Var is a function-scoped, it can be redeclared and do not comply with block scope, on the other let and const are block-scoped but the value of const cannot be changed. Let value can be changed again.

Map() makes new array and every item changes. forEach() only executes loop but anything not return. filter() creates a new array with items that match the condition.

Arrow Function (=>) is a short function syntax that retains its own this, is used instead of the function keyword, and differs from regular functions. In regular functions, this changes dynamically, and arguments is an object.

JavaScript Promise is an object, which asynchronous task returns a result if it succeeds or fails. It can be in three states—pending, fulfilled, and rejected.

JavaScript closures occur when a function can access variables from its outer function, even after the outer function has been executed. It keeps the data inside the function private and helps with state management.