V8 (and most modern JS engines) uses a generational approach to garbage collection. The insight: most objects die young (“the generational hypothesis”).

Newly created objects live here:

function process() {
  // These objects are created in the young generation
  const temp = { data: 'temporary' };
  const result = temp.data.toUpperCase();
  return result;
  // Both 'temp' and the upper case string are likely
  // collected in the next minor GC
}

• Small region of memory
• Collected frequently (minor GC)
• Uses Scavenge algorithm (fast copy collector)
• Objects surviving 2 GC cycles → promoted to old generation

Long-lived objects live here:

// These objects survive and get promoted
const appConfig = { theme: 'dark', lang: 'en' };
const cache = new Map();

function setup() {
  const handler = () => console.log('ready');
  window.addEventListener('load', handler);
  // handler survives because it's referenced by the event system
}

• Larger region of memory
• Collected infrequently (major GC)
• Uses Mark-Sweep-Compact algorithm
• More expensive to collect

flowchart LR
    subgraph "Young Generation (Nursery)"
        New["New Objects\nfrequent GC"]
        Surviving["Surviving Objects\npromoted after 2 GCs"]
    end

    subgraph "Old Generation"
        Old["Long-lived Objects\ninfrequent major GC"]
    end

    New -->|"Minor GC (Scavenge)"| Surviving
    Surviving -->|"Promotion"| Old
    Old -->|"Major GC (Mark-Sweep-Compact)"| Final

Modern V8 performs GC work incrementally and concurrently to avoid long pauses:

• Incremental: Break GC work into small chunks between JS tasks
• Concurrent: GC work runs in parallel with your code (multi-core)

This means you rarely notice GC pauses in modern browsers.

• Create objects only when needed (avoid temporary objects in hot loops)
• Reuse objects rather than creating new ones
• Large caches can cause old gen GC pressure
• Modern V8 handles most patterns well

• V8: Trash Talk
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