Resolving the Not assignable to parameter of type never TypeScript Error in Vue Store

Learn how to fix the common TypeScript error "Not assignable to parameter of type never" caused by Vue store array declarations in your projects.
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Resolving the Not assignable to parameter of type never TypeScript Error in Vue Store

Understanding the Error

The crux of the problem lies in TypeScript’s attempt to infer types when a type is not explicitly declared. In the specific case highlighted, you’re dealing with a mutation method in your Vuex store that tries to find the index of a section based on its ID. Here's the critical line of code that triggers the error:

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TypeScript is having a hard time determining the type of elements in the sections array because the initial state declaration may give the impression that the items are always empty and hence, inferred as never[]—an array that cannot contain any values.

Step-by-Step Solution

Let’s walk through the steps you can take to resolve this TypeScript error.

1. Specify the Return Type of the State

A common solution is to explicitly define the return type of the state function. By specifying the return type, TypeScript can better understand the structure of your state. Here’s how you can do it:

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In this snippet, the return type is clearly defined as an object containing a sections property that is an array of section objects.

2. Use an Explicit Interface for State

If your state becomes more complex, or if you prefer clearer code, consider defining a separate interface for your state. This promotes readability and maintains type safety. Here’s an example:

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3. Ensure All Properties Are Defined

Next, TypeScript may throw another error if the structure of your section objects does not match the defined section interface. In the given example, your interface stipulates that each section should have an additional property named section_body, which is missing in the original state declaration. You have two choices here:

Add the section_body property to section objects in your state:

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Modify the interface to make the section_body property optional (if it may not always be present):

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4. Improve the Type Safety of sectionItems

Lastly, if you want to enhance the type safety of your sectionItems property, consider changing its type from any to Array<any> or a more specific object type that reflects the expected structure of the items.

Conclusion

With these adjustments, your store should now function without any errors, allowing you to focus on building your Vue application effectively!