Error handling is an essential aspect of software development that often gets overlooked, yet it can make a significant difference in the robustness of your applications. In TypeScript, error handling can be efficiently managed due to its strong typing system and flexibility. In this post, we'll explore different methods and techniques for handling errors in TypeScript.

Understanding Errors in TypeScript

In TypeScript, errors can be broadly categorized into two types: compile-time errors and runtime errors.

• Compile-time errors are issues that can be caught by the TypeScript compiler before the code is executed, such as type mismatches or syntax errors. These errors are straightforward because they alert you to problems while writing any TypeScript program.

• Runtime errors, on the other hand, occur when the program is executing. This includes issues like accessing properties of undefined, failing API calls, or unexpected input from users.

The try-catch Statement

One of the most fundamental techniques for error handling in TypeScript is the try-catch statement. This allows you to test a block of code for errors and handle those errors gracefully.

Here's how it works:

function parseJson(jsonString: string) {
  try {
    const result = JSON.parse(jsonString);
    console.log(result);
  } catch (error) {
    console.error("Invalid JSON format:", error);
  }
}

// Example usage
parseJson('{"name": "John"}'); // Works fine
parseJson('{"name": "John"');   // Catches error

In this example, the try block attempts to parse a JSON string. If the string is invalid, the code jumps into the catch block where you can log or handle the error as needed.

Custom Error Classes

While JavaScript and TypeScript come with built-in error types, there are times when you may want to throw custom errors. This is where custom error classes come into play.

You can create a custom error class that extends the base Error class in TypeScript:

class ValidationError extends Error {
  constructor(message: string) {
    super(message);
    this.name = "ValidationError";
  }
}

// Usage
function validateInput(input: any) {
  if (!input) {
    throw new ValidationError("Input cannot be empty.");
  }
}

try {
  validateInput(null);
} catch (error) {
  if (error instanceof ValidationError) {
    console.error(error.message);
  }
}

Here, we created a ValidationError class that allows us to throw specific validation-related errors. By checking the instance of the error in the catch block, we can respond accordingly.

The never Type for Error Handling

TypeScript's never type is often used to represent a function that never returns. This is particularly useful in error handling, as it can make your intent clear when your code encounters an unrecoverable situation, leading to cleaner and more predictable flow.

function throwError(message: string): never {
  throw new Error(message);
}

function processData(data: any) {
  if (!data) {
    throwError("No data provided!");
  }
}

try {
  processData(null);
} catch (error) {
  console.error(error.message); // Output: No data provided!
}

In this case, throwError will never return a value since it always throws an error, which adds clarity to your code.

Asynchronous Error Handling

Handling errors in asynchronous code (like promises and async/await) can be tricky but manageable. With async/await, errors can be caught using standard try-catch blocks.

Here's an example:

async function fetchData(url: string): Promise<void> {
  try {
    const response = await fetch(url);
    if (!response.ok) {
      throw new Error(`HTTP error! status: ${response.status}`);
    }
    const data = await response.json();
    console.log(data);
  } catch (error) {
    console.error("Fetch error:", error);
  }
}

// Example usage
fetchData("https://jsonplaceholder.typicode.com/posts")
  .catch(error => console.error("Unexpected error:", error));

In this example, any errors during the fetch process or while parsing the response are gracefully handled, ensuring that the application continues to run smoothly.

Conclusion

Proper error handling in TypeScript significantly enhances the quality and reliability of your applications. By using the techniques outlined in this blog—such as try-catch statements, custom error classes, and the never type—you can create a robust error handling strategy that prepares your code to face unexpected situations systematically.

Always keep in mind that how you handle errors is as crucial as the logic of your code itself, as clear and effective communication of error states can aid in debugging and maintaining your application in the long run.