.
2. Using Non-Explicit Dependencies: If you have a controller or service with dependencies that are not explicitly injected (i.e., without array notation), AngularJS will not be able to resolve those dependencies, resulting in errors.
3. Minifying Code Without Annotations: Minification can rename your variables, causing the DI mechanism to break. Ensure that all components are annotated with explicit dependencies to avoid this issue.

Example of Strict DI Mode Fix:

Before Strict DI:

angular.module('app')
  .controller('MyController', function($scope, MyService) {
    // Controller logic
  });

After Strict DI (with Array Notation):

angular.module('app')
  .controller('MyController', ['$scope', 'MyService', function($scope, MyService) {
    // Controller logic
  }]);

Step 5: Debugging and Tools

1. Use ng-annotate to help automate the process of adding annotations to your code, especially if you are dealing with a large codebase.
2. Check for Minification Issues: Always test your code after minification to ensure that your DI works correctly. Minified code without explicit dependency injection will often break in production.
3. Check the AngularJS Developer Tools: AngularJS has debugging tools that allow you to inspect the dependency injection in your application. These can help you track down errors related to DI.
4. Ensure Consistency Across the Codebase: When working in teams, ensure that everyone follows the strict DI syntax for consistency. This helps prevent errors, especially in large applications.