JavaScriptMed
JavaScript Closures — Complete Guide with Interactive Visualizations
A closure is a function that remembers the variables from its outer scope even after the outer function has returned. Think of it as a function carrying a "backpack" of variables wherever it goes.
9 concepts·4 practice problems·Intermediate level
Key Concepts
- •A closure is created every time a function is created
- •Inner functions have access to outer function variables
- •The outer variables are captured by reference, not by value
- •Closures enable data privacy and stateful functions
Topics Covered
ClosuresFunctions that remember their scopeScope Basics: Global, Function & BlockWhere variables live and are accessibleLexical Scoping & Scope ChainScope is determined at write time, not runtimeWhat is a Closure?Functions that remember their outer scopePractical Closure PatternsData privacy, factories, and stateful functionsThe Infamous Loop Closure BugWhy var + closures in loops cause bugsClosures & Memory ManagementWhen closures prevent garbage collectionThe Module Pattern (IIFE + Closure)Encapsulation with IIFE and closuresPartial Application & CurryingFixing function arguments with closures
Practice Problems
Common Mistakes to Avoid
- ×Loop variable capture - all callbacks share the same variable
- ×Memory leaks from closures holding large objects
- ×Forgetting that closures capture by reference, not value
Interview Tips
- ✓Use the "backpack" metaphor to explain closures
- ✓Show practical uses: data privacy, function factories, memoization
- ✓Be ready to solve the classic loop closure problem