WeakMap and WeakRef: Memory-Efficient Data Structures

WeakMap and WeakRef: Memory-Efficient Data Structures | DevPrep

Memory management is critical at Google scale. WeakMap and WeakRef are essential tools for avoiding memory leaks in long-running applications.

WeakMap vs Map

Feature	Map	WeakMap
Key types	Any	Objects only
Enumerable	Yes (.keys(), .values())	No
GC behavior	Keys prevent GC	Keys are weakly held
Size property	Yes	No

Use Case: Private Data

Use Case: DOM Metadata

WeakRef and FinalizationRegistry

WeakRef holds a weak reference to an object. FinalizationRegistry lets you run cleanup when an object is garbage collected.

Production Warning

WeakRef and FinalizationRegistry are non-deterministic — you can't predict when GC runs. Never use them for critical logic. They're optimization tools, not correctness tools.

const privateData = new WeakMap();

class User {
  constructor(name, ssn) {
    this.name = name;
    privateData.set(this, { ssn });
  }
  getSSN() {
    return privateData.get(this).ssn;
  }
}
// When User instance is GC'd, private data is too

const elementData = new WeakMap();

function trackElement(el) {
  elementData.set(el, {
    clickCount: 0,
    firstSeen: Date.now()
  });
}
// When element is removed from DOM and GC'd,
// metadata is automatically cleaned up

const cache = new Map();
const registry = new FinalizationRegistry((key) => {
  cache.delete(key);
});

function cacheResult(key, value) {
  const ref = new WeakRef(value);
  cache.set(key, ref);
  registry.register(value, key);
}

function getCached(key) {
  const ref = cache.get(key);
  return ref?.deref(); // Returns undefined if GC'd
}

WeakMap and WeakRef: Memory-Efficient Data Structures

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WeakMap vs Map

Use Case: Private Data

Use Case: DOM Metadata

WeakRef and FinalizationRegistry

Production Warning