Скачать или смотреть Fixing the undefined Error When Updating Arrays in Svelte Stores

Learn how to properly update arrays in Svelte stores to avoid the common `undefined` error, ensuring your Svelte components work seamlessly.
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Solving the undefined Error When Updating Arrays with Svelte Stores

If you're venturing into the world of Svelte and custom stores, you might find yourself facing an annoying issue: getting an undefined error when trying to update a writable array in your stores. Understanding why this happens and how to fix it is crucial for ensuring your Svelte components function properly. In this guide, we'll dissect the problem and walk through a solution step-by-step.

Understanding the Problem

When using Svelte's writable store to manage state, updating an array incorrectly can lead to issues that affect components subscribed to that store. In essence, when we try to update our array, we're unintentionally returning undefined, which confuses Svelte and triggers errors in the component that relies on that data.

Common Scenario That Causes Errors

Let's take a look at a simple example where we have a writable store with an array:

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

In this example, activeData is a writable store containing a single string in an array. Now, when we implement a component to update this store by pushing a new value ("pushed!"), we run into trouble:

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

The problem arises from the line val = val;. The update callback should return the new value, but here we're inadvertently returning undefined. This causes our other components that rely on activeData to fail with an error: "{# each} only iterates over array-like objects."

Solution to the Problem

To fix the issue, we need to ensure that the update callback returns the modified array. Instead of pushing to the existing array in place, we can use the spread operator to create a new array that includes the existing values plus the new element. Here's the corrected line of code to achieve that:

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

Updated Component Code

After implementing the fix, your handlePush function will look as follows:

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

What Changed and Why It Works

Returning the New Array: By using the spread operator, we're returning a new array that contains both the old values from val and the new "pushed!" string. Svelte correctly recognizes this updated data structure as an array.

Avoiding Mutations: The approach avoids direct mutations to the original array, which is a good practice in programming to maintain immutability and help prevent bugs.

Conclusion

Dealing with undefined errors while updating arrays in Svelte can be frustrating, especially for those new to the framework. However, by understanding the importance of what your update function returns, you can avoid many of these pitfalls. Remember, when you're updating arrays in Svelte stores, always ensure you're returning the new value so that your components behave correctly. Happy coding!