Error Handling with Promises in TypeScript

Error handling is crucial in TypeScript, especially when working with asynchronous operations. Promises provide an elegant way to manage errors in asynchronous code. This guide explores how to effectively handle errors using Promises in TypeScript.

Understanding Promises and Error Handling

Promises represent the eventual completion or failure of an asynchronous operation. They offer built-in mechanisms for handling both successful results and errors. In TypeScript, Promises are strongly typed, enhancing code reliability.

Basic Promise Syntax

Here's a simple example of a Promise in TypeScript:

const myPromise = new Promise<string>((resolve, reject) => {
    // Asynchronous operation
    if (/* operation successful */) {
        resolve("Success!");
    } else {
        reject(new Error("Something went wrong"));
    }
});

Handling Errors with .catch()

The .catch() method is the primary way to handle errors in Promises. It captures any errors thrown in the Promise chain.

myPromise
    .then(result => console.log(result))
    .catch(error => console.error("Error:", error.message));

Using try-catch with async/await

When using Async/Await in TypeScript, you can use traditional try-catch blocks for error handling:

async function fetchData() {
    try {
        const result = await myPromise;
        console.log(result);
    } catch (error) {
        console.error("Error:", error.message);
    }
}

Best Practices for Error Handling with Promises

• Always include error handling in Promise chains or async functions.
• Use specific error types for better error management.
• Avoid swallowing errors silently; log or report them appropriately.
• Consider using a global error handler for unhandled Promise rejections.

Custom Error Types

Creating custom error types can enhance error handling:

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

async function fetchData(url: string): Promise<string> {
    try {
        const response = await fetch(url);
        if (!response.ok) {
            throw new NetworkError("Failed to fetch data");
        }
        return await response.text();
    } catch (error) {
        if (error instanceof NetworkError) {
            console.error("Network issue:", error.message);
        } else {
            console.error("Unexpected error:", error);
        }
        throw error;
    }
}

Chaining Promises and Error Propagation

Errors in Promise chains propagate to the nearest catch handler. This allows for centralized error handling:

fetchData("https://api.example.com/data")
    .then(processData)
    .then(saveData)
    .catch(error => {
        console.error("Error in data pipeline:", error);
        // Handle or rethrow the error
    });

Conclusion

Effective error handling with Promises in TypeScript is essential for robust application development. By leveraging TypeScript's strong typing and Promise-based error handling mechanisms, developers can create more reliable and maintainable asynchronous code.

Remember to always handle potential errors in your Promise-based operations, whether using .catch() or try-catch blocks with async/await. This practice ensures that your application gracefully manages unexpected situations and provides a better user experience.