Quick Answer

Fix AngularJS module errors: check dependencies, verify module names, and resolve circular references.

Understanding the Issue

The "[$injector:modulerr]" error in AngularJS indicates a problem with module loading or dependency injection. Common causes include missing module dependencies, typos in module names, incorrect script loading order, or circular dependencies between modules. This error prevents the AngularJS application from bootstrapping properly. Understanding AngularJS module system, dependency management, and proper module declaration patterns helps resolve these initialization errors.

The Problem

This code demonstrates the issue:

Javascript Error 
-- Problem 1: Missing module dependency
angular.module("myApp", ["nonExistentModule"]) -- Module doesn't exist
    .controller("MainController", function($scope) {
        $scope.message = "Hello World";
    });

-- Problem 2: Typo in module name or missing script
angular.module("myApp", ["ui.router"]) -- ui-router script not loaded
    .config(function($stateProvider) {
        -- Configuration code
    });

-- HTML trying to bootstrap non-existent module
-- <div ng-app="wrongModuleName">

The Solution

Here's the corrected code:

Javascript Fixed 
-- Solution 1: Proper module declaration and dependency management
-- First, declare the module without dependencies if it's standalone
angular.module("myApp", []);

-- Or with correct dependencies
angular.module("myApp", ["ngRoute", "ui.bootstrap"])
    .controller("MainController", function($scope) {
        $scope.message = "Hello World";
    });

-- Ensure all dependency scripts are loaded before your app
-- <script src="angular.js"></script>
-- <script src="angular-route.js"></script>
-- <script src="ui-bootstrap.js"></script>
-- <script src="app.js"></script>

-- Solution 2: Check and verify module dependencies
-- Create modules in correct order
angular.module("myServices", [])
    .service("DataService", function($http) {
        return {
            getData: function() {
                return $http.get("/api/data");
            }
        };
    });

angular.module("myControllers", ["myServices"])
    .controller("MainController", function($scope, DataService) {
        DataService.getData().then(function(response) {
            $scope.data = response.data;
        });
    });

-- Main app module with all dependencies
angular.module("myApp", [
    "ngRoute",
    "myServices", 
    "myControllers"
])
.config(function($routeProvider) {
    $routeProvider
        .when("/", {
            templateUrl: "home.html",
            controller: "MainController"
        })
        .otherwise({
            redirectTo: "/"
        });
});

-- Debug module loading issues
function checkModuleDependencies() {
    const appModule = "myApp";
    
    try {
        const module = angular.module(appModule);
        console.log("Module loaded successfully:", module.name);
        console.log("Dependencies:", module.requires);
    } catch (error) {
        console.error("Module loading failed:", error.message);
    }
}

-- Safe module retrieval
function getOrCreateModule(moduleName, dependencies = []) {
    try {
        return angular.module(moduleName);
    } catch (error) {
        console.log("Creating new module:", moduleName);
        return angular.module(moduleName, dependencies);
    }
}

-- Usage
const myModule = getOrCreateModule("myUtilModule");
myModule.service("UtilService", function() {
    return {
        formatDate: function(date) {
            return new Date(date).toLocaleDateString();
        }
    };
});

Key Takeaways

Verify all module dependencies exist and scripts are loaded. Check module names for typos. Ensure proper script loading order. Use browser dev tools to debug missing dependencies.