Fundamentals Interview Questions

JavaScript is a high-level, interpreted programming language commonly used for web development. Key features include dynamic typing, first-class functions, event-driven programming, support for asynchronous operations (like Promises and async/await), and the ability to manipulate the DOM (Document Object Model) in web pages.

`var` has function scope and can be redeclared or updated, potentially leading to issues. `let` is block-scoped and can be updated but not redeclared within the same scope. `const` is also block-scoped and cannot be updated or redeclared, ensuring constant values.

A closure is a function that retains access to its lexical scope, even when the function is executed outside that scope. Closures allow you to create private variables and functions. For example: ```js function outer() { let count = 0; return function() { count++; return count; }; } const increment = outer(); console.log(increment()); // 1 console.log(increment()); // 2 ```

The event loop is a mechanism in JavaScript that handles asynchronous operations. It continuously checks the call stack for functions to execute. If the call stack is empty, it processes tasks from the message queue (e.g., callbacks from Promises, setTimeout, etc.). This non-blocking behavior allows JavaScript to handle tasks asynchronously.

Arrow functions are a concise syntax for defining functions in JavaScript, using `=>`. Unlike regular functions, arrow functions do not have their own `this` value; instead, they inherit `this` from their lexical context. This makes them useful for cases where you want to retain the value of `this` in a callback.