The transition to mid-level is marked by a shift from 'making it work' to 'making it efficient and scalable'.
1The Lexical Fortress
Closures aren't just a quirk; they are a feature. They allow you to create 'private' variables that cannot be accessed from the outside, providing a layer of security and structure to your modules.
2The Prototype Chain
When you use a method like .map() or .filter(), you are using the prototype. Understanding this chain allows you to add shared functionality to thousands of objects without wasting a single byte of memory.
3Asynchronous Discipline
Moving beyond simple .then() chains to async/await with robust error handling is a hallmark of senior-leaning developers. It makes your code readable, maintainable, and resilient to network failures.
