javascript

var is function-scoped and can be redeclared, let is block-scoped and can be reassigned, const is block-scoped and cannot be reassigned after initialization.

JavaScript has 7 primitive data types: string, number, bigint, boolean, undefined, null, and symbol.

map() transforms each element and returns a new array of the same length, filter() returns elements that pass a test, and reduce() accumulates array elements into a single value.

== performs type coercion before comparison (loose equality), while === compares both value and type without coercion (strict equality).

Arrow functions have a shorter syntax, don't have their own 'this' binding (they inherit from the enclosing scope), cannot be used as constructors, and don't have access to the 'arguments' object.