How to Fix 'Property 'map' does not exist on type 'never'' in Next.js with TypeScript

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This error typically arises when TypeScript is unable to infer the correct type for a variable that you are trying to use the .map function on. Below is a detailed guide to understand and fix this error.

Understanding the Error

The error "Property 'map' does not exist on type 'never'" occurs because TypeScript has inferred the type of the variable as never. The never type in TypeScript represents the type of values that never occur. This type is often the result of an empty array or an inconsistent state in type inference.

Common Scenarios Leading to the Error

Uninitialized State:
If your initial state is not set properly, TypeScript may infer it as never.

Incorrect Type Inference:
This can happen if TypeScript cannot infer the type of your data correctly, leading it to assume never.

Solution Guide

Here are some steps to resolve this error effectively:

Initialize State Correctly

When using React state, make sure to provide a proper initial state:

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In this case, items is correctly initialized as an array of strings.

Explicit Type Annotation

Provide explicit type annotation to avoid type inference issues:

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By defining the ItemType and annotating the state appropriately, TypeScript can infer the types correctly, preventing the never type issue.

Ensure Non-Empty Arrays

Make sure you are not attempting to map over an array that is possibly undefined or null. Use conditional checks or default values:

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Alternatively:

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Conclusion

We hope this guide helps you troubleshoot and resolve the "Property 'map' does not exist on type 'never'" error in your projects. Happy coding!