Скачать или смотреть Resolving the Promise Call Invoked Twice Issue in Angular 11

Learn how to solve the problem of duplicate service calls in Angular 11 by understanding component instantiation and proper structuring of your application.
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Resolving the Promise Call Invoked Twice Issue in Angular 11

In the world of Angular development, you may encounter situations that can lead to unpredictable behavior, such as a service call being invoked multiple times. One common problem that many developers face is when they notice their promise calls are being executed more than once. This frequently happens due to the way components are instantiated within the application. Let’s dive into this issue and explore how to effectively resolve it.

Understanding the Issue

First, let's look at the problem in context. Suppose you have implemented a service call in Angular to fetch data, such as a list of countries. When testing, you might intentionally shut down the backend to see how your application handles errors. Instead of a simple error message, you notice that the service call is executed twice. This can be confusing, and you're left wondering why this is happening.

Sample Code

Here’s a simplified version of the code that was used in the instance described:

[[See Video to Reveal this Text or Code Snippet]]

Alongside the service code, which may look like this:

[[See Video to Reveal this Text or Code Snippet]]

The Root Cause

After examining the code, the root cause of the issue was identified as follows:

The component was instantiated twice due to improper configuration in the Angular module.

In the app.module.ts, the component was added to the bootstrap array in addition to being referenced in the application’s template (app.component.html).

Code Example of the Issue

Here’s a snippet from the app.module.ts showing the bootstrap issue:

[[See Video to Reveal this Text or Code Snippet]]

And in your app.component.html, you may have:

[[See Video to Reveal this Text or Code Snippet]]

The presence of both entries is what led to the duplicate instantiation of the MyComponent, which in turn resulted in the service call being made more than once.

The Solution

The solution to this issue is quite straightforward. You need to ensure that your component is only instantiated once in your application. Here’s how you can do that:

Step 1: Adjust Component Bootstrapping

Remove the component from the bootstrap array in your app.module.ts. Change it to only include the primary component, like so:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Continue Using in Template

Ensure that you still keep your component reference in app.component.html like this:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Verify Changes

After making these adjustments, test your application again. You should observe that the service call to fetch countries is now executed only once, preventing unnecessary multiple requests and enhancing the efficiency of your application.

Conclusion

In conclusion, understanding how Angular instantiates components is crucial for effective development. By ensuring that each component is only instantiated once, you can avoid issues like multiple service calls. This problem is common, especially for those new to Angular, but with the right configuration, you can streamline your application and avoid redundant server requests.

If you find yourself facing similar situations, take a close look at how your components are structured and instantiated – it could save you from unexpected behavior and inefficiencies in your application.