Common Mistakes in JavaScript Async/Await and How to Fix Them

JavaScript's async/await syntax revolutionized asynchronous programming by providing a cleaner, more readable alternative to traditional promises and callbacks. However, even experienced developers often encounter pitfalls when using async/await. This guide examines common pitfalls developers encounter, their underlying causes, and actionable remedies. By tackling these issues, you can ensure your asynchronous code is both efficient and error-free.

1. Forgetting to Use await with Async Functions

One of the most common mistakes is neglecting to use await when calling an asynchronous function, leading to unexpected promise objects instead of resolved values.

Example of the Mistake:

async function fetchData() {
    return "data";
}

async function main() {
    const result = fetchData(); // Missing await
    console.log(result); // Logs: Promise {<resolved>: "data"}
}
main();

Why It Happens:

Calling an async function without await results in a promise being returned rather than the resolved value.

How to Fix:

Always use await or explicitly handle the promise using .then().

Corrected Code:

async function main() {
    const result = await fetchData(); // Properly awaits the promise
    console.log(result); // Logs: "data"
}
main();

2. Using await Outside of an Async Function

You cannot use await directly in non-async functions or the global scope (except in environments like Node.js with top-level await).

Example of the Mistake:

function fetchData() {
    return new Promise(resolve => setTimeout(() => resolve("data"), 1000));
}

// SyntaxError: Unexpected identifier
const result = await fetchData();

Why It Happens:

The await keyword cannot be used within outside async function.

How to Fix:

Wrap the code inside an async function.

Corrected Code:

async function main() {
    const result = await fetchData();
    console.log(result); // Logs: "data"
}
main();

3. Not Handling Errors with try/catch

Uncaught errors in async functions can crash your application.

Example of the Mistake:

async function fetchData() {
    throw new Error("Something went wrong!");
}

async function main() {
    const result = await fetchData(); // Unhandled error
    console.log(result);
}
main(); // Error: Something went wrong!

Why It Happens:

await automatically propagates rejected promises unless explicitly caught.

How to Fix:

Enclose the await call within a try/catch block or manage errors using .catch().

Corrected Code:

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

4. Running Async Functions Sequentially Instead of Concurrently

Using await in a loop or multiple sequential calls can lead to performance bottlenecks.

Example of the Mistake:

async function fetchItem(item) {
    return new Promise(resolve => setTimeout(() => resolve(item), 1000));
}

async function fetchAll() {
    const items = ["a", "b", "c"];
    for (const item of items) {
        console.log(await fetchItem(item)); // Sequential execution
    }
}
fetchAll();
// Takes approximately 3 seconds to complete

Why It Happens:

Each await pauses execution until the current promise resolves, causing unnecessary delays.

How to Fix:

Use Promise.all() to run multiple async tasks concurrently.

Corrected Code:

async function fetchAll() {
    const items = ["a", "b", "c"];
    const results = await Promise.all(items.map(fetchItem));
    results.forEach(result => console.log(result));
}
// Takes approximately 1 second to complete

5. Mixing Async/Await and .then()

Combining async/await with .then() unnecessarily complicates the code and can lead to confusion.

Example of the Mistake:

async function fetchData() {
    return "data";
}

async function main() {
    fetchData().then(result => console.log(result)); // Unnecessary use of .then()
}
main();

Why It Happens:

Developers accustomed to promises may accidentally combine both patterns.

How to Fix:

Choose a single approach—async/await is generally better for creating cleaner and more maintainable code.

Corrected Code:

async function main() {
    const result = await fetchData();
    console.log(result);
}
main();

6. Ignoring Unhandled Promise Rejections

Uncaught promise rejections can lead to silent issues within your application.

Example of the Mistake:

async function fetchData() {
    return Promise.reject("Fetch failed!");
}

async function main() {
    const result = await fetchData(); // Rejection not handled
    console.log(result);
}
main();
// Logs: UnhandledPromiseRejectionWarning

Why It Happens:

When an error occurs, JavaScript raises an unhandled rejection warning but does not crash the script.

How to Fix:

Always handle rejections with try/catch or .catch().

Corrected Code:

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

7. Forgetting to return Values in Async Functions

When an async function doesn't explicitly return a value, it resolves with undefined.

Example of the Mistake:

async function fetchData() {
    const data = "data";
    // Missing return statement
}

async function main() {
    const result = await fetchData();
    console.log(result); // Logs: undefined
}
main();

Why It Happens:

Async functions default to returning a promise that resolves with undefined if no value is returned.

How to Fix:

Always return a value from async functions.

Corrected Code:

async function fetchData() {
    const data = "data";
    return data; // Explicitly return the value
}

Conclusion

Async/await makes asynchronous code in JavaScript much easier to read and maintain, but improper usage can lead to performance issues and hard-to-debug errors. By understanding these common mistakes and their solutions, you can write robust, efficient async code.

Make these practices part of your coding routine, and you'll avoid potential pitfalls while leveraging the full power of async/await in JavaScript.