The Hidden Debugging Benefit of 'return await' in Async Functions

When writing async functions in JavaScript, you might have encountered the subtle choice between these two patterns:

// Option 1: Return the promise directly
async function fetchData() {
  const response = await fetch("/api/data");
  return response.json();
}

// Option 2: Await before returning
async function fetchData() {
  const response = await fetch("/api/data");
  return await response.json();
}

At first glance, these two approaches appear functionally equivalent—both return a promise that resolves to the parsed JSON. However, there's a critical difference that becomes apparent when errors occur: debuggability.

The Stack Trace Problem

When you use return response.json() without await, the async function immediately returns the promise from response.json(). This means your function exits the call stack before the promise resolves or rejects.

If an error occurs during JSON parsing (e.g., invalid JSON, network error), the stack trace will not include your function. You'll see an error that appears to originate from the promise itself, making it harder to trace back to where it was called in your code.

Example: Poor Stack Trace

async function fetchUser() {
  const response = await fetch("/api/user");
  return response.json(); // Function exits the stack immediately
}

fetchUser().catch(console.error);

Error output:

SyntaxError: Unexpected token < in JSON at position 0
    at Response.json()
    at <anonymous>

Notice how fetchUser is missing from the stack trace. This makes it difficult to determine which fetch call failed, especially in larger codebases with multiple API calls.

The Solution: Use return await

By using return await response.json(), the async function stays in the call stack until the promise resolves or rejects. This provides a much clearer stack trace when errors occur.

Example: Better Stack Trace

async function fetchUser() {
  const response = await fetch("/api/user");
  return await response.json(); // Function remains in stack
}

fetchUser().catch(console.error);

Error output:

SyntaxError: Unexpected token < in JSON at position 0
    at Response.json()
    at fetchUser (app.js:3:28)
    at <anonymous>

Now fetchUser appears in the stack trace, making it immediately clear where the error originated.

When Does This Matter Most?

This pattern is especially valuable when:

• Multiple async operations are chained together
• Error handling needs to pinpoint the exact location of failures
• Production debugging requires clear error logs
• Large codebases have many similar async functions

The Performance Trade-off

You might wonder: does return await have a performance cost?

In modern JavaScript engines (V8, SpiderMonkey), the performance difference is negligible. The improved debuggability far outweighs any minimal overhead, especially considering that most async operations involve I/O which dwarfs any micro-optimizations.

Best Practices

1. Use return await in async functions when you want better error traces
2. Enable ESLint rule no-return-await only if your team prefers minimal stack traces (not recommended)
3. Consider return await mandatory in try-catch blocks where you're handling errors locally

async function fetchUserWithErrorHandling() {
  try {
    const response = await fetch("/api/user");
    return await response.json(); // Critical for catching errors in this try block
  } catch (error) {
    console.error("Failed to fetch user:", error);
    throw error;
  }
}

Conclusion

While return response.json() and return await response.json() are functionally equivalent in happy-path scenarios, the latter provides significantly better debugging capabilities. The stack traces produced when using await make it far easier to identify where errors occur in your code, saving valuable time during development and production debugging.

Next time you write an async function, remember: await for debuggability.