You Don't Know JS: Scope & Closures. K. Simpson

 You’ll learn how and why they work, and how an understanding of closures can be a powerful part of your development skillset. Like other books in the "You Don’t Know JS" series, Scope and Closures dives into trickier parts of the language that many JavaScript programmers simply avoid. Armed with this knowledge, you can achieve true JavaScript mastery.Learn about scope, a set of rules to help JavaScript engines locate variables in your codeGo deeper into nested scope, a series of containers for variables and functionsExplore function- and block-based scope, “hoisting”, and the patterns and benefits of scope-based hidingDiscover how to use closures for synchronous and asynchronous tasks, including the creation of JavaScript libraries.