JavaScript Scope, Closures, and Prototypes Explained for Deeper Runtime Understanding

JavaScript hoisting behaves differently depending on whether var, let, or const is used — var is hoisted and initialized to undefined, while let and const enter a Temporal Dead Zone and cannot be accessed before their declaration. Unlike var, which is function-scoped and leaks out of blocks like loops and conditionals, let and const are block-scoped, preventing common bugs such as shared loop variables. Closures allow inner functions to retain access to variables from their outer scope even after that scope has finished executing, enabling patterns like data encapsulation, function factories, and memoization. A practical example shows a bank account object where the balance variable remains private and is only accessible through returned methods, thanks to closure. These concepts — hoisting rules, block scoping, and closure mechanics — form the foundation that distinguishes developers who merely use JavaScript from those who understand how it executes at runtime.