var: Function-scoped, hoisted, can be re-declared and updated.
let: Block-scoped, not hoisted like var, can be updated but not re-declared in the same scope.
const: Block-scoped, cannot be updated or re-declared (must be initialized).

map(): Returns a new array after applying a function to each element.
forEach(): Executes a function on each element but returns undefined.
filter(): Returns a new array with elements that pass a condition.

Arrow functions are shorter (()=>{}) and do not bind this.
Regular functions have their own this context.
Arrow functions are best for callbacks or non-method functions.

A Promise represents a value that will be available in the future (async). It can be in three states: pending, fulfilled, or rejected. Use .then() for success, .catch() for errors.

A closure is when a function remembers variables from its outer scope, even after that scope has finished executing.Used in private data, callbacks, and more.