Why You Should Use Return Await in JavaScript Functions

As a seasoned developer, I’ve spent years navigating the intricacies of JavaScript’s asynchronous behavior. One topic that often sparks debate is the use of return await in async functions. Let’s delve into this concept and clarify when it’s beneficial to use return await and when it’s unnecessary.

Understanding Async Functions and Promises

In JavaScript, an async function always returns a promise. This means that whether you explicitly return a value or a promise, JavaScript wraps it in a promise. For example:

async function fetchData() {
    return 'Data fetched';
}

Calling fetchData() returns a promise that resolves to ‘Data fetched’. Now, consider this function:

async function fetchData() {
    return await getDataFromAPI();
}

Here, getDataFromAPI() is an asynchronous function returning a promise. The await keyword pauses the execution of fetchData() until getDataFromAPI() resolves. However, since fetchData() is an async function, it inherently returns a promise. Thus, the await seems redundant.

When return await Is Redundant

In straightforward scenarios, using return await adds unnecessary complexity. For instance:

async function getUser() {
    return await fetchUserFromDatabase();
}

Here, fetchUserFromDatabase() returns a promise. The await pauses getUser() until the promise resolves, and then getUser() returns the resolved value. But since getUser() is an async function, it automatically returns a promise. Therefore, return await fetchUserFromDatabase() is functionally equivalent to return fetchUserFromDatabase(), making the await redundant.

When return await Is Essential

However, there are scenarios where return await is crucial, particularly in error handling. Consider the following example:

async function processData() {
    try {
        return await fetchData();
    } catch (error) {
        console.error('Error fetching data:', error);
        throw error;
    }
}

In this case, await ensures that if fetchData() throws an error, it’s caught within the try...catch block. Without await, the promise rejection would bypass the catch block, and the error would need to be handled by the function’s caller.

Practical Example: Cleaning Up Resources

Let’s explore a practical scenario where return await is beneficial. Suppose we have a function that creates a temporary directory, performs operations within it, and then cleans up:

const fs = require('fs').promises;

async function withTempDir(callback) {
    const dir = await fs.mkdtemp('/tmp/dir-');
    try {
        return await callback(dir);
    } finally {
        await fs.rmdir(dir, { recursive: true });
    }
}

In this example, await ensures that the temporary directory is removed after the callback has completed its operations. Omitting await could result in the directory being deleted before the callback finishes, leading to potential errors.

Performance Considerations

Some developers express concerns about the performance implications of using return await. While it’s true that adding await introduces a microtask tick, in most real-world applications, this overhead is negligible. The clarity and explicit error handling it provides often outweigh the minimal performance cost.

Best Practices

• Use return await within try...catch blocks: This ensures that any errors thrown are caught and handled appropriately within the function.
• Avoid return await in simple return statements: If there’s no error handling or additional logic, returning the promise directly is more concise and efficient.
• Be mindful of resource cleanup: When working with resources that require cleanup (e.g., file systems, network connections), use await within finally blocks to ensure proper resource management.

Conclusion

Understanding when to use return await in JavaScript async functions is essential for effective error handling and resource management. While it may seem redundant in some cases, its judicious use within try...catch and try...finally blocks ensures that your asynchronous code behaves as intended, leading to more robust and maintainable applications.