How does garbage collection work in JavaScript?

JavaScript uses automatic garbage collection — memory is freed when objects become unreachable from the "roots" (globals + active call stack).

Mark-and-Sweep algorithm:

1. Start from roots (global scope + call stack)
2. Mark every reachable object
3. Sweep — free everything NOT marked

let user = { name: 'Alice' }; // reachable via 'user'
user = null;                  // reference dropped → unreachable → GC'd

// Circular reference — NOT a problem for modern mark-and-sweep
let a = {}; let b = {};
a.ref = b; b.ref = a; // circular — but if a and b lose all external refs, both are GC'd

Common memory leak sources:

• Forgotten setInterval holding references to DOM elements
• Detached DOM nodes still referenced in JS variables
• Event listeners never removed (removeEventListener)
• Closures unintentionally capturing large objects
• Unbounded caches / global arrays that grow forever