Hey folks,
Did you know that you can filter your data in react query? Noooo! Ok, it's time to learn it!

To filter the data in your useQuery hook, you have to handle the select option. This option accepts a function that is used to filter the data.
The function has one parameter, and this parameter contains the result of the query; the scope of the select function is to manipulate the query's result and returns a new result that respects some rules.

Let's take a look at this example

import { useQuery } from '@tanstack/react-query';
import { Dispatch, SetStateAction, useCallback, useState } from 'react';
import { QUERY_KEY } from 'src/app/constants/queryKeys';
import { Todo } from 'src/app/models';
import { ResponseError } from '../../../../utils/Errors/ResponseError';

const fetchTodos = async (): Promise<Todo[]> => {
  const response = await fetch('api/tasks');
  if (!response.ok) {
    throw new ResponseError('Failed to fetch todos', response);
  }
  return await response.json();
};

interface UseTodos {
  todos: Todo[];
  isLoading: boolean;
  isFetching: boolean;
  error?: string;
  setUserFilter: Dispatch<SetStateAction<number | null>>;
}

function mapError(error: unknown | undefined): undefined | string {
  if (!error) return undefined;
  if (error instanceof ResponseError) return error.response.statusText;
  if (error instanceof Error) return error.message;
  return 'Unknown error';
}

export const useTodos = (): UseTodos => {
  const [userFilter, setUserFilter] = useState<number | null>(null);

  const filterTodoByAssignee = useCallback(
    (todos: Todo[]) => {
      if (!userFilter) return todos;
      return todos.filter((todo) => todo.assigneeId === userFilter);
    },
    [userFilter]
  );

  const {
    data: todos = [],
    isLoading,
    isFetching,
    error,
  } = useQuery([QUERY_KEY.todos], fetchTodos, {
    refetchOnWindowFocus: false,
    retry: 2,
    select: filterTodoByAssignee,
  });

  return {
    todos,
    isLoading,
    isFetching,
    error: mapError(error),
    setUserFilter,
  };
};

As you can notice, this example has some key points to analyse.
First, the filterTodoByAssignee function is the select function passed to the useQuery hook. This function checks the userFilter state, and if it is empty, the function returns the entire list; else, it filters the result and returns only the todos that have the assigneeId that matches with the userFilter value.
It's important, in this case, to use the useCallback hook and pass to it the right dependencies to prevent a decrease in the performance of the application.
Then another important thing is the setUserFilter function that it's used to set the userFilter value. This function is exposed by the custom hook to permit the users to change the filter.

And that's all; in these two simple steps, you have a select function to filter the dataset got from the query. You can use this solution also to create a client-side pagination in your react query application (for the server-side pagination, there will be a post in the future)

If you want to find out more, check my youtube video about this topic

Ok, that's all from the select option!

I hope you enjoyed this content!

See you soon folks

Bye Bye 👋

p.s. you can find the code of the video here