TypeScript Type Error When Transforming API Response Format to Different Format Using Select in useInfiniteQuery

If you’re a developer working with React Query and TypeScript, you might have encountered a frustrating issue: a type error when transforming API response data using the select option in useInfiniteQuery. Don’t worry, you’re not alone! In this article, we’ll dive into the problem, explain why it happens, and provide a step-by-step solution to overcome this hurdle.

Understanding the Problem

When using React Query’s useInfiniteQuery hook, you might want to transform the API response data into a different format before displaying it to the user. This is where the select option comes in. It allows you to specify a function that takes the API response data as an input and returns the transformed data.

import { useInfiniteQuery } from '@tanstack/react-query';

interface ApiResponse {
  data: {
    items: string[];
  };
}

interface TransformedData {
  items: string[];
  totalCount: number;
}

function fetchData(page: number) {
  return fetch(`https://api.example.com/data?page=${page}`);
}

function transformData(response: ApiResponse) {
  return {
    items: response.data.items,
    totalCount: response.data.totalCount,
  };
}

function MyComponent() {
  const { data, error, isLoading } = useInfiniteQuery(
    ['myData'],
    ({ pageParam = 1 }) => fetchData(pageParam),
    {
      select: transformData,
    }
  );

  if (isLoading) return 
Loading...
;
  if (error) return 
Error: {error.message}
;

  return (
    

      {data.items.map((item, index) => (
        
{item}
      ))}
    
  );
}

In the example above, we’re using the select option to transform the API response data into a new format using the transformData function. However, this is where the type error occurs.

Type Error Explanation

The type error occurs because TypeScript is complaining about the type mismatch between the API response data and the transformed data. The select option expects the transformation function to return data of the same type as the API response data. In our example, the API response data is of type ApiResponse, but the transformed data is of type TransformedData.

This type mismatch causes TypeScript to throw an error, complaining that the select option is trying to return a value of type TransformedData when it expects ApiResponse.

Solution: Using the `select` Option with Type Casting

To overcome this type error, we need to cast the transformed data to the correct type using the as keyword. We’ll also add type annotations to the select option to ensure that TypeScript understands the correct return type.

import { useInfiniteQuery } from '@tanstack/react-query';

interface ApiResponse {
  data: {
    items: string[];
  };
}

interface TransformedData {
  items: string[];
  totalCount: number;
}

function fetchData(page: number) {
  return fetch(`https://api.example.com/data?page=${page}`);
}

function transformData(response: ApiResponse) {
  return {
    items: response.data.items,
    totalCount: response.data.totalCount,
  } as TransformedData; // Add type casting
}

function MyComponent() {
  const { data, error, isLoading } = useInfiniteQuery(
    ['myData'],
    ({ pageParam = 1 }) => fetchData(pageParam),
    {
      // Add type annotation for the select option
      select: (data: ApiResponse) => transformData(data) as TransformedData,
    }
  );

  if (isLoading) return 
Loading...
;
  if (error) return 
Error: {error.message}
;

  return (
    

      {data.items.map((item, index) => (
        
{item}
      ))}
    
  );
}

By adding the type casting and annotation, we’re telling TypeScript that the transformed data is of type TransformedData, and it should be treated as such. This resolves the type mismatch and allows us to use the transformed data without any issues.

Alternative Solution: Using a Separate Transformation Function

If you prefer not to use type casting, you can create a separate transformation function that returns the transformed data with the correct type. This approach can be useful when you need to perform complex transformations or when you want to keep the transformation logic separate from the useInfiniteQuery hook.

import { useInfiniteQuery } from '@tanstack/react-query';

interface ApiResponse {
  data: {
    items: string[];
  };
}

interface TransformedData {
  items: string[];
  totalCount: number;
}

function fetchData(page: number) {
  return fetch(`https://api.example.com/data?page=${page}`);
}

function transformApiResponseToTransformedData(apiResponse: ApiResponse): TransformedData {
  return {
    items: apiResponse.data.items,
    totalCount: apiResponse.data.totalCount,
  };
}

function MyComponent() {
  const { data, error, isLoading } = useInfiniteQuery(
    ['myData'],
    ({ pageParam = 1 }) => fetchData(pageParam)
  );

  const transformedData = data ? transformApiResponseToTransformedData(data) : undefined;

  if (isLoading) return 
Loading...
;
  if (error) return 
Error: {error.message}
;

  return (
    

      {transformedData && transformedData.items.map((item, index) => (
        
{item}
      ))}
    
  );
}

In this example, we’ve created a separate transformation function transformApiResponseToTransformedData that takes the API response data as an input and returns the transformed data with the correct type. We then use this function to transform the data after the useInfiniteQuery hook has fetched the data.

Conclusion

In this article, we’ve explored the issue of TypeScript type errors when transforming API response data using the select option in useInfiniteQuery. We’ve provided two solutions to overcome this problem: using type casting and creating a separate transformation function. By following these solutions, you should be able to transform your API response data without any type errors and enjoy a seamless development experience with React Query and TypeScript.

If you have any questions or need further assistance, feel free to ask in the comments below!

Section	Description
Understanding the Problem	Explains the problem and provides an example code snippet
Type Error Explanation	Explains the type error and why it occurs
Solution 1: Using the `select` Option with Type Casting	Provides a solution using type casting and annotations
Solution 2: Using a Separate Transformation Function	Provides an alternative solution using a separate transformation function
Conclusion	Summarizes the article and provides a conclusion

• Remember to add type annotations for the select option to ensure TypeScript understands the correct return type.
• Use type casting sparingly and only when necessary, as it can lead to type inconsistencies.
• Keep your transformation logic separate from the useInfiniteQuery hook to maintain code organization and reusability.

Happy coding!

Frequently Asked Question

Get answers to the most common questions about dealing with TypeScript type errors when transforming API response formats using select in useInfiniteQuery.