10 Async Await JavaScript Interview Questions and Answers

Async/Await in JavaScript has revolutionized the way developers handle asynchronous operations, making code more readable and easier to manage. This modern syntax, built on top of Promises, allows for writing asynchronous code that looks and behaves more like synchronous code, reducing the complexity often associated with callbacks and promise chains. Mastery of Async/Await is crucial for developing efficient, non-blocking applications, especially in environments where performance and responsiveness are key.

This article provides a curated selection of interview questions focused on Async/Await in JavaScript. By working through these questions and their detailed answers, you will gain a deeper understanding of asynchronous programming patterns and be better prepared to demonstrate your expertise in technical interviews.

Async Await JavaScript Interview Questions and Answers

1. How does async/await improve code readability compared to traditional promise chaining?

Async/await enhances code readability by allowing developers to write asynchronous code that resembles synchronous code, making it easier to understand and maintain. Traditional promise chaining can lead to deeply nested structures, which are harder to follow.

Example using traditional promise chaining:

fetchData()
  .then(response => processData(response))
  .then(result => saveData(result))
  .then(finalResult => console.log('Data saved:', finalResult))
  .catch(error => console.error('Error:', error));

Example using async/await:

async function handleData() {
  try {
    const response = await fetchData();
    const result = await processData(response);
    const finalResult = await saveData(result);
    console.log('Data saved:', finalResult);
  } catch (error) {
    console.error('Error:', error);
  }
}

handleData();

2. Write a function that fetches data from an API endpoint using async/await.

Async/await is syntactic sugar over Promises, simplifying asynchronous code. It allows you to write code that looks synchronous, which is useful for operations like fetching data from an API.

Example function fetching data from an API endpoint:

async function fetchData(url) {
    try {
        const response = await fetch(url);
        if (!response.ok) {
            throw new Error('Network response was not ok');
        }
        const data = await response.json();
        return data;
    } catch (error) {
        console.error('There has been a problem with your fetch operation:', error);
    }
}

// Example usage:
fetchData('https://api.example.com/data')
    .then(data => console.log(data));

3. Explain how error handling works with async/await.

Error handling with async/await is straightforward, using try/catch blocks similar to synchronous code. This approach results in cleaner and more readable code compared to traditional promise-based error handling.

Example:

async function fetchData() {
    try {
        let response = await fetch('https://api.example.com/data');
        if (!response.ok) {
            throw new Error('Network response was not ok');
        }
        let data = await response.json();
        return data;
    } catch (error) {
        console.error('There has been a problem with your fetch operation:', error);
    }
}

fetchData();

4. Write a function that handles errors when fetching data using async/await.

When fetching data asynchronously, handling potential errors is important for application robustness. This can be achieved using try/catch blocks within an async function.

Example:

async function fetchData(url) {
    try {
        let response = await fetch(url);
        if (!response.ok) {
            throw new Error('Network response was not ok');
        }
        let data = await response.json();
        return data;
    } catch (error) {
        console.error('There has been a problem with your fetch operation:', error);
    }
}

5. Write a function that executes multiple asynchronous operations concurrently using async/await.

When dealing with multiple asynchronous operations, you can use async/await with Promise.all to execute them concurrently. This ensures all promises are resolved before proceeding, improving efficiency.

Example:

async function fetchData(urls) {
    const fetchPromises = urls.map(url => fetch(url).then(response => response.json()));
    const results = await Promise.all(fetchPromises);
    return results;
}

const urls = [
    'https://api.example.com/data1',
    'https://api.example.com/data2',
    'https://api.example.com/data3'
];

fetchData(urls).then(data => {
    console.log(data);
});

6. How would you handle a situation where you need to wait for multiple promises to resolve before proceeding?

To wait for multiple promises to resolve before proceeding, use the Promise.all method with async/await. Promise.all takes an array of promises and returns a single promise that resolves when all have resolved.

Example:

async function fetchData() {
    const promise1 = fetch('https://api.example.com/data1');
    const promise2 = fetch('https://api.example.com/data2');
    const promise3 = fetch('https://api.example.com/data3');

    const [response1, response2, response3] = await Promise.all([promise1, promise2, promise3]);

    const data1 = await response1.json();
    const data2 = await response2.json();
    const data3 = await response3.json();

    return [data1, data2, data3];
}

fetchData().then(data => {
    console.log(data);
});

7. Write a function that retries an asynchronous operation up to three times using async/await.

To retry an asynchronous operation up to three times using async/await, handle the operation within a loop and catch any errors. Retry the operation until the maximum number of retries is reached.

async function retryAsyncOperation(operation, retries = 3) {
    for (let attempt = 1; attempt <= retries; attempt++) {
        try {
            return await operation();
        } catch (error) {
            if (attempt === retries) {
                throw error;
            }
        }
    }
}

// Example usage:
const exampleOperation = async () => {
    if (Math.random() > 0.5) {
        throw new Error('Operation failed');
    }
    return 'Operation succeeded';
};

retryAsyncOperation(exampleOperation)
    .then(result => console.log(result))
    .catch(error => console.error(error));

8. Write a function that uses async/await to read multiple files concurrently and return their contents.

Async/await can be useful for reading multiple files concurrently. Here is an example:

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

async function readFilesConcurrently(filePaths) {
    try {
        const readPromises = filePaths.map(path => fs.readFile(path, 'utf8'));
        const fileContents = await Promise.all(readPromises);
        return fileContents;
    } catch (error) {
        console.error('Error reading files:', error);
    }
}

// Example usage:
const filePaths = ['file1.txt', 'file2.txt', 'file3.txt'];
readFilesConcurrently(filePaths).then(contents => {
    console.log(contents);
});

9. Write a function that implements a timeout for an async operation using async/await.

To implement a timeout for an async operation, use the Promise.race method, which allows you to race multiple promises against each other and resolve or reject based on the first one that settles.

Example:

async function asyncOperationWithTimeout(asyncOperation, timeout) {
    const timeoutPromise = new Promise((_, reject) => {
        setTimeout(() => {
            reject(new Error('Operation timed out'));
        }, timeout);
    });

    return Promise.race([asyncOperation, timeoutPromise]);
}

// Example usage
async function exampleAsyncOperation() {
    return new Promise((resolve) => {
        setTimeout(() => {
            resolve('Operation completed');
        }, 2000);
    });
}

async function runExample() {
    try {
        const result = await asyncOperationWithTimeout(exampleAsyncOperation(), 1000);
        console.log(result);
    } catch (error) {
        console.error(error.message);
    }
}

runExample();

10. Discuss the performance implications of using async/await in JavaScript.

Async/await provides a more readable way to handle asynchronous operations. However, consider the performance implications:

• Event Loop and Non-blocking Nature: Async/await does not block the main thread. When an async function encounters an await expression, it pauses execution until the awaited Promise resolves, allowing other operations to continue.
• Error Handling: Async/await simplifies error handling by using try/catch blocks, leading to cleaner code. This can indirectly improve performance by reducing bugs and making the code easier to debug.
• Microtasks and Macrotasks: Awaited Promises are scheduled as microtasks, which have higher priority than macrotasks, leading to more efficient execution.
• Stack Traces: A potential downside is that async/await can make stack traces less informative, as the stack trace may not include the full call stack.

Example:

async function fetchData(url) {
    try {
        let response = await fetch(url);
        let data = await response.json();
        return data;
    } catch (error) {
        console.error('Error fetching data:', error);
    }
}