In JavaScript, var, let, and const are used to declare variables, but they differ in scope and mutability: var is function-scoped and can be re-declared and updated, while let and const are block-scoped, with let allowing updates but not re-declaration, and const preventing both.

In JavaScript, map(), forEach(), and filter() are array methods that iterate through an array, but they differ in their purpose and return values: map() transforms elements and returns a new array, forEach() executes a function for each element but doesn't return anything, and filter() creates a new array with elements that meet a condition.

Arrow functions, introduced in ES6, offer a concise syntax for writing functions in JavaScript, especially for simple expressions, while regular functions provide a traditional syntax with more flexibility, especially when dealing with complex logic or needing to use this dynamically.

It represent the eventual result of an asynchronous operation. Have three states: pending, fulfilled, or rejected. Use .then() to handle successful results and .catch() for errors. Enable cleaner, chained asynchronous code, avoiding "callback hell." Essentially, they provide a structured way to deal with tasks that take time, like fetching data, without blocking your program.

A JavaScript closure is when an inner function retains access to the outer function's variables even after the outer function has finished executing. It "remembers" its lexical environment.