javascript

A closure is a function that has access to variables from its outer (enclosing) scope, even after the outer function has returned. They're useful for data privacy, state management, and creating function factories.

A Promise is an object representing the eventual completion or failure of an asynchronous operation. It has three states: pending, fulfilled, or rejected, and provides .then(), .catch(), and .finally() methods for handling results.

async/await is syntactic sugar over Promises. An async function always returns a Promise, and await pauses execution until a Promise resolves, making asynchronous code look and behave more like synchronous code.

The Event Loop is JavaScript's mechanism for handling asynchronous operations. It continuously checks the call stack and task queues, executing callbacks from the microtask queue (Promises) before the macrotask queue (setTimeout, events).

The value of 'this' depends on how a function is called: in a method, it refers to the object; in a regular function, it's the global object (or undefined in strict mode); in arrow functions, it's lexically inherited from the enclosing scope.

A shallow copy duplicates only top-level properties while nested objects share references; a deep copy recursively duplicates all nested objects creating fully independent copies.