Implementing APP_INITIALIZER in Angular: A Beginner’s Guide 

When building modern web applications, you often need to set up your app with essential data or configurations before it starts. This is the work of Angular APP_INITIALIZER. It helps you run initialization tasks before the main app loads, in a way ensuring everything is ready to go when users interact with your application.

At its core, APP_INITIALIZER is an Angular feature that allows you to run specific functions during the app’s startup phase. These functions can perform tasks like fetching configuration data, checking user authentication, or loading essential services. In simple terms, it’s like a to-do list of tasks you want to run when your app is first loading.

So, how does Angular APP_INITIALIZER really work?

How Does Angular APP_INITIALIZER Work?

Think that you have a function that needs to load some important data, like user settings, or check the server for updates before users can start interacting with the app. The functions provided to APP_INITIALIZER are injected when your app begins its startup process.

These functions can delay the application’s initialization by returning a Promise or an Observable. Why is this important? Sometimes, you need to wait for certain actions to complete—like fetching data from an API—before the app is ready for the user.

Here’s a simple code example:

Why Use Angular APP_INITIALIZER?

Using APP_INITIALIZER gives you control over your app’s startup behavior. Instead of letting everything run without order, you can ensure that important tasks like loading configurations, setting user preferences, or checking API readiness are completed first. 

These are some common use cases:

1. Loading configuration files: If your app relies on external configurations, you can use Angular APP_INITIALIZER to fetch and load them before the app begins.
2. Setting up global services: When certain services (like authentication) need to be ready from the start, you can initialize them before the app loads.
3. Performing initial data fetches: You can use it to make sure essential data is available to your app’s components before they start rendering.

APP_INITIALIZER provides a more reliable and user-friendly experience. It provides a smoother experience for your users, as they won’t run into issues due to incomplete or missing data. 

Using APP_INITIALIZER

One of the main reasons for using Angular APP_INITIALIZER is to handle asynchronous operations. For example data fetching, when your app is loading. You want to make sure everything is set up correctly before your users start interacting with the app.

Here’s how you can use APP_INITIALIZER step-by-step:

1. Create a Factory Function 

The first step in using Angular APP_INITIALIZER is to create a factory function. This function is responsible for loading the data you need before the application starts. 

For example, let’s say you need to fetch some configuration data from a server before the app begins. Your factory function might look something like this:

In this example, the loadAppConfig function uses a service (in this case, ConfigService) to fetch configuration data. The function returns a Promise, ensuring that the app waits for the data to load before it continues initializing.

2. Provide the Factory Function to APP_INITIALIZER Token

The next step is to provide it to Angular APP_INITIALIZER in your module. This tells Angular that the function should be executed during the app’s startup phase.

Here’s how you can set that up:

In this code:

• useFactory tells Angular to use the loadAppConfig factory function.
• deps specifies that ConfigService is a dependency that needs to be injected.
• multi: true allows you to register multiple initializers if necessary.

Once the APP_INITIALIZER is set up, the function is executed when the application starts. This helps you make sure that any critical data or resources are available before the app fully boots up.

For example, if you’re loading a configuration file, the app will wait until the configuration is fetched before it displays anything to the user. 

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Now, let’s look at some real-world scenarios where Angular APP_INITIALIZER comes in handy.

Common Scenarios for Using APP_INITIALIZER

There are several practical cases where you might want to use Angular APP_INITIALIZER to load necessary data before your Angular app starts.

Fetching the Base URL for HTTP Calls from an External Source

In large applications, the API’s base URL may change based on the environment (development, staging, or production). Using APP_INITIALIZER, you can fetch the base URL dynamically from an external source before the app starts making HTTP calls. Hence, all services will point to the correct URL without the need for manual updates.

Loading Main Menus and Sub-Menus from a Repository

Imagine a scenario where your app’s navigation menus are stored in a central repository or fetched dynamically based on user roles. Using Angular APP_INITIALIZER, you can load the main menus and sub-menus before the app displays them.

Fetching User Login Information or Authentication Details

For applications that rely on user authentication, it’s important to check whether the user is already logged in or needs to be redirected to a login page. APP_INITIALIZER can be used to verify the user’s authentication status before the app loads its main components.

Loading Application Details Like Name and Version

Some applications display information like the app’s name, version number, or release notes on the startup screen. Using APP_INITIALIZER, you can fetch this information from a server or a configuration file and display it as part of the app’s initial view.

Fetching Feature Toggles

Feature toggles are commonly used to enable or disable specific functionality in an application. If you’re deploying different features to different user groups, you can use APP_INITIALIZER to fetch these toggles from a server or configuration file before the app fully starts.

Restricting Application Loading

In some cases, you may want to restrict access to your application based on the user’s geographical location. With Angular APP_INITIALIZER, you can fetch location data and apply restrictions before the app loads. 

Displaying Different Data for Different Countries

For global applications, it’s common to show different currencies, units of measurement, or localized content based on the user’s location. APP_INITIALIZER can be used to fetch country-specific data, such as exchange rates or currency formats.

Loading Themes for Specific Scenarios

In some applications, you might offer different themes or UI styles based on user preferences, the time of day, or specific scenarios (like dark mode or high contrast for accessibility). With APP_INITIALIZER, you can fetch and apply the correct theme before the app’s UI is rendered.

Check out this blog: Is Angular Best Used for Frontend or Backend in Your Project?

Let’s dive into how to implement code in Angular APP_INITIALIZER.

Code Implementation

Whether you’re working on a traditional NgModule-based Angular app or experimenting with standalone components using the angular app_initializer, this section will guide you through the process.

1. At the heart of APP_INITIALIZER is a function that performs the tasks needed before the app loads. This function can return a Promise or an Observable, so you can handle asynchronous operations like fetching data or making HTTP requests.
2. In a typical NgModule-based Angular application, you’ll need to register this function within the ‘providers’ array of your ‘AppModule’. By doing so, Angular knows to run this function during the app’s initialization process.
3. With Angular’s recent support for standalone components, you might be building an app that doesn’t rely on NgModules. In such cases, you can bind your initialization function directly to the ‘bootstrapApplication’ function.

One of the most common use cases for APP_INITIALIZER is fetching configuration data that the app needs to run properly. Let’s take a look at this example.

Practical Example: Loading Configuration Data

Whether it’s API endpoints, app settings, or user preferences, configuration data needs to be available before the app initializes. 

1. Create a ConfigurationService

The first step is to create a ConfigurationService that will be responsible for fetching configuration data from an external source, typically via an HTTP GET request. This service ensures that all critical settings are loaded before the app begins.

2. Provide the ConfigurationService to APP_INITIALIZER

The next step is to register this configuration loading function with Angular APP_INITIALIZER in your AppModule. This tells Angular to run the loadConfigurationData function before the app is fully initialized.

3. Ensure APP_INITIALIZER Completes Before the App Fully Initializes

The beauty of APP_INITIALIZER is that it ensures all asynchronous tasks (like fetching configuration data) are completed before the app loads. If the Promise returned by loadConfigurationData is still pending, Angular will delay the app’s startup until the data is ready.

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One practical use case of APP_INITIALIZER is configuring routes dynamically based on whether a user is authenticated.

Dynamic Routes Based on Authentication

Here’s how you can set up dynamic routing based on authentication using Angular APP_INITIALIZER.

1. The first step is to define an initialization function that checks the user’s authentication status and configures the app’s routes accordingly. This function will interact with your authentication service and route settings to determine which pages the user can access.
2. Once you’ve defined the initialization function, the next step is to provide this function to APP_INITIALIZER in the AppModule. This ensures that Angular runs the function during the app’s startup phase, dynamically configuring routes before the app loads fully.

The real magic happens when the APP_INITIALIZER delays the app’s initialization until the route configuration is updated. 

• If the user is authenticated, they are directed to secure pages like the dashboard or profile.
• If the user is not authenticated, they are directed to the login page.

In some cases, you may need to run multiple initialization tasks before your app starts. This is where using multiple APP_INITIALIZER tokens comes into play. Let’s see how.

Handling Multiple APP_INITIALIZER Tokens

You can ensure that everything runs smoothly without one task blocking another by handling each task separately.

1. The first step in handling multiple Angular APP_INITIALIZER tokens is to create separate functions for each initialization task. Each function should return a Promise or an Observable to ensure Angular knows when the task has been completed.
2. Once you’ve defined your initialization functions, the next step is to provide them as separate APP_INITIALIZER tokens in your AppModule. This way, Angular can execute each initialization function independently.
3. In some cases, you may need to ensure that certain initialization tasks are completed before others. You can manage the order of initialization by ensuring that one APP_INITIALIZER completes before the next one begins. You can achieve this by chaining Promises or by setting dependencies between the functions.

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Conclusion

There are many practical ways to implement Angular APP_INITIALIZER in your applications. For example:

• Loading Configuration Data: Fetching settings or API endpoints from a server before the app starts is a common use case.
• Setting Up Routes: Dynamically configuring routes based on user authentication ensures the app is secure and tailored to each user’s needs.
• Customizing Initial Data Loads: You can preload essential data for services or components to make sure the app runs smoothly from the start.

One of the main advantages of using APP_INITIALIZER is the ability to delay the app’s full initialization until critical configuration tasks are completed. To achieve a seamless application startup, it’s crucial to manage dependencies and asynchronous operations properly.

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Also read: Build Your First Angular App: A Simple Tutorial.