What Are Unhandled Promise Rejections?

Unhandled promise rejections occur when a Promise is rejected but no catch handler or .then failure callback is attached to handle the rejection. In modern versions of Node.js, unhandled rejections are treated as fatal errors by default.

Causes of Unhandled Promise Rejections

1. Missing catch Block

A common cause is forgetting to attach a catch block to a promise chain:

// Incorrect
fetchData().then((data) => process(data)); // No error handling

// Correct
fetchData().then((data) => process(data)).catch((error) => console.error(error));

2. Async/Await Without Try-Catch

Using async/await without a try-catch block can result in unhandled rejections:

// Incorrect
async function main() {
    const data = await fetchData(); // No error handling
    process(data);
}

// Correct
async function main() {
    try {
        const data = await fetchData();
        process(data);
    } catch (error) {
        console.error(error);
    }
}

3. Forgotten Return Statements

Forgetting a return statement in a promise chain can prevent error handling from being executed:

// Incorrect
function processRequest() {
    fetchData()
        .then((data) => process(data))
        .catch((error) => console.error(error));
    console.log("Request complete"); // Executes before fetchData completes
}

// Correct
function processRequest() {
    return fetchData()
        .then((data) => process(data))
        .catch((error) => console.error(error));
}

4. Ignored Promises

When a promise is created but not used or handled:

// Incorrect
fetchData(); // No handling for rejection

// Correct
fetchData().then((data) => process(data)).catch((error) => console.error(error));

Impact of Unhandled Rejections

In Node.js, unhandled promise rejections were previously treated as warnings. However, since Node.js 15, they are considered fatal and terminate the process by default. This behavior can lead to application crashes, disrupted services, and potential data loss.

How to Handle Unhandled Promise Rejections

1. Use Global Handlers

Catch unhandled promise rejections globally to prevent application crashes:

process.on("unhandledRejection", (reason, promise) => {
    console.error("Unhandled Rejection at:", promise, "reason:", reason);
    // Decide whether to exit or continue
});

2. Enable Strict Rejection Handling

Set the --unhandled-rejections=strict flag to enforce strict handling:

node --unhandled-rejections=strict app.js

3. Refactor Code for Explicit Handling

• Ensure every promise chain ends with a catch block.
• Wrap all async/await calls in try-catch blocks.

4. Use Utility Libraries

Leverage libraries like Bluebird to add enhanced error-handling mechanisms:

const Promise = require('bluebird');
Promise.onPossiblyUnhandledRejection((error) => {
    console.error("Unhandled Error:", error);
});

Best Practices to Avoid Unhandled Rejections

• Always handle promise rejections explicitly with catch or try-catch.
• Use global handlers as a safety net but avoid relying on them exclusively.
• Write unit tests to simulate rejection scenarios and validate error handling.
• Enable strict rejection handling during development and testing.

Conclusion

Unhandled promise rejections can have severe consequences in Node.js applications, especially in production environments. By adopting best practices and robust error-handling strategies, you can ensure the stability and reliability of your application.

FAQs

1. What are unhandled promise rejections?

They occur when a promise is rejected without a catch or equivalent handler to manage the error.

2. How does Node.js handle unhandled rejections?

In Node.js 15 and later, unhandled promise rejections terminate the application by default.

3. How can I debug unhandled rejections?

Use the process.on("unhandledRejection") event to log and debug rejected promises.

4. Why should I use try-catch with async/await?

try-catch ensures that any rejection from the await call is properly handled, preventing unhandled errors.

5. What tools can help with promise debugging?

Libraries like Bluebird and Node.js's built-in debugging tools are useful for tracking promise rejections.