🔒 Closures in JavaScript — The Complete Guide
Category: js / general-concepts
Difficulty: hard
Interview Importance: 🔴 Critical — Closures are asked in 90% of JavaScript interviews and form the foundation for understanding scope, data privacy, and functional patterns 1️⃣ What is a Closure? A closure is a function that remembers and has access to variables from its outer (enclosing) scope, even after the outer function has finished executing. The Core Concept [code example] Real-World Analogy: The Backpack 🎒 Think of a closure like a backpack that a function carries with it: [code example] Simple Example [code example] 2️⃣ Why Do Closures Matter? Use Cases Table Solution with Closures Encapsulate state that can't be accessed directly Function remembers values between calls Create specialized functions from templates Pre-fill some arguments Expose public API, hide internals Maintain state in async operations Benefits [code example] 3️⃣ How Closures Work — Implementation Basic Implementation: Counter [code example] 🔍 Dry Run — Step-by-Step Execution [code example] 4️⃣ Understanding Key Concepts Lexical Scoping — The Foundation of Closures [code example] Scope Chain Visualization [code example] What Makes a Closure? Three conditions must be met: [code example] What Breaks Without Closures? [code example] 5️⃣ Advanced Implementations Production-Ready Private State Module [code example] Function Factory with Closures [code example] 6️⃣ Real-World Examples React Hooks Using Closures [code example] Event Handler Factory [code example] Module Pattern [code example] 7️⃣ Closures vs Other Patterns Comparison Table Closure Plain Object ✅ True privacy ❌ No Memory per instance Higher ✅ None ✅ None Inheritance ✅ Built-in ❌ No (functions) ✅ Yes TypeScript support ✅ Excellent Visual Comparison [code example] When to Use Which [code example] 8️⃣ Common Interview Questions Q1: What is a closure? Explain with an example. Answer: [code example] Q2: What will this code output and why? [code example] Answer: [code example] Q3: Create a function that can only be called once Answe...