Advanced JavaScript Interview Questions and Answers

A Promise is an object that represents the eventual completion (or failure) of an asynchronous operation and its resulting value. Promises provide a cleaner and more manageable way to handle asynchronous operations compared to callbacks, which can lead to "callback hell."

Example:

const promise = new Promise((resolve, reject) => {
    setTimeout(() => resolve("Success"), 1000);
});

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

Answer:
The event loop is a mechanism that allows JavaScript to perform non-blocking I/O operations despite being single-threaded. It continuously checks the call stack and the task queue. When the call stack is empty, it pushes tasks from the queue to the stack for execution.

Key components:

• Call Stack: Tracks function calls.

• Task Queue: Holds asynchronous callbacks (e.g., setTimeout, Promise).

• Microtask Queue: Holds higher-priority tasks like Promise resolutions.

Example

console.log("Start");
setTimeout(() => console.log("Timeout"), 0);
Promise.resolve().then(() => console.log("Promise"));
console.log("End");

// Output:
// Start
// End
// Promise
// Timeout