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Learn how to fix TypeScript inline errors in VSCode when using Deno, including handling return types and adjusting TypeScript configurations.
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Resolving VSCode Inline TypeScript Errors with Deno

If you've recently installed Deno and started coding in TypeScript, you might have encountered a pesky issue where VSCode isn't showing inline TypeScript errors. This can be frustrating, especially if you're following a guide and want to catch mistakes on-the-go. In this post, we’ll dive into the problem, understand why it’s happening, and explore effective solutions to ensure you get the most out of your coding experience.

Identifying the Problem

You’re working on a simple TypeScript function to check even or odd numbers, and it’s expected to return a string. However, you're facing an inline warning stating:

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

This message is confusing because, while you're running your code correctly, the editor isn't providing feedback as expected.

Here's the sample code you're working with:

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Understanding the Issue

The primary reason for this warning revolves around TypeScript's handling of function return types. In your case, the function someFunc may return undefined if the input isn't even, which leads to the warning when you attempt to call substring on the possibly undefined value. Here's why this happens:

Return Types: Your function's default return type is inferred as string | undefined since you may not be returning a value in all code paths (i.e., when n is not even).

Strict Function Types: The TypeScript configuration may have strictFunctionTypes enabled in your tsconfig.json. This option enforces stricter checks regarding function return types, leading to the observed error.

Solutions to Fix the Error

1. Handle All Return Cases

One straightforward way to deal with the warning is to ensure that all possible cases are handled in your function. Here’s one way to adjust your function:

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By ensuring that your function returns a value regardless of the input, TypeScript can safely conclude that the return type is a string rather than string | undefined.

2. Adjust Your TypeScript Configuration

If you prefer a workaround that lets you keep the current function logic while silencing the warning, you can turn off strict function type checking in your tsconfig.json. Here's how to do it:

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Note: While this allows you to avoid the immediate issue, it’s important to remember that disabling strictFunctionTypes might not be ideal, especially in larger projects where type safety is crucial.

Conclusion

Dealing with inline errors in TypeScript while using VSCode and Deno can seem intimidating at first. However, by understanding the underlying causes and implementing the solutions outlined above, you can ensure a smoother development experience. Whether you choose to modify your function or adjust TypeScript settings, being aware of these aspects will help you avoid confusion and catch errors effectively. Happy coding!