Var is function-scoped, hoisted, and allows re-declaration and reassignment, while let and const are block-scoped and not initialized before use. let allows reassignment but not re-declaration, whereas const prevents both but allows modifications to objects and arrays.

map(), forEach(), and filter() are array methods with different purposes. map() creates a new array by transforming each element, forEach() executes a function on each element without returning a new array, and filter() returns a new array containing only elements that meet a specified condition.

Arrow functions in JavaScript are a shorter syntax for writing functions using =>, and they do not have their own this, arguments, super, or new.target. Unlike regular functions, arrow functions inherit this from their surrounding scope, making them ideal for callbacks and methods where preserving context is important.

JavaScript Promises handle asynchronous operations by representing a value that may be available now, later, or never. They have three states—pending, fulfilled, or rejected—and use .then() for success handling and .catch() for errors.

A closure in JavaScript occurs when a function remembers and accesses variables from its outer scope even after the outer function has finished executing. This allows functions to maintain private variables and is commonly used for data encapsulation and callbacks.