Скачать или смотреть Understanding Why Your Angular Service Returns null Values and How to Fix It

Learn the common pitfalls of Angular services returning `null` and how to resolve them effectively for smoother functionality.
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This video is based on the question https://stackoverflow.com/q/68756596/ asked by the user 'Arnaud VDR' ( https://stackoverflow.com/u/16453649/ ) and on the answer https://stackoverflow.com/a/68757030/ provided by the user 'Michael D' ( https://stackoverflow.com/u/6513921/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Why Your Angular Service Returns null Values and How to Fix It

When working with Angular applications, you may encounter instances where a service unexpectedly returns null, causing issues in your front-end functionality. This can be frustrating, especially when the operations in the backend appear to execute correctly. In this guide, we'll explore a common scenario that leads to this problem, examine the provided code, and outline the steps to identify and resolve the issue effectively.

The Problem: Unruly Null Values

In a specific situation, a developer faced the challenge of an Angular service returning null values. This impeded the ability to push data into an array, leading to the need for a manual page refresh to display the updates. The developer faced an error where the next callback within the observable pipeline reported that the object x was null.

Here's an example of the relevant section of their code:

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Key Observations:

The developer received valid data in the console logs but did not see a return value for produit in their subscription callback.

The error message read: ERROR TypeError: x is null.

Understanding the Solution

To remedy this issue, several factors need to be taken into account:

1. Ensure Proper Typing

When working with TypeScript, it’s crucial to define types for data structures to access properties reliably using dot notation. The tap operator should explicitly indicate the type of the variable being handled.

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2. Console Logs Are Asynchronous

It’s important to recognize that the order of console log statements can be asynchronous, meaning the sequence of events may not follow your expectations. In this scenario, the logs can be ordered as 1 (form log), 3 (post request start), and then 2 (inside the subscription callback). Thus, the result in log 2 isn’t guaranteed to come immediately after log 3, leading to confusion.

3. Angular Change Detection

When updating arrays, simply using the .push() method will not trigger Angular's change detection, as the reference to the original array has not changed. To address this:

You can create a new array using destructuring:

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By doing so, Angular will recognize the change and update the view accordingly.

Conclusion

Encountering null values in an Angular service can be a misunderstanding of data flow between your front end and back end. By ensuring clear typing, understanding the asynchronous nature of logs, and utilizing Angular’s change detection correctly, you can create a smoother user experience without needing to refresh the page manually. With these best practices, you can tackle similar challenges confidently.

Thanks for reading, and happy coding!