Understand closures for data privacy and the module system (import/export) for organizing code.
Closures are a fundamental and powerful concept in JavaScript. A closure is formed when a function is defined inside another function, giving the inner function access to the outer function's variables and scope, even after the outer function has finished executing. This creates a persistent, private state. Closures are the mechanism behind many common JavaScript patterns, such as creating private variables and methods in object-oriented programming. For example, you can create a counter function where the count variable is not accessible from the global scope, preventing it from being accidentally modified. Building on the idea of encapsulation, JavaScript modules provide a native way to organize code into separate, reusable files. Before modules, developers often resorted to patterns like IIFEs (Immediately Invoked Function Expressions) or external libraries to avoid polluting the global namespace. The ES6 module system introduces the `export` and `import` keywords. You can `export` functions, variables, or classes from one file (a module) to make them available for use in other files. Then, in another file, you use `import` to bring in that exported functionality. This system promotes a modular architecture, making codebases easier to manage, maintain, and scale. It encourages breaking down a large application into smaller, focused pieces, each with a single responsibility.