JavaScript Closures, Lexical Scope, and IIFEs: A Practical Explainer

Scope in JavaScript determines where variables can be accessed, and the language uses lexical scope, meaning a variable's accessibility is defined by where the code is written rather than where it runs. Closures occur when an inner function retains access to its parent function's variables even after the parent has finished executing, enabling patterns like private data and stateful functions. Immediately Invoked Function Expressions (IIFEs) are functions that execute the moment they are defined, creating isolated scopes that prevent variables from leaking into the global namespace. Before ES6 introduced block-scoped declarations, IIFEs were a primary tool for scope management, and they remain useful today when combined with closures to build self-contained modules. Together, closures and lexical scope underpin key JavaScript capabilities including data encapsulation, currying, event handling, and modular code design.