Скачать или смотреть Understanding Vue Composition API: Why .value Returns null and How to Fix It

Learn why `console.error(error.value)` returns `null` in Vue Composition API and how to correctly handle async functions with axios.
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This video is based on the question https://stackoverflow.com/q/68480320/ asked by the user 'Mr.Brownies' ( https://stackoverflow.com/u/7342010/ ) and on the answer https://stackoverflow.com/a/68480881/ provided by the user 'Estus Flask' ( https://stackoverflow.com/u/3731501/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Vue Composition API: Why .value Returns null and How to Fix It

Vue.js is a popular framework for building user interfaces, and the Composition API has introduced a new way to compose component logic. However, developers sometimes encounter unexpected results, such as when the .value property of a reference returns null. In this post, we'll dive into a specific issue regarding the Vue Composition API and provide clear solutions.

The Problem

Imagine you have a composable function that fetches data using Axios. After implementing your useFetch function, you encounter a puzzling situation:

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When you execute this line, it outputs null, even though earlier logs suggest there was an error captured in the error object. Let's explore why this happens.

The Heart of the Issue

Reference Behavior:

When you log console.error(error), you're outputting the entire error object, which at that moment may contain an error message.

However, console.error(error.value) fetches the value of the error immediately. If the fetch operation has not yet completed (because it's asynchronous), this value will still be null.

Asynchronous Execution:

The useFetch function executes an asynchronous fetch request. By the time you try to access error.value, the request may not have completed, hence returning null.

The Solution

To effectively deal with this, you can use one of the following approaches:

Option 1: Use a watch to Monitor Loading State

The first solution involves using Vue's watch function to monitor the loading state. When the loading is complete, you can safely log the error value:

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This way, console.error(error.value) is called only after the fetch operation has completed, ensuring that you are accessing the updated value.

Option 2: Return a Promise

Another approach is to modify your composable useFetch to return a Promise, allowing you to chain it and handle the result afterward:

Update your fetch function to return the promise:

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Additionally, utilize the promise to capture the result within the onMounted lifecycle method:

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This method provides more control over how to handle async data, aligning well with the Composition API while ensuring you access values only when data is ready.

Conclusion

In Vue.js, understanding how to properly access reactive references is crucial, especially within asynchronous contexts. If you encounter a situation where .value returns null, remember to check if the operation has completed. Utilize watch or return a Promise to handle these cases effectively.

Now, with this knowledge, you can troubleshoot similar issues in your Vue applications and correctly manage asynchronous data fetching with the Composition API. Happy coding!