Working with React and TypeScript (TSX) offers many benefits, including type safety and better tooling. However, developers can still run into unhandled runtime errors. These errors can be frustrating and time-consuming to debug. Here are some common mistakes that cause unhandled runtime errors in React TSX and how to avoid them.

1. Accessing Undefined or Null Values

Mistake:

Attempting to access properties on an undefined or null value is a frequent cause of runtime errors.

Example:

const UserProfile: React.FC<{ user: User | undefined }> = ({ user }) => {
  return <div>{user.name}</div>;
};

If user is undefined, accessing user.name will throw an error.

Solution:

Use optional chaining or conditional rendering to ensure the value is defined.

const UserProfile: React.FC<{ user: User | undefined }> = ({ user }) => {
  return <div>{user?.name ?? 'Guest'}</div>;
};

2. Incorrectly Using Array Methods

Mistake:

Using array methods like map, filter, or reduce on a variable that is not an array can lead to errors.

Example:

const ItemList: React.FC<{ items: Item[] | undefined }> = ({ items }) => {
  return (
    <ul>
      {items.map(item => (
        <li key={item.id}>{item.name}</li>
      ))}
    </ul>
  );
};

If items is undefined, items.map will throw an error.

Solution:

Provide a default value to ensure the variable is always an array.

const ItemList: React.FC<{ items: Item[] | undefined }> = ({ items = [] }) => {
  return (
    <ul>
      {items.map(item => (
        <li key={item.id}>{item.name}</li>
      ))}
    </ul>
  );
};

3. Failing to Handle Promises Correctly

Mistake:

Not properly handling promises, especially in asynchronous operations within components, can lead to runtime errors.

Example:

const DataFetchingComponent: React.FC = () => {
  const [data, setData] = React.useState<Data | null>(null);

  React.useEffect(() => {
    fetchData().then(response => setData(response.data));
  }, []);

  return <div>{data.name}</div>;
};

If fetchData fails, data will be null, and accessing data.name will throw an error.

Solution:

Handle the loading and error states properly.

const DataFetchingComponent: React.FC = () => {
  const [data, setData] = React.useState<Data | null>(null);
  const [loading, setLoading] = React.useState(true);
  const [error, setError] = React.useState<Error | null>(null);

  React.useEffect(() => {
    fetchData()
      .then(response => setData(response.data))
      .catch(error => setError(error))
      .finally(() => setLoading(false));
  }, []);

  if (loading) return <div>Loading...</div>;
  if (error) return <div>Error: {error.message}</div>;

  return <div>{data?.name}</div>;
};

4. Mismanaging Component State

Mistake:

Not initializing state properly or updating state in a way that causes inconsistencies can lead to errors.

Example:

const Counter: React.FC = () => {
  const [count, setCount] = React.useState<number>();

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
};

If count is undefined, count + 1 will throw an error.

Solution:

Initialize state with a valid value.

const Counter: React.FC = () => {
  const [count, setCount] = React.useState<number>(0);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
};

5. Ignoring TypeScript Warnings and Errors

Mistake:

Ignoring TypeScript's type warnings and errors can lead to unexpected runtime errors.

Example:

const Greeting: React.FC<{ name: string }> = ({ name }) => {
  return <div>Hello, {name.toUpperCase()}</div>;
};

If name is optional but not treated as such, this can lead to runtime errors when name is undefined.

Solution:

Pay attention to TypeScript warnings and enforce strict type checking.

const Greeting: React.FC<{ name?: string }> = ({ name = 'Guest' }) => {
  return <div>Hello, {name.toUpperCase()}</div>;
};

6. Not Handling Default Props Correctly

Mistake:

Failing to provide default values for optional props can cause errors when those props are accessed without being passed.

Example:

const WelcomeMessage: React.FC<{ message?: string }> = ({ message }) => {
  return <div>{message.toUpperCase()}</div>;
};

If message is undefined, message.toUpperCase() will throw an error.

Solution:

Provide default values for optional props.

const WelcomeMessage: React.FC<{ message?: string }> = ({ message = 'Welcome!' }) => {
  return <div>{message.toUpperCase()}</div>;
};

Conclusion

Unhandled runtime errors can be a significant hindrance in developing robust React applications with TypeScript. By being aware of these common mistakes and following best practices, you can avoid many of these pitfalls. Ensure that you handle undefined values, manage component state correctly, and pay attention to TypeScript warnings. Doing so will lead to more reliable and maintainable code.