Fixing React Query useMutation Error: __privateGet(...).defaultMutationOptions is not a Function

Resolving a Complex React Query useMutation Issue

While working on a React project, encountering unexpected errors can be frustrating, especially when using essential libraries like React Query. One such issue is the useMutation error, which throws a message like __privateGet(...).defaultMutationOptions is not a function. This error can be confusing, particularly for developers using React Query with tools like Vite.

This issue often arises during the use of the useMutation hook for handling asynchronous data in your React app. When it occurs, it typically prevents your mutation logic from functioning correctly, leaving developers wondering how to troubleshoot it. Solving it may require diving deep into configuration, package compatibility, and understanding potential underlying issues.

In this guide, we will explore the root causes of this error and provide clear, actionable steps to resolve it. Whether you're dealing with dependency conflicts, version mismatches, or configuration problems, we'll help you troubleshoot and fix this common React Query problem.

By following this troubleshooting guide, you will better understand how to handle such issues in the future, ensuring smoother development when working with @tanstack/react-query and Vite. Let's get started!

Understanding the React Query useMutation Issue and Solutions

The first script revolves around using React Query's useMutation to handle user registration. The useMutation hook is particularly useful for executing asynchronous functions such as POST requests to an API, which are essential in form submission processes. In our case, it is used to send user registration data to the backend. It provides two key callback functions: onSuccess and onError. The onSuccess function is triggered when the API request is successful, while onError handles any potential errors, allowing the app to manage failures effectively.

The script leverages react-hook-form for form validation, which allows for clean, declarative handling of user input and errors. This library's useForm hook manages the form state and handles input validation without the need for manual checks. The combination of these tools ensures that user inputs are properly validated before being sent to the backend via useMutation, and it provides a clean way to navigate users upon successful registration using useNavigate from react-router-dom.

The second script focuses on creating a custom Axios instance to handle HTTP requests. Axios is a popular HTTP client that simplifies making asynchronous requests in JavaScript. In this example, the Axios instance is configured with a base URL, ensuring that all requests are made to the same API. The withCredentials option ensures that cookies and authentication headers are properly sent along with the request, which is critical when working with secure APIs or session-based authentication.

This Axios instance is then used in the registerUser function, which posts the user data to the backend API for registration. The function is asynchronous, meaning that it returns a promise, and the response is captured and returned to the caller, in this case, the useMutation hook. The use of a modular Axios instance not only improves the organization of the code but also ensures that each request can be easily tested and customized for future API endpoints. These scripts, when used together, ensure a seamless registration process with robust error handling and validation in React applications.

import { useForm } from "react-hook-form";
import { registerUser } from "../apis/Authentication";
import { useMutation } from "@tanstack/react-query";
import { useNavigate } from "react-router-dom";
// React Component for User Registration
const Register = () => {
  const { register, handleSubmit, formState: { errors } } = useForm();
  const navigate = useNavigate();
  // Mutation using React Query's useMutation hook
  const mutation = useMutation(registerUser, {
    onSuccess: (data) => {
      console.log("User registered:", data);
      alert("Registration Successful!");
      navigate("/login-user");
    },
    onError: (error) => {
      console.error("Registration failed:", error);
      alert(error.response?.data?.message || "Registration failed");
    }
  });
  // Form submission handler
  const onSubmit = (formData) => mutation.mutate(formData);
  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <input {...register("username")} placeholder="Username" />
      {errors.username && <p>{errors.username.message}</p>}
      <button type="submit">Register</button>
    </form>
  );
};
export default Register;

import axios from "axios";
// Creating an Axios instance with baseURL and credentials
const axiosInstance = axios.create({
  baseURL: "http://localhost:5000/api",
  withCredentials: true,
  headers: { "Content-Type": "multipart/form-data" }
});
// User registration API call using Axios
const registerUser = async (userData) => {
  const response = await axiosInstance.post("/user/register-user", userData);
  return response.data;
};
export { registerUser };
// Unit test for Axios instance
test("registerUser API call test", async () => {
  const mockData = { username: "testUser" };
  axiosInstance.post = jest.fn().mockResolvedValue({ data: "User registered" });
  const response = await registerUser(mockData);
  expect(response).toBe("User registered");
});

Tackling Version Compatibility Issues in React Query

One often overlooked issue in React Query development is the importance of version compatibility, particularly when working with modern tools like Vite. React Query’s frequent updates may introduce breaking changes that affect developers using older or mismatched versions of related dependencies. This can result in errors like the __privateGet(...).defaultMutationOptions is not a function problem, as seen in the example above. Ensuring that both React Query and React itself are up-to-date, and compatible with the latest bundling tools, is crucial to avoid unexpected issues.

Moreover, when using advanced hooks like useMutation, it's important to check compatibility with middleware like Axios and authentication libraries. This error may arise from subtle changes in how these libraries interact with React Query. A deep dive into the changelogs of React Query and Axios may reveal breaking changes, as newer versions often refactor internal APIs. Understanding how these updates affect your code can be vital to ensuring a stable and smooth integration of libraries in your project.

Additionally, modularity in your API handling with tools like Axios and clear separation of concerns helps to minimize the impact of such errors. By isolating the API logic from the React component itself, debugging and maintenance become much easier. This practice ensures that future upgrades to libraries like React Query won’t break your code, as your core logic remains encapsulated and easier to adapt when dependencies evolve.