Resolving TypeError: Undefined Properties in TypeScript Login Forms

Understanding and Solving Undefined Errors in Login Forms

Encountering runtime errors can be frustrating, especially when it seems like everything in your code is in place. One of the common challenges in TypeScript applications is the infamous TypeError: Cannot read properties of undefined, especially when building forms or authentication flows. This error often pops up due to minor oversights in asynchronous function responses or unexpected API returns.

Imagine implementing a login form that allows users to sign in seamlessly. Everything appears to be working—users can log in, and you receive confirmation. However, out of nowhere, a lingering error message appears, making the interface seem broken to users. Even after successful authentication, errors like these can make the experience confusing and disrupt the flow. 😓

In this article, we’ll break down why such errors occur, particularly when handling data from asynchronous calls in TypeScript. We’ll explore how mismatches in expected and actual data structures can lead to undefined property errors. Along the way, I’ll show practical examples to help you identify and fix these issues in your own projects.

Let’s dive into some troubleshooting techniques, including safe data handling practices, to prevent and resolve this TypeError. These strategies will allow your login form to handle different states reliably, ensuring a smooth user experience without the sudden pop-up of confusing errors.

Handling Undefined Property Errors in TypeScript Login Forms

In our TypeScript and React login form setup, we encountered a common runtime error, the TypeError, specifically "Cannot read properties of undefined." This issue typically arises when the application expects data that isn't returned or processed as anticipated. Here, we have a login function that returns either a success or error message based on the authentication result. The frontend component, however, sometimes fails to handle undefined responses gracefully, resulting in the error we see. By implementing both frontend and backend solutions, including better error handling and validation checks, we can make sure that undefined properties are managed properly, thereby avoiding unexpected runtime errors.

The login function, located on the server, performs authentication by calling NextAuth’s signIn function. Before signing in, it first validates the form data using Zod's validation schema, ensuring the data conforms to the required structure. If the data fails validation, the function immediately returns an error. In the frontend LoginForm component, we utilize React’s useState hooks to manage success and error messages dynamically. The useTransition hook, a lesser-known but useful feature, is used to handle concurrent state updates, allowing for smoother state changes without disrupting the main UI rendering. This is especially helpful for operations like login, where background transitions should not hinder the user interface experience.

When users submit the form, the login function is called within a startTransition function, allowing React to prioritize immediate user interaction while handling other updates in the background. Once the server returns a response, we attempt to display the error or success message by updating the error and success states accordingly. However, as the error message may sometimes be missing in cases of unexpected responses, we handle this by adding conditional checks, such as verifying if data.error exists before trying to set it. This type of defensive programming ensures that even if the backend fails to deliver a specific response property, our frontend won’t crash, resulting in a smoother, more robust user experience. 🎉

Unit tests were also added to verify that error and success messages display correctly based on various login scenarios. By using testing tools like React Testing Library, we simulate form submissions with both valid and invalid credentials, checking that the appropriate feedback appears for each case. For example, by intentionally entering wrong credentials, we ensure that the "Invalid credentials" message displays as expected. These tests also allow us to confirm that changes to the backend (such as error message updates) are correctly reflected on the frontend without causing any unexpected crashes. In real-world applications, having thorough unit tests is invaluable, as it helps catch potential issues before deployment.

This approach not only prevents undefined errors but also reinforces a smoother, more resilient login experience. Whether dealing with common issues like missing fields or specific authentication errors, following this method equips developers with reliable techniques for managing various edge cases and improving TypeScript login functionality. Implementing these strategies not only fixes runtime errors but also contributes to a polished user experience, ensuring that login interactions are as smooth and frustration-free as possible. 🚀

import React, { useState } from "react";
import { useTransition } from "react";
import { useForm } from "react-hook-form";
import { z } from "zod";
import { zodResolver } from "@hookform/resolvers/zod";
import { login } from "./authService";
import { LoginSchema } from "./schemas";
export const LoginForm = () => {
  const [error, setError] = useState<string | undefined>("");
  const [success, setSuccess] = useState<string | undefined>("");
  const [isPending, startTransition] = useTransition();
  const form = useForm<z.infer<typeof LoginSchema>>({
    resolver: zodResolver(LoginSchema),
    defaultValues: { email: "", password: "" },
  });
  const onSubmit = (values: z.infer<typeof LoginSchema>) => {
    setError("");
    setSuccess("");
    startTransition(() => {
      login(values)
        .then((data) => {
          setError(data?.error || "");
          setSuccess(data?.success || "");
        })
        .catch(() => setError("An unexpected error occurred."));
    });
  };
  return (
    <form onSubmit={form.handleSubmit(onSubmit)}>
      <input {...form.register("email")} placeholder="Email" />
      <input {...form.register("password")} placeholder="Password" type="password" />
      <button type="submit" disabled={isPending}>Login</button>
      {error && <p style={{ color: "red" }}>{error}</p>}
      {success && <p style={{ color: "green" }}>{success}</p>}
    </form>
  );
};

import { z } from "zod";
import { AuthError } from "next-auth";
import { signIn } from "@/auth";
import { LoginSchema } from "@/schemas";
import { DEFAULT_LOGIN_REDIRECT } from "@/routes";
export const login = async (values: z.infer<typeof LoginSchema>) => {
  const validatedFields = LoginSchema.safeParse(values);
  if (!validatedFields.success) {
    return { error: "Invalid fields!" };
  }
  const { email, password } = validatedFields.data;
  try {
    await signIn("credentials", {
      email,
      password,
      redirectTo: DEFAULT_LOGIN_REDIRECT
    });
    return { success: "Login successful!" };
  } catch (error) {
    if (error instanceof AuthError) {
      switch (error.type) {
        case "CredentialsSignin":
          return { error: "Invalid credentials!" };
        default:
          return { error: "Something went wrong!" };
      }
    }
    throw error;
  }
};

import { render, screen, fireEvent } from "@testing-library/react";
import { LoginForm } from "./LoginForm";
import "@testing-library/jest-dom";
describe("LoginForm", () => {
  it("displays error when login fails", async () => {
    render(<LoginForm />);
    fireEvent.change(screen.getByPlaceholderText("Email"), {
      target: { value: "invalid@example.com" }
    });
    fireEvent.change(screen.getByPlaceholderText("Password"), {
      target: { value: "wrongpassword" }
    });
    fireEvent.click(screen.getByRole("button", { name: /login/i }));
    const errorMessage = await screen.findByText("Invalid credentials!");
    expect(errorMessage).toBeInTheDocument();
  });
});

Improving Error Handling and Debugging in TypeScript Authentication

In TypeScript-based authentication flows, a common issue is handling undefined properties gracefully. When working with login forms, undefined errors like the infamous TypeError often occur if a property—such as an error message—is absent in the response. While catching such issues can be tricky, employing safe coding patterns is essential for avoiding runtime issues and improving the user experience. This challenge highlights the importance of comprehensive error handling and defensive programming techniques. For example, using conditional checks around data assignments ensures that our application won’t attempt to read missing properties, which helps prevent these annoying errors from occurring.

Another crucial technique to handle undefined errors is implementing server-side validation using libraries like Zod. Zod provides type-safe schema validation, making it easier to enforce data requirements before they reach the client. In our login function, we use Zod’s safeParse method to ensure that fields like email and password meet specified formats before sending the data to the authentication service. If the input fails this validation, our function instantly returns a meaningful error message. On the client side, by utilizing frameworks like React Hook Form, we can set up real-time form validation that prevents the user from even attempting a login with invalid fields, saving both user and server time.

Finally, effective debugging and testing practices can catch undefined errors early in the development process. Using testing libraries like Jest and React Testing Library, developers can simulate various login scenarios and validate that all expected responses, such as error and success messages, display correctly. Writing unit tests that simulate incorrect login attempts (like entering invalid credentials) allows developers to verify that all undefined scenarios are covered. By addressing errors in the testing phase, the code becomes more robust and user-friendly, ensuring a smoother experience for users who rely on stable login features. 🛠️