Writing Benchmarks

When to Use This Skill

Comparing implementations: Measuring old vs new approach after an optimization
Regression testing: Verifying a refactor doesn't degrade performance
Comparing with published version: Benchmarking workspace code against the latest published npm package

Do NOT Use This Skill For

General app-level performance optimization (use jazz-performance)
Profiling or debugging slow user-facing behavior

Directory Structure

All benchmarks live in the bench/ directory at the repository root:

bench/
├── package.json              # Dependencies: cronometro, cojson, jazz-tools, vitest
├── jazz-tools/               # jazz-tools benchmarks
│   └── *.bench.ts

File Naming

Benchmark files follow the pattern: <subject>.<operation>.bench.ts

Each file should focus on a single benchmark comparing multiple implementations (e.g., @latest vs @workspace).

Examples:

comap.create.jazz-tools.bench.ts — benchmarks CoMap creation
filestream.getChunks.bench.ts — benchmarks FileStream.getChunks()
filestream.asBase64.bench.ts — benchmarks FileStream.asBase64()
binaryCoStream.write.bench.ts — benchmarks binary stream writes

Benchmark Library: cronometro

Benchmarks use cronometro, which runs each test in an isolated worker thread for accurate measurement.

Basic Template

import cronometro from "cronometro";

const TOTAL_BYTES = 5 * 1024 * 1024;
let data: SomeType;

await cronometro(
  {
    "operation - @latest": {
      async before() {
        // Setup — runs once before the test iterations
        data = prepareTestData(TOTAL_BYTES);
      },
      test() {
        // The code being benchmarked — runs many times
        latestImplementation(data);
      },
      async after() {
        // Cleanup — runs once after all iterations
        cleanup();
      },
    },
    "operation - @workspace": {
      async before() {
        data = prepareTestData(TOTAL_BYTES);
      },
      test() {
        workspaceImplementation(data);
      },
      async after() {
        cleanup();
      },
    },
  },
  {
    iterations: 50,
    warmup: true,
    print: {
      colors: true,
      compare: true,
    },
    onTestError: (testName: string, error: unknown) => {
      console.error(`\nError in test "${testName}":`);
      console.error(error);
    },
  },
);

Single Cronometro Instance Per Benchmark

Each benchmark file should have a single cronometro() call that compares multiple implementations of the same operation. This makes results easier to read and compare:

import cronometro from "cronometro";

const TOTAL_BYTES = 5 * 1024 * 1024;
let data: InputType;

await cronometro(
  {
    "operationName - @latest": {
      async before() {
        data = generateInput(TOTAL_BYTES);
      },
      test() {
        latestImplementation(data);
      },
      async after() {
        cleanup();
      },
    },
    "operationName - @workspace": {
      async before() {
        data = generateInput(TOTAL_BYTES);
      },
      test() {
        workspaceImplementation(data);
      },
      async after() {
        cleanup();
      },
    },
  },
  {
    iterations: 50,
    warmup: true,
    print: { colors: true, compare: true },
    onTestError: (testName: string, error: unknown) => {
      console.error(`\nError in test "${testName}":`);
      console.error(error);
    },
  },
);

Key principles:

One file = one benchmark (e.g., getChunks, asBase64, write)
One cronometro call comparing @latest vs @workspace (or old vs new)
Fixed data size at the top of the file (e.g., const TOTAL_BYTES = 5 * 1024 * 1024)
Descriptive test names with format "operation - @implementation"

Comparing workspace vs published package

To compare current workspace code against the latest published version:

1. Add npm aliases to bench/package.json:

{
  "dependencies": {
    "cojson": "workspace:*",
    "cojson-latest": "npm:cojson@0.20.7",
    "jazz-tools": "workspace:*",
    "jazz-tools-latest": "npm:jazz-tools@0.20.7"
  }
}

Then run pnpm install in bench/.

2. Import both versions:

import * as localTools from "jazz-tools";
import * as latestPublishedTools from "jazz-tools-latest";
import { WasmCrypto as LocalWasmCrypto } from "cojson/crypto/WasmCrypto";
import { WasmCrypto as LatestPublishedWasmCrypto } from "cojson-latest/crypto/WasmCrypto";

3. Use @ts-expect-error when passing the published package since the types won't match the workspace version:

ctx = await createContext(
  // @ts-expect-error version mismatch
  latestPublishedTools,
  LatestPublishedWasmCrypto,
);

Benchmarking with a Jazz context

When benchmarking CoValues (not standalone functions), create a full Jazz context. Use this helper pattern:

async function createContext(tools: typeof localTools, wasmCrypto: typeof LocalWasmCrypto) {
  const ctx = await tools.createJazzContextForNewAccount({
    creationProps: { name: "Bench Account" },
    peers: [],
    crypto: await wasmCrypto.create(),
    sessionProvider: new tools.MockSessionProvider(),
  });
  return { account: ctx.account, node: ctx.node };
}

Key points:

Pass peers: [] — benchmarks don't need network sync
Use MockSessionProvider — avoids real session persistence
Call (ctx.node as any).gracefulShutdown() in after() to clean up

Test data strategy

Define a fixed data size constant at the top of the file, then generate test data inside the before hook:

const TOTAL_BYTES = 5 * 1024 * 1024; // 5MB

let chunks: Uint8Array[];

await cronometro({
  "operationName - @workspace": {
    async before() {
      chunks = makeChunks(TOTAL_BYTES, CHUNK_SIZE);
    },
    test() {
      doWork(chunks);
    },
  },
}, options);

Choose a size large enough to measure meaningfully. Small data (e.g., 100KB) may complete so fast that measurement noise dominates. 5MB is typically a good default for file/stream operations.

All fixture generation must be done inside the before hook, not at module level. This ensures data is created in the same worker thread that runs the test.

Running Benchmarks

Add a script entry to bench/package.json:

{
  "scripts": {
    "bench:mytest": "node --experimental-strip-types --no-warnings ./jazz-tools/mytest.jazz-tools.bench.ts"
  }
}

Then run from the bench/ directory:

cd bench
pnpm run bench:mytest

Critical Gotchas

1. Use `node --experimental-strip-types`, NOT `tsx`

Cronometro spawns worker threads that re-import the benchmark file. Workers don't inherit tsx's custom ESM loader, so the TypeScript import fails silently and the benchmark hangs forever.

Use node --experimental-strip-types --no-warnings instead:

"bench:foo": "node --experimental-strip-types --no-warnings ./jazz-tools/foo.bench.ts"

2. `before`/`after` hooks MUST be `async` or accept a callback

Cronometro's lifecycle hooks expect either:

An async function (returns a Promise)
A function that accepts and calls a callback parameter

A plain synchronous function that does neither will silently prevent the test from ever starting, causing the benchmark to hang indefinitely:

// BAD — test never starts, benchmark hangs
{
  before() {
    data = generateInput();  // sync, no callback, no promise
  },
  test() { ... },
}

// GOOD — async function returns a Promise
{
  async before() {
    data = generateInput();
  },
  test() { ... },
}

// ALSO GOOD — callback style
{
  before(cb: () => void) {
    data = generateInput();
    cb();
  },
  test() { ... },
}

3. `test()` can be sync or async

Unlike before/after, the test function works correctly as a plain synchronous function. Make it async only if the code under test is genuinely asynchronous.

4. TypeScript constraints under `--experimental-strip-types`

Node's type stripping handles annotations, as casts, and ! assertions. But it does not support:

enum declarations (use const objects instead)
namespace declarations
Parameter properties in constructors (constructor(private x: number))
Legacy import = / export = syntax

Keep benchmark files to simple TypeScript that only uses type annotations, interfaces, type aliases, and casts.

Example: Full Benchmark

This example shows a benchmark comparing getChunks() between the published package and workspace code:

import cronometro from "cronometro";
import * as localTools from "jazz-tools";
import * as latestPublishedTools from "jazz-tools-latest";
import { WasmCrypto as LocalWasmCrypto } from "cojson/crypto/WasmCrypto";
import { cojsonInternals } from "cojson";
import { WasmCrypto as LatestPublishedWasmCrypto } from "cojson-latest/crypto/WasmCrypto";

const CHUNK_SIZE = cojsonInternals.TRANSACTION_CONFIG.MAX_RECOMMENDED_TX_SIZE;
const TOTAL_BYTES = 5 * 1024 * 1024;

function makeChunks(totalBytes: number, chunkSize: number): Uint8Array[] {
  const chunks: Uint8Array[] = [];
  let remaining = totalBytes;
  while (remaining > 0) {
    const size = Math.min(chunkSize, remaining);
    const chunk = new Uint8Array(size);
    for (let i = 0; i < size; i++) {
      chunk[i] = Math.floor(Math.random() * 256);
    }
    chunks.push(chunk);
    remaining -= size;
  }
  return chunks;
}

type Tools = typeof localTools;

async function createContext(tools: Tools, wasmCrypto: typeof LocalWasmCrypto) {
  const ctx = await tools.createJazzContextForNewAccount({
    creationProps: { name: "Bench Account" },
    peers: [],
    crypto: await wasmCrypto.create(),
    sessionProvider: new tools.MockSessionProvider(),
  });
  return { account: ctx.account, node: ctx.node, FileStream: tools.FileStream };
}

function populateStream(ctx: Awaited<ReturnType<typeof createContext>>, chunks: Uint8Array[]) {
  let totalBytes = 0;
  for (const c of chunks) totalBytes += c.length;
  const stream = ctx.FileStream.create({ owner: ctx.account });
  stream.start({ mimeType: "application/octet-stream", totalSizeBytes: totalBytes });
  for (const chunk of chunks) stream.push(chunk);
  stream.end();
  return stream;
}

const benchOptions = {
  iterations: 50,
  warmup: true,
  print: { colors: true, compare: true },
  onTestError: (testName: string, error: unknown) => {
    console.error(`\nError in test "${testName}":`);
    console.error(error);
  },
};

let readCtx: Awaited<ReturnType<typeof createContext>>;
let readStream: ReturnType<typeof populateStream>;

await cronometro(
  {
    "getChunks - @latest": {
      async before() {
        readCtx = await createContext(
          // @ts-expect-error version mismatch
          latestPublishedTools,
          LatestPublishedWasmCrypto,
        );
        readStream = populateStream(readCtx, makeChunks(TOTAL_BYTES, CHUNK_SIZE));
      },
      test() {
        readStream.getChunks();
      },
      async after() {
        (readCtx.node as any).gracefulShutdown();
      },
    },
    "getChunks - @workspace": {
      async before() {
        readCtx = await createContext(localTools, LocalWasmCrypto);
        readStream = populateStream(readCtx, makeChunks(TOTAL_BYTES, CHUNK_SIZE));
      },
      test() {
        readStream.getChunks();
      },
      async after() {
        (readCtx.node as any).gracefulShutdown();
      },
    },
  },
  benchOptions,
);

benchmarking