What are the differences between Map/Set and WeakMap/WeakSet in JavaScript?
JavaScript provides collections for managing key-value pairs (Map, WeakMap) and unique values (Set, WeakSet). While Map and Set are general-purpose, WeakMap and WeakSet have specialized use cases with distinct features.
Map and Set
Definition
Map: A collection of key-value pairs where keys can be of any data type.Set: A collection of unique values.
Features
- Strong References: Keys and values remain in memory until explicitly removed.
- Iterability: Supports
forEach,for...of, and spread syntax. - Use Cases:
Map: When you need a dictionary-like structure.Set: When you need a collection of unique values.
Examples
Map
const map = new Map();
map.set('key1', 'value1');
map.set(42, 'value2');
console.log(map.get('key1')); // "value1"
console.log(map.size); // 2
Set
const set = new Set();
set.add(1);
set.add(2);
set.add(2); // Duplicate ignored
console.log(set.has(2)); // true
console.log([...set]); // [1, 2]
WeakMap and WeakSet
Definition
WeakMap: A collection of key-value pairs where keys must be objects.WeakSet: A collection of unique objects.
Features
- Weak References: Objects used as keys can be garbage-collected when no other references exist.
- Non-Iterability: Cannot be iterated over; no methods like
forEach. - Use Cases:
- Storing metadata or caching data associated with objects without preventing their garbage collection.
- Managing temporary data for DOM elements.
Examples
WeakMap
const weakMap = new WeakMap();
let obj = { id: 1 };
weakMap.set(obj, 'metadata');
console.log(weakMap.get(obj)); // "metadata"
obj = null; // The object and its value in the WeakMap can be garbage-collected.
WeakSet
const weakSet = new WeakSet();
let obj = { id: 2 };
weakSet.add(obj);
console.log(weakSet.has(obj)); // true
obj = null; // The object can be garbage-collected.
Use Cases and Best Practices
When to Use Map/Set
- Storing and iterating over data with a variety of key or value types.
- Use cases requiring strong references.
When to Use WeakMap/WeakSet
- Associating metadata or auxiliary information with objects.
- Managing data that should not prevent object garbage collection.
Avoid Common Pitfalls
- Avoid using
WeakMap/WeakSetwhen you need iteration. - Avoid using
Map/Setfor temporary data tied to objects, as it prevents garbage collection.
Conclusion
While Map and Set are general-purpose and suitable for most scenarios, WeakMap and WeakSet excel in memory-sensitive cases where objects may need to be garbage-collected. Understanding the nuances between these collections helps in selecting the right tool for the job.