JavaScript Sets, Maps, and Hashing

Sun, 09 Feb 2025 00:00:00 +0000

TL;DR

Notes captured on 2025-02-09 during Deveficiente algorithms study.
Set = unique values with O(1) add/remove; Map = key-value store with O(1) lookups.
Hashing is reference-based for objects; custom equality needs manual hashing/serialization.
Useful for caching and deduplication; V8 resolves collisions with internal linked lists/trees.
C# exposes GetHashCode; in JS you must implement equivalent behaviour explicitly.

Sets

Store unique values; duplicates are ignored.
Backed by hash tables in engines like V8.
Typical operations (add, delete, has) run in O(1) on average.

const set = new Set();
set.add("value1");
set.add("value2");
console.log(set.has("value1")); // true
console.log(set.size); // 2

Maps

Store key-value pairs; keys can be any data type.
Also implemented with hashing for O(1) lookups.

const map = new Map();
const key = { id: 1 };
map.set(key, "object");
console.log(map.get(key)); // 'object'

Hashing behaviour

Objects/arrays are compared by reference. Two objects with identical structure are not equal unless they share the same reference.
For custom hash keys (similar to C# GetHashCode), stringify values or build hashing utilities.

function hashObject(obj) {
 return JSON.stringify(obj);
}
const cache = new Map();
const obj = { id: 1, name: "test" };
cache.set(hashObject(obj), "cached");

Cache patterns

const cache = new Map();

function getData(key) {
 if (cache.has(key)) {
 return cache.get(key);
 }
 const value = fetchFromSource(key);
 cache.set(key, value);
 return value;
}

Internal details (V8)

Hash collisions handled via linked lists or adaptive structures (e.g., small arrays, balanced trees).
Sparse arrays may convert to hash tables internally for memory efficiency.

Set vs Map recap

Set: unique values.
Map: key-value pairs, versatile for caching, memoization.

References

MDN: Set, Map.
V8 blog on hash tables and inline caching.

[[javascript-closures-and-curries|Closures and Curries]]
[[react-forwardRef|Forwarding Ref]]

Data-Structures on Osmar Petry

JavaScript Sets, Maps, and Hashing

TL;DR

Sets

Maps

Hashing behaviour

Cache patterns

Internal details (V8)

Set vs Map recap

References

Related Notes