TypeError: Cannot Read Properties Of Undefined (reading 'result') When Redirecting Before Fetcher Finished Loading

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Introduction

When working with React Router, it's not uncommon to encounter issues related to asynchronous data loading and redirects. In this article, we'll explore a specific scenario where a TypeError occurs when redirecting before the fetcher has finished loading. We'll delve into the root cause of the issue, provide a step-by-step guide to reproduce the problem, and offer a solution to resolve the error.

Reproduction

To replicate the issue, follow these steps:

  1. Visit the StackBlitz link: https://stackblitz.com/edit/github-mhvzvdkl
  2. Click on the first button to load the data
  3. Before the load ends, click on the second button that will submit a form and redirect the user
  4. Open the console to see the following error

Error Screenshot

Image

System Information

System

  • OS: macOS 14.5
  • CPU: (10) arm64 Apple M1 Pro
  • Memory: 64.88 MB / 16.00 GB
  • Shell: 5.9 - /bin/zsh

Binaries

  • Node: 22.11.0 - ~/.nvm/versions/node/v22.11.0/bin/node
  • Yarn: 1.22.22 - ~/Library/pnpm/yarn
  • npm: 10.9.0 - ~/.nvm/versions/node/v22.11.0/bin/npm
  • pnpm: 10.7.0 - ~/Library/pnpm/pnpm
  • bun: 1.0.29 - ~/.bun/bin/bun

Browsers

  • Brave Browser: 122.1.63.169
  • Chrome: 135.0.7049.96
  • Edge: 135.0.3179.85
  • Safari: 17.5

Used Package Manager

  • pnpm

Expected Behavior

No error should pop in the console.

Actual Behavior

There is a "Cannot read properties of undefined" error.

Root Cause of the Issue

The root cause of the issue lies in the way React Router handles redirects and asynchronous data loading. When the user clicks on the second button, the form is submitted, and the user is redirected before the fetcher has finished loading. This causes the result property to be undefined, resulting in the TypeError.

Solution

To resolve the issue, we need to ensure that the fetcher has finished loading before redirecting the user. We can achieve this by using the useEffect hook to wait for the fetcher to complete before redirecting.

Here's an updated code snippet that demonstrates the solution:

import { useState, useEffect } from 'react';
import { useNavigate } from 'react-router-dom';

function App() {
  const [data, setData] = useState(null);
  const [loading, setLoading] = useState(true);
  const navigate = useNavigate();

 useEffect(() => {
    const fetchData = async () => {
      const response = await fetch('/api/data');
      const result = await response.json();
      setData(result);
      setLoading(false);
    };
    fetchData();
  }, []);

  const handleSubmit = (event) => {
    event.preventDefault();
    navigate('/redirect');
  };

  if (loading) {
    return <div>Loading...</div>;
  }

  return (
    <div>
      <button onClick={handleSubmit}>Submit Form</button>
      <div>{data}</div>
    </div>
  );
}

In this updated code snippet, we've added a useEffect hook to wait for the fetcher to complete before redirecting the user. We've also added a loading state to indicate whether the fetcher is still loading.

Conclusion

In this article, we've explored a specific scenario where a TypeError occurs when redirecting before the fetcher has finished loading. We've provided a step-by-step guide to reproduce the problem and offered a solution to resolve the error. By using the useEffect hook to wait for the fetcher to complete before redirecting, we can ensure that the result property is defined, and the TypeError is resolved.

Additional Tips

  • Always ensure that the fetcher has finished loading before redirecting the user.
  • Use the useEffect hook to wait for the fetcher to complete before redirecting.
  • Add a loading state to indicate whether the fetcher is still loading.
  • Use a try-catch block to catch any errors that may occur during the fetcher process.
    TypeError: Cannot read properties of undefined (reading 'result') when redirecting before fetcher finished loading: Q&A ===========================================================

Q: What is the root cause of the TypeError: Cannot read properties of undefined (reading 'result') when redirecting before fetcher finished loading?

A: The root cause of the issue lies in the way React Router handles redirects and asynchronous data loading. When the user clicks on the second button, the form is submitted, and the user is redirected before the fetcher has finished loading. This causes the result property to be undefined, resulting in the TypeError.

Q: How can I reproduce the issue?

A: To replicate the issue, follow these steps:

  1. Visit the StackBlitz link: https://stackblitz.com/edit/github-mhvzvdkl
  2. Click on the first button to load the data
  3. Before the load ends, click on the second button that will submit a form and redirect the user
  4. Open the console to see the following error

Q: What is the expected behavior?

A: No error should pop in the console.

Q: What is the actual behavior?

A: There is a "Cannot read properties of undefined" error.

Q: How can I resolve the issue?

A: To resolve the issue, you need to ensure that the fetcher has finished loading before redirecting the user. You can achieve this by using the useEffect hook to wait for the fetcher to complete before redirecting.

Q: What is the updated code snippet that demonstrates the solution?

A: Here's an updated code snippet that demonstrates the solution:

import { useState, useEffect } from 'react';
import { useNavigate } from 'react-router-dom';

function App() {
  const [data, setData] = useState(null);
  const [loading, setLoading] = useState(true);
  const navigate = useNavigate();

 useEffect(() => {
    const fetchData = async () => {
      const response = await fetch('/api/data');
      const result = await response.json();
      setData(result);
      setLoading(false);
    };
    fetchData();
  }, []);

  const handleSubmit = (event) => {
    event.preventDefault();
    navigate('/redirect');
  };

  if (loading) {
    return <div>Loading...</div>;
  }

  return (
    <div>
      <button onClick={handleSubmit}>Submit Form</button>
      <div>{data}</div>
    </div>
  );
}

Q: What are some additional tips to prevent this issue?

A: Here are some additional tips to prevent this issue:

  • Always ensure that the fetcher has finished loading before redirecting the user.
  • Use the useEffect hook to wait for the fetcher to complete before redirecting.
  • Add a loading state to indicate whether the fetcher is still loading.
  • Use a try-catch block to catch any errors that may occur during the fetcher process.

Q: Can you provide more information about the useEffect hook?

A: The useEffect hook is a built-in React hook that allows you to run side effects (e.g., fetching data, setting timers) after rendering a component. It takes two arguments: a function to run and an array of dependencies. When the dependencies change, the function is re-run.

Q: Can you provide more information about the loading state?

A: The loading state is a boolean variable that indicates whether the fetcher is still loading. It's used to display a loading message to the user while the fetcher is still running.

Q: Can you provide more information about try-catch blocks?

A: A try-catch block is a way to catch and handle errors that occur during the execution of a block of code. The try block contains the code that may throw an error, and the catch block contains the code that will be executed if an error occurs.