JavaScript Asynchronous Programming Interview Questions

Here are some common JavaScript asynchronous programming interview questions along with their answers:

1. What is asynchronous programming in JavaScript?

Asynchronous programming allows JavaScript to perform tasks without blocking the main thread. It enables functions to execute in the background while allowing the rest of the code to run. This is essential in JavaScript because it runs in a single-threaded environment, especially when handling I/O operations like network requests or file reading.

2. What are callbacks in JavaScript?

Callbacks are functions that are passed as arguments to other functions and are executed after some operation is completed. They are a fundamental way to handle asynchronous operations in JavaScript.

function fetchData(callback) {
    setTimeout(() => {
        callback("Data received");
    }, 1000);
}

fetchData((data) => {
    console.log(data); // "Data received"
});

3. What is a Promise?

A Promise is an object that represents the eventual completion (or failure) of an asynchronous operation and its resulting value. It can be in one of three states: pending, fulfilled, or rejected.

let promise = new Promise((resolve, reject) => {
    // Asynchronous operation
    let success = true;
    if (success) {
        resolve("Success!");
    } else {
        reject("Error!");
    }
});

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

4. What is the difference between Promise.all and Promise.race?

• Promise.all: Takes an array of promises and returns a single Promise that resolves when all of the promises in the array have resolved, or rejects if any promise rejects.

Promise.all([promise1, promise2])
    .then(results => console.log(results))
    .catch(error => console.log(error));

Promise.race: Returns a promise that resolves or rejects as soon as one of the promises in the array resolves or rejects.

Promise.race([promise1, promise2])
    .then(result => console.log(result))
    .catch(error => console.log(error));

5. What are async/await?

async and await are syntactic sugars built on top of Promises, making asynchronous code easier to write and read. An async function always returns a Promise, and await is used to pause execution until the Promise resolves.

async function fetchData() {
    try {
        let response = await fetch('https://api.example.com/data');
        let data = await response.json();
        console.log(data);
    } catch (error) {
        console.error(error);
    }
}

6. How does error handling work in async/await?

Error handling in async/await can be done using try/catch blocks. If a Promise is rejected, the execution will jump to the catch block.

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();
        console.log(data);
    } catch (error) {
        console.error('Fetch error:', error);
    }
}

7. What is the Event Loop in JavaScript?

The Event Loop is a mechanism that allows JavaScript to perform non-blocking I/O operations by offloading operations to the system kernel whenever possible. It handles asynchronous operations by placing callbacks into a queue that will be executed once the call stack is clear.

8. What is the difference between synchronous and asynchronous code?

• Synchronous Code: Executes line by line and waits for each operation to complete before moving to the next one. This can lead to blocking the main thread and performance issues.
• Asynchronous Code: Allows operations to run in the background while the program continues executing subsequent code, improving efficiency and responsiveness.

9. Can you explain the concept of “callback hell”?

Callback hell refers to the situation where callbacks are nested within each other, leading to code that is difficult to read and maintain. It often results in “pyramid” structures in code, making it challenging to handle errors or logic.

asyncFunction1((result1) => {
    asyncFunction2(result1, (result2) => {
        asyncFunction3(result2, (result3) => {
            // More nested callbacks...
        });
    });
});

10. How can you avoid callback hell?

We can avoid callback hell by:

• Using Promises instead of callbacks
• Utilizing async/await for clearer, more linear code structure.
• Modularizing code into smaller functions that handle specific tasks.

async function fetchData() {
    try {
        const result1 = await asyncFunction1();
        const result2 = await asyncFunction2(result1);
        const result3 = await asyncFunction3(result2);
    } catch (error) {
        console.error(error);
    }
}