Hashing - Bun

Bun provides high-performance hashing functions for checksums, data integrity, and hash tables. All implementations are optimized with SIMD instructions where available.

Quick Start

import { hash } from "bun";

// Default hash (Wyhash)
const h = hash("Hello World");
console.log(h); // Fast 64-bit hash

// With seed
const h2 = hash("Hello World", 42);

Non-Cryptographic Hashes

Fast hashes for hash tables, checksums, and data structures.

Wyhash

Default hash function, extremely fast:

import { hash } from "bun";

// 64-bit hash
const h = hash.wyhash("data");
const h2 = hash.wyhash("data", seed);

// Also available as default export
const h3 = hash("data");

Characteristics:

Speed: ~25 GB/s
Output: 64-bit
Collision resistance: Excellent
Use for: Hash tables, general purpose

CityHash

Google’s CityHash family:

import { hash } from "bun";

// 32-bit variant
const h32 = hash.cityHash32("data");
const h32_seeded = hash.cityHash32("data", seed);

// 64-bit variant  
const h64 = hash.cityHash64("data");
const h64_seeded = hash.cityHash64("data", seed);

Characteristics:

Speed: ~20 GB/s
Output: 32-bit or 64-bit
Use for: Hash tables, databases

xxHash

Extremely fast hash with excellent distribution:

import { hash } from "bun";

// xxHash32
const h32 = hash.xxHash32("data");
const h32_seeded = hash.xxHash32("data", seed);

// xxHash64
const h64 = hash.xxHash64("data");
const h64_seeded = hash.xxHash64("data", seed);

// xxHash3 (latest, fastest)
const h3 = hash.xxHash3("data");
const h3_seeded = hash.xxHash3("data", seed);

Characteristics:

Speed: xxHash3 ~30 GB/s, others ~15 GB/s
Output: 32-bit or 64-bit
Collision resistance: Excellent
Use for: Checksums, hash tables, caching

Murmur Hash

Popular hash used in many databases:

import { hash } from "bun";

// MurmurHash2 32-bit
const h = hash.murmur32v2("data", seed);

// MurmurHash2 64-bit
const h64 = hash.murmur64v2("data", seed);

// MurmurHash3 32-bit
const h3 = hash.murmur32v3("data", seed);

Characteristics:

Speed: ~5 GB/s
Output: 32-bit or 64-bit
Use for: Hash tables, Bloom filters

CRC32

Cyclic redundancy check, hardware accelerated:

import { hash } from "bun";

const checksum = hash.crc32("data");
const checksum2 = hash.crc32("data", seed);

Characteristics:

Speed: ~10 GB/s (hardware accelerated)
Output: 32-bit
Use for: Data integrity, checksums

Adler-32

Simpler checksum algorithm:

import { hash } from "bun";

const checksum = hash.adler32("data");
const checksum2 = hash.adler32("data", seed);

Characteristics:

Speed: ~15 GB/s
Output: 32-bit
Use for: Checksums (faster but less reliable than CRC32)

Input Types

All hash functions accept multiple input types:

Strings

const h = hash("Hello World");

TypedArrays

const data = new Uint8Array([1, 2, 3, 4]);
const h = hash(data);

ArrayBuffer

const buffer = new ArrayBuffer(1024);
const h = hash(buffer);

Blob

const blob = new Blob(["data"]);
const h = hash(blob);

Cryptographic Hashes

For security-sensitive applications, use cryptographic hash functions:

SHA-256

import { SHA256 } from "bun";

const hasher = new SHA256();
hasher.update("Hello ");
hasher.update("World");

const digest = hasher.digest();
console.log(digest.toString("hex"));

// One-shot hashing
const hash = SHA256.hash("data");
console.log(hash.toString("hex"));

SHA-512

import { SHA512 } from "bun";

const hash = SHA512.hash("data");
console.log(hash.toString("hex"));

Other SHA Variants

import { SHA1, SHA224, SHA384, SHA512_256 } from "bun";

const h1 = SHA1.hash("data");
const h224 = SHA224.hash("data");
const h384 = SHA384.hash("data");
const h512_256 = SHA512_256.hash("data");

MD4 and MD5

⚠️ Warning: MD4 and MD5 are cryptographically broken. Only use for legacy compatibility.

import { MD4, MD5 } from "bun";

const md4 = MD4.hash("data");
const md5 = MD5.hash("data");

Performance Comparison

Throughput Benchmark

const data = new Uint8Array(1024 * 1024); // 1MB

const algorithms = [
  ["wyhash", hash.wyhash],
  ["xxHash3", hash.xxHash3],
  ["xxHash64", hash.xxHash64],
  ["cityHash64", hash.cityHash64],
  ["crc32", hash.crc32],
  ["adler32", hash.adler32],
];

for (const [name, fn] of algorithms) {
  const start = performance.now();
  for (let i = 0; i < 1000; i++) {
    fn(data);
  }
  const elapsed = performance.now() - start;
  const throughput = (1000 / elapsed) * 1024; // MB/s
  
  console.log(`${name}: ${throughput.toFixed(0)} MB/s`);
}

// Typical results on M1 Mac:
// wyhash: 25000 MB/s
// xxHash3: 30000 MB/s
// xxHash64: 15000 MB/s
// cityHash64: 20000 MB/s
// crc32: 10000 MB/s
// adler32: 15000 MB/s

Common Use Cases

Hash Table Keys

import { hash } from "bun";

class HashMap<V> {
  buckets = new Map<number, Array<[string, V]>>();
  
  set(key: string, value: V) {
    const h = hash(key);
    const bucket = this.buckets.get(h) ?? [];
    bucket.push([key, value]);
    this.buckets.set(h, bucket);
  }
  
  get(key: string): V | undefined {
    const h = hash(key);
    const bucket = this.buckets.get(h);
    if (!bucket) return undefined;
    
    for (const [k, v] of bucket) {
      if (k === key) return v;
    }
  }
}

File Integrity

import { hash } from "bun";

async function verifyFile(path: string, expectedHash: number) {
  const file = Bun.file(path);
  const contents = await file.arrayBuffer();
  const actualHash = hash(contents);
  
  return actualHash === expectedHash;
}

const isValid = await verifyFile("data.bin", 0x1234567890abcdef);

Content Addressing

import { hash } from "bun";

class ContentStore {
  store = new Map<string, Uint8Array>();
  
  async put(data: Uint8Array): Promise<string> {
    const h = hash(data);
    const id = h.toString(16);
    this.store.set(id, data);
    return id;
  }
  
  get(id: string): Uint8Array | undefined {
    return this.store.get(id);
  }
}

const store = new ContentStore();
const id = await store.put(new Uint8Array([1, 2, 3]));
const data = store.get(id);

Deduplication

import { hash } from "bun";

class Deduplicator {
  seen = new Set<number>();
  
  isDuplicate(data: string | Uint8Array): boolean {
    const h = hash(data);
    if (this.seen.has(h)) {
      return true;
    }
    this.seen.add(h);
    return false;
  }
}

const dedup = new Deduplicator();
console.log(dedup.isDuplicate("foo")); // false
console.log(dedup.isDuplicate("foo")); // true
console.log(dedup.isDuplicate("bar")); // false

Cache Keys

import { hash } from "bun";

class Cache<T> {
  data = new Map<number, T>();
  
  key(...args: any[]): number {
    const str = JSON.stringify(args);
    return hash(str);
  }
  
  get(...args: any[]): T | undefined {
    return this.data.get(this.key(...args));
  }
  
  set(value: T, ...args: any[]) {
    this.data.set(this.key(...args), value);
  }
}

const cache = new Cache<string>();
cache.set("result", "arg1", 42, true);
const result = cache.get("arg1", 42, true);

Password Hashing

⚠️ Warning: Use proper password hashing like bcrypt or Argon2. Never use fast hashes for passwords.

// DON'T DO THIS:
import { hash } from "bun";
const passwordHash = hash(password); // INSECURE!

// Instead, use:
import { password } from "bun";
const passwordHash = await password.hash(password);
const isValid = await password.verify(password, passwordHash);

Seeds and Determinism

All hash functions support seeds for deterministic results:

import { hash } from "bun";

// Without seed - uses random seed
const h1 = hash("data");
const h2 = hash("data"); // Different on each run

// With seed - deterministic
const h3 = hash("data", 0);
const h4 = hash("data", 0); // Always same

console.log(h3 === h4); // true

Reproducible Builds

const SEED = 0x1234567890abcdef;

function hashFile(path: string): number {
  const data = Bun.file(path).arrayBuffer();
  return hash(data, SEED);
}

// Always produces same hash for same file
const h1 = hashFile("input.js");
const h2 = hashFile("input.js");
console.log(h1 === h2); // true

Hash Quality

Collision Resistance

Test hash distribution:

function testCollisions(hashFn: Function, count: number) {
  const seen = new Set<number>();
  let collisions = 0;
  
  for (let i = 0; i < count; i++) {
    const h = hashFn(String(i));
    if (seen.has(h)) collisions++;
    seen.add(h);
  }
  
  console.log(`Collisions: ${collisions}/${count}`);
  console.log(`Rate: ${(collisions / count * 100).toFixed(4)}%`);
}

testCollisions(hash.wyhash, 1_000_000);
// Typical result: 0 collisions

Avalanche Effect

Small input changes should produce large hash changes:

const h1 = hash("test");
const h2 = hash("Test"); // One bit different
const diff = h1 ^ h2;
const bitCount = diff.toString(2).split('1').length - 1;

console.log(`Bits changed: ${bitCount}/64`);
// Good hash: ~32 bits

Best Practices

Choose the right hash for your use case
- Hash tables: wyhash, xxHash3
- Checksums: crc32, xxHash64
- Security: SHA256, SHA512
- Legacy: MD5 (only if required)

Use seeds for reproducibility

const SEED = 0;
const h = hash(data, SEED);

Consider hash size

// 32-bit: Good for small datasets
const h32 = hash.cityHash32(data);

// 64-bit: Better for large datasets
const h64 = hash.cityHash64(data);

Benchmark for your data

// Test with your actual data patterns
const start = performance.now();
for (const item of myData) {
  hash(item);
}
console.log(`${performance.now() - start}ms`);

Platform Differences

Hardware Acceleration

x86_64: CRC32, SHA use CPU instructions
ARM64: CRC32, SHA use NEON instructions
Other: Software fallback

Endianness

Hashes are consistent across platforms:

// Same hash on little-endian and big-endian systems
const h = hash("test");

API Reference

hash()

Default fast hash function (Wyhash):

hash(data: string | TypedArray | ArrayBuffer, seed?: number): number

hash.wyhash()

hash.wyhash(data: string | TypedArray | ArrayBuffer, seed?: number): number

hash.xxHash3()

hash.xxHash3(data: string | TypedArray | ArrayBuffer, seed?: number): number

hash.crc32()

hash.crc32(data: string | TypedArray | ArrayBuffer, seed?: number): number

See FFI documentation for low-level hash implementations.

Core Runtime

APIs

HTTP & WebSocket

Data & Storage

System & Process

Utilities

Documentation Index

​Quick Start

​Non-Cryptographic Hashes

​Wyhash

​CityHash

​xxHash

​Murmur Hash

​CRC32

​Adler-32

​Input Types

​Strings

​TypedArrays

​ArrayBuffer

​Blob

​Cryptographic Hashes

​SHA-256

​SHA-512

​Other SHA Variants

​MD4 and MD5

​Performance Comparison

​Throughput Benchmark

​Common Use Cases

​Hash Table Keys

​File Integrity

​Content Addressing

​Deduplication

​Cache Keys

​Password Hashing

​Seeds and Determinism

​Reproducible Builds

​Hash Quality

​Collision Resistance

​Avalanche Effect

​Best Practices

​Platform Differences

​Hardware Acceleration

​Endianness

​API Reference

​hash()

​hash.wyhash()

​hash.xxHash3()

​hash.crc32()

Build docs developers (and LLMs) love

Quick Start

Non-Cryptographic Hashes

Wyhash

CityHash

xxHash

Murmur Hash

CRC32

Adler-32

Input Types

Strings

TypedArrays

ArrayBuffer

Blob

Cryptographic Hashes

SHA-256

SHA-512

Other SHA Variants

MD4 and MD5

Performance Comparison

Throughput Benchmark

Common Use Cases

Hash Table Keys

File Integrity

Content Addressing

Deduplication

Cache Keys

Password Hashing

Seeds and Determinism

Reproducible Builds

Hash Quality

Collision Resistance

Avalanche Effect

Best Practices

Platform Differences

Hardware Acceleration

Endianness

API Reference

hash()

hash.wyhash()

hash.xxHash3()

hash.crc32()