TracePerf

See where the time goes in your code. It prints the call tree while you develop, keeps cheap per-function statistics in production, and exports to Perfetto or OpenTelemetry when you need more.

• No code changes needed. node --require traceperf/auto app.js tracks every request, outgoing call and database query, and tells you why slow requests were slow when the process exits.
• Explains, not just measures. traceperf explain points out blocked event loops (and which function blocked it), awaits that could run in parallel and N+1 queries.
• Call trees, not just timers. Tracked calls inside tracked calls nest automatically, including across await on Node.js.
• Honest numbers. You get total, self and synchronous time for each call. Detailed mode flags calls that blocked the event loop.
• Production-ready. Stats are bounded per label (p50/p95/p99), sampling is built in, and enabled: false makes tracking a pass-through costing ~40 ns.
• Gets data out. Chrome/Perfetto traces, OpenTelemetry spans, JSON lines, or your own onComplete hook.
• Zero-effort coverage. One line tracks every fetch, HTTP request or query, for pg, mysql2, MongoDB/Mongoose, ioredis, Prisma and the AWS SDK.
• Built for teams. It feeds Prometheus, catches regressions in CI (traceperf compare --fail), shows up in Chrome DevTools, and produces HTML flame-chart reports.
• Fits your stack. Express, Fastify, Hono, Koa, NestJS, Next.js route handlers, decorators, CommonJS, ESM, TypeScript, browsers, Node.js 14+, Bun, Deno, Cloudflare Workers and Vercel Edge. There are no runtime dependencies.

Try it in the browser: the playground is an Express API with a hidden N+1 query problem.

Upgrading from 0.x? The tracking API is unchanged; only the old undocumented logging classes were removed. See MIGRATION.md.

Install

npm install traceperf

Quick start

const { createTracePerf } = require('traceperf');
const tp = createTracePerf();

const fetchUser = tp.createTrackable(async (id) => {
  await new Promise(r => setTimeout(r, 20)); // pretend this is a database call
  return { id };
}, { label: 'fetchUser' });

const render = tp.createTrackable((user) => `<h1>${user.id}</h1>`, { label: 'render' });

await tp.track(async () => render(await fetchUser(42)), { label: 'handleRequest' });

Prints one tree when the outermost call finishes:

└─ handleRequest: 26.412 ms 📊 +83.34 KB
  ├─ fetchUser: 20.312 ms 📊 +28.79 KB
  └─ render: 0.314 ms 📊 +1.53 KB

📊 is the change in V8 heap during the call. A negative value is marked (gc), because it means a garbage collection ran during the call.

ES modules work the same way:

import tp, { createTracePerf, TrackingMode } from 'traceperf';

tp.track(() => JSON.parse('{"a":1}'), { label: 'parse' }); // the default instance

Why is my API slow? traceperf run

One command, no code changes. It starts your app with tracking on, sends it concurrent traffic, and tells you why requests were slow:

npx traceperf run server.js --path /report --path /users

traceperf: sending 5 s of traffic to /report, /users on port 3000, 10 at a time

load: 230 requests in 5.0 s (46/s), 230× 200
...per-route and per-query p50/p95/p99...

Why it was slow:
🔥 GET /report ran 48 ms of synchronous code, blocking everything else in the process for that long; CPU time: buildReport (server.js:10) 100%
⏳ GET /users waited ~165 ms (95% of its 174 ms) while GET /report blocked the event loop, in buildReport (server.js:10)
● SELECT * FROM users took 100% of GET /users (174 ms of 174 ms), but ~165 ms of that was waiting for blocking code elsewhere

The database was fast; buildReport was blocking it. That only shows up when requests overlap, which is why run sends concurrent traffic.

• The blocking function is named even though nothing tracks it: run samples the CPU (the V8 profiler) during the synchronous part of each blocking call. Native work is named by the app function that called it (hashPassword → pbkdf2Sync), code V8 inlined is credited to the inlined function, and garbage-collection pauses are reported as such. --no-profile turns this off.

• Queueing the app can't see: a request that arrives while the event loop is blocked waits in the socket before its handler starts. run compares what its clients waited with the app's own timings and reports the difference.

• --path /x (repeatable): what to request. Without --path, use the app yourself and press Ctrl-C for the report.

• -c 10 concurrency, -d 5s duration or -n 500 requests, --method POST, --port 3000 (detected by default).

• --html report.html, --output calls.jsonl (keep the trees), --json.

• --warmup 3: untimed requests per path first, so cold-start work doesn't look like a problem.

• --no-profile: don't sample the CPU.

• Any start command works: npx traceperf run -- npm start.

examples/slow-apps/ has small apps with one planted problem each (blocking in your code, blocking in built-ins, sequential awaits, N+1, a slow dependency, errors); CI checks that run names each one.

The same naming works without run: TRACEPERF_PROFILE_BLOCKING=1 node --require traceperf/auto server.js adds the function names to the exit summary (sampling costs a few percent CPU, so use it while investigating). In code: profileSlowCalls({ select: root => blockingRanges(root), mergeInlined: true, onProfile }), then explain(roots, { hotspots }).

No code changes: traceperf/auto

Start your app with traceperf preloaded. Every incoming HTTP request becomes a tree, with the outgoing http/fetch calls and the pg, mysql2, MongoDB/Mongoose and ioredis queries it made nested under it:

node --require traceperf/auto server.js            # CommonJS or ESM apps
node --import traceperf/auto server.mjs            # ESM, Node.js >= 18.19
NODE_OPTIONS="--require traceperf/auto" npm start  # through any script

traceperf: tracking server, http, fetch, pg
└─ GET /users/:id: 84.112 ms
  ├─ pg SELECT users: 12.331 ms
  └─ fetch GET billing.internal/invoices: 70.950 ms

When the process exits (including Ctrl-C), it prints the statistics table and an explanation of what made calls slow (see traceperf explain). Ids in paths are collapsed (/users/42 becomes /users/:id), so labels stay few.

Database clients are only instrumented when your package.json lists them. Configure it with environment variables:

Variable	Effect
TRACEPERF_OUTPUT=calls.jsonl	Append every request tree as JSON lines (for traceperf report, explain, compare, watch). Console trees are then off.
TRACEPERF_THRESHOLD=50	Only report calls taking at least 50 ms
TRACEPERF_SAMPLE_RATE=0.1	Track 10% of requests
TRACEPERF_IGNORE=^/health	Regular expression of request paths not to track
TRACEPERF_DISABLE=http,fetch	Skip targets: server, http, fetch, pg, mysql2, mongodb, ioredis
TRACEPERF_SILENT=1 / TRACEPERF_SUMMARY=0 / TRACEPERF_QUIET=1	No per-request trees / no exit summary / no startup line
TRACEPERF_ENABLED=0	Load nothing

Your own tp.track(...) calls (with require('traceperf') or import tp from 'traceperf') nest inside the request trees too. In code, the same thing is autoInstrument(tp), and request tracking alone is instrumentHttpServer(tp). Don't combine it with a framework middleware from Frameworks, or each request is tracked twice.

Ways to track

const { createTracePerf } = require('traceperf');
const tp = createTracePerf();

// 1. Wrap a function once, call it as usual (keeps `this`, name and length)
const add = tp.createTrackable((a, b) => a + b, { label: 'add' });
add(2, 3);

// 2. Track a single call
const total = tp.track(() => [1, 2, 3].reduce((a, b) => a + b), { label: 'sum' });

// 3. Track every method of an object or class instance (a proxy; state stays on the original)
class Cart {
  items = [];
  add(item) { this.items.push(item); return this.items.length; }
}
const cart = tp.trackObject(new Cart());
cart.add('book'); // reported as "Cart.add"

// 4. Track work that is not one function call (a "span")
const span = tp.start('import batch');
span.run(() => add(1, 1)); // tracked calls inside become children
span.end();

With TypeScript (or any decorator-capable build), methods can be decorated. Both TypeScript 5 standard decorators and legacy experimentalDecorators are supported:

import { traced } from 'traceperf';

class UserService {
  @traced()                       // label: "UserService.getUser"
  async getUser(id: string) { /* ... */ }

  @traced({ threshold: 50 })      // only report slow calls
  async search(q: string) { /* ... */ }
}

registerModule(obj) is still available. It returns a copy of a plain object with its own methods tracked; trackObject is usually the better choice.

Auto-instrumentation

Track I/O without wrapping each call. Every instrument* call patches in place, is safe to call twice, and returns an uninstall function.

const { createTracePerf, instrumentFetch, instrumentHttp, instrumentMethods } = require('traceperf');
const tp = createTracePerf({ trackMemory: false });

instrumentFetch(tp);   // every fetch(): "fetch GET api.example.com/users"
instrumentHttp(tp);    // node http/https (axios, got, ...): "http GET api.example.com/users"

// Any client: patch its prototype, with per-call labels
class Db { async query(sql) { return []; } }
instrumentMethods(tp, Db.prototype, ['query'], {
  labelFor: (_method, [sql]) => `db ${String(sql).trim().split(/\s+/)[0]}`, // "db SELECT"
});

await tp.track(async () => { await new Db().query('SELECT * FROM users'); }, { label: 'handler' });

└─ handler: 0.112 ms
  └─ db SELECT: 0.031 ms

Ready-made helpers for popular clients. Pass the module you already use; traceperf doesn't depend on any of them:

const { instrumentPg, instrumentMysql2, instrumentMongo, instrumentRedis, instrumentAwsClient, prismaExtension } = require('traceperf');

instrumentPg(tp, require('pg'));                    // "pg SELECT users" (Pool.query included)
instrumentMysql2(tp, require('mysql2'));            // "mysql UPDATE orders" (callback and promise APIs)
instrumentMongo(tp, require('mongodb'));            // "mongo find users", "mongo insertOne users"
instrumentMongo(tp, require('mongoose').mongo);     // Mongoose uses its own copy of the driver
instrumentRedis(tp, require('ioredis'));            // "redis GET"
instrumentAwsClient(tp, s3Client);                  // "aws S3.GetObject"
const prisma = new PrismaClient().$extends(prismaExtension(tp)); // "prisma.User.findMany"

SQL labels are the verb plus the first table (sqlLabel()), never the values, so stats stay grouped. These are tested against the real client libraries, with Postgres and MongoDB running for real.

fetch timing ends when the response headers arrive. http timing ends when the body has been read. Responses with status ≥ 500 mark the call failed.

Nested tracking

Any tracked call made while another tracked call is running becomes its child. You don't need to pass anything through:

const { createTracePerf } = require('traceperf');
const traceperf = createTracePerf({ trackMemory: false });

async function fetchData() {
  return traceperf.track(async () => {
    await new Promise(r => setTimeout(r, 10));
    return [1, 2, 3];
  }, { label: 'fetchData' });
}

function transformData(data) {
  return traceperf.track(() => data.map(x => x * 2), { label: 'transformData' });
}

async function processData() {
  return traceperf.track(async () => transformData(await fetchData()), { label: 'processData' });
}

processData();

└─ processData: 11.912 ms
  ├─ fetchData: 11.217 ms
  └─ transformData: 0.075 ms

On Node.js this uses AsyncLocalStorage, so children started after an await are attached correctly and concurrent requests never get mixed up. In browsers, nesting follows the synchronous call stack: calls made after the first await of a tracked async function become separate trees.

Configuration

const { createTracePerf, TrackingMode } = require('traceperf');

const tp = createTracePerf({
  trackingMode: TrackingMode.BALANCED, // PERFORMANCE | BALANCED (default) | DETAILED | DEBUG
  silent: false,            // no console output (stats, hooks and exporters still run)
  trackMemory: true,        // heap delta per call
  enableNestedTracking: true,
  threshold: 0,             // only report calls taking at least this many ms
  sampleRate: 1,            // fraction of calls to track (0..1)
  enabled: true,            // false: tracked functions are plain pass-through calls
  blockingThreshold: 50,    // DETAILED: flag calls whose sync part runs >= 50 ms
  stats: true,              // per-label statistics ({ reservoirSize, maxLabels } to tune)
});

Every option except stats, plugins and reporter can also be passed per call to track/createTrackable as { label, silent, trackMemory, trackingMode, threshold, sampleRate, captureArgs, enableNestedTracking }. Anything you leave out follows the instance's current settings, so tp.setConfig(), tp.enableSilentMode() and tp.setSampleRate() also apply to functions wrapped earlier.

Options are strictly typed: { treshold: 50 } doesn't compile.

Tracking modes

Mode	Console output	Memory	Use for
PERFORMANCE	none	no	production: stats, hooks and exporters only
BALANCED (default)	tree with duration and heap delta	yes	everyday development
DETAILED / DEBUG	adds self time, sync time, event-loop blocking and error messages	yes	finding the actual bottleneck

DETAILED separates waiting from working:

const { createTracePerf, TrackingMode } = require('traceperf');
const tp = createTracePerf({ trackingMode: TrackingMode.DETAILED, trackMemory: false });
const sleep = ms => new Promise(r => setTimeout(r, ms));

const parseConfig = tp.createTrackable(() => {
  const start = Date.now();
  while (Date.now() - start < 60) {} // CPU-bound work
}, { label: 'parseConfig' });

await tp.track(async () => {
  parseConfig();
  await Promise.all([
    tp.track(() => sleep(30), { label: 'loadA' }),
    tp.track(() => sleep(30), { label: 'loadB' }),
  ]);
}, { label: 'startup' });

└─ startup: 93.880 ms (self 0.533 ms, sync 60.720 ms) ⚠ blocked event loop 60.720 ms
  ├─ parseConfig: 59.891 ms ⚠ blocked event loop 59.891 ms
  ├─ loadA: 29.756 ms (sync 0.200 ms)
  └─ loadB: 33.208 ms (sync 0.059 ms)

• self: time not covered by tracked children. Overlapping children (as with Promise.all) are counted once.
• sync: for async calls, the part that ran before the first await. That part blocks everything else.

Thresholds

const { createTracePerf } = require('traceperf');
const tracker = createTracePerf({ threshold: 50 }); // calls under 50 ms print nothing

tracker.track(slowFunction, { label: 'slowFunction' });        // reported if >= 50 ms
tracker.track(criticalFunction, { threshold: 10 });            // stricter for a hot path
tracker.track(backgroundTask, { threshold: 200 });             // looser for background work

└─ slowFunction: 60.950 ms 📊 +144.98 KB

Logging arguments

const { createTracePerf } = require('traceperf');
const tp = createTracePerf({ trackMemory: false });

const getUser = tp.createTrackable((id, opts) => ({ id }), { label: 'getUser', captureArgs: true });
getUser(123, { cache: true });

└─ getUser(123, {"cache":true}): 0.051 ms

Arguments are rendered safely and kept short. Circular, huge or exotic values never throw and never get walked in full.

Statistics

Every tracked call (including in PERFORMANCE mode and when silent) feeds per-label statistics with bounded memory:

const { createTracePerf } = require('traceperf');
const tp = createTracePerf({ silent: true });
const sleep = ms => new Promise(r => setTimeout(r, ms));

const query = tp.createTrackable(async (ms) => sleep(ms), { label: 'db.query' });
const handler = tp.createTrackable(async () => { await query(2); await query(3); }, { label: 'GET /users' });

for (let i = 0; i < 20; i++) await handler();

tp.printStats();
console.log(tp.getStats('db.query').p95); // ms

label       calls  errors  total ms   mean    p50    p95    p99    max  self mean
----------  -----  ------  --------  -----  -----  -----  -----  -----  ---------
GET /users     20       0     119.6  5.979  5.761  7.207  8.257  8.257      0.134
db.query       40       0     116.9  2.922  3.124  4.189  4.782  4.782      2.922

count, errors, total, min and max are exact. Percentiles come from a 512-sample reservoir per label. Past 1,000 distinct labels, new labels are folded into (other). resetStats() starts a new window.

Why was it slow? traceperf explain

explain() reads finished trees and says, in plain English, what probably made them slow:

const { createTracePerf, explain, formatFindings } = require('traceperf');
const roots = [];
const tp = createTracePerf({ silent: true, onComplete: r => roots.push(r) });
const sleep = ms => new Promise(r => setTimeout(r, ms));
const getUser = tp.createTrackable(() => sleep(20), { label: 'getUser' });
const getOrders = tp.createTrackable(() => sleep(30), { label: 'getOrders' });
const getPrice = tp.createTrackable(() => sleep(2), { label: 'pg SELECT prices' });

await tp.track(async () => {
  await getUser();
  await getOrders();
  for (let i = 0; i < 10; i++) await getPrice();
}, { label: 'GET /dashboard' });

console.log(formatFindings(explain(roots)));

⇉ GET /dashboard awaited 12 calls one after another (getUser, getOrders, pg SELECT prices, pg SELECT prices, ...: 78 ms); if they are independent, Promise.all could save up to ~47 ms
🔁 GET /dashboard called pg SELECT prices 10 times (27 ms total); batching them into one call may help (N+1 pattern)

It looks for:

• Blocked event loop: "req2 waited ~60 ms while cpu in req1, req3 blocked the event loop". Synchronous work in one request delays every other request, and it names the function that did it.
• Sequential awaits that could run in parallel, with the most time Promise.all could save.
• Repeated calls under one parent (N+1 queries).
• A dominant child, or time spent outside any tracked call.
• Errors, reported once, where they happened.

The same works on saved data: traceperf explain calls.jsonl (or a Chrome trace, or - for stdin), with --json, --max N, --blocking ms and --min ms. Findings are heuristics with their numbers, so check them against the tree before acting on them.

Getting data out

Perfetto / Chrome trace

const fs = require('fs');
const { createTracePerf } = require('traceperf');
const tp = createTracePerf({ silent: true });

const recording = tp.startTrace();
await tp.track(async () => {
  await Promise.all([1, 2, 3].map(i => tp.track(() => new Promise(r => setTimeout(r, 10 * i)), { label: `job ${i}` })));
}, { label: 'batch' });
fs.writeFileSync('trace.json', JSON.stringify(recording.stop()));

Open trace.json in ui.perfetto.dev, chrome://tracing, or the DevTools Performance panel. Concurrent async calls are put on separate tracks, so the flame chart stays valid.

OpenTelemetry

const otel = require('@opentelemetry/api');
const { createTracePerf, openTelemetryExporter } = require('traceperf');

const tp = createTracePerf({ silent: true, plugins: [openTelemetryExporter(otel)] });

Each finished tree becomes spans with the real start and end times, parent/child links, error status and traceperf.* attributes. Top-level spans are parented to the OpenTelemetry context that was active when the call started, so they join your existing request traces in Jaeger, Honeycomb, Datadog or Grafana. traceperf doesn't depend on @opentelemetry/api; you pass in the one your app uses.

Live mode (openTelemetryExporter(otel, { live: true })) opens each span when the call starts and makes it the active context while the call runs, across await. Spans created by your other instrumentation (pg, http, Redis) then appear inside traceperf's spans.

Prometheus

const { createTracePerf, prometheusExporter } = require('traceperf');

const prom = prometheusExporter();
const tp = createTracePerf({ silent: true, plugins: [prom] });
tp.track(() => JSON.stringify({ ok: true }), { label: 'serialize' });

// app.get('/metrics', prom.handler);
console.log(prom.metrics().split('\n').filter(l => l.includes('_count')).join('\n'));

traceperf_call_duration_seconds_count{name="serialize"} 1

This exposes a traceperf_call_duration_seconds histogram and a traceperf_call_errors_total counter per label. Use rootsOnly: true for request-level metrics only.

Slow-call alerts

const { createTracePerf } = require('traceperf');
const tp = createTracePerf({ silent: true });

tp.onSlow({ 'db.query': 50, '*': 500 }, (call, root) => {
  console.warn(`slow: ${call.label} took ${call.duration.toFixed(0)} ms (in ${root.label})`);
});

onSlow checks every call in each finished tree, nested ones included, in every mode.

Hooks, JSON lines and custom output

const { createTracePerf, jsonLinesExporter, formatTree } = require('traceperf');

const tp = createTracePerf({
  silent: true,
  plugins: [jsonLinesExporter(line => {})],          // one JSON document per finished tree
  onComplete: (root) => {                             // every finished top-level call
    if (root.duration > 100) console.warn(`slow: ${root.label}`);
  },
});

tp.use({ name: 'my-sink', onStart: (e) => {}, onComplete: (root) => {} }); // returns an unsubscribe function

// Replace console output entirely, e.g. to route it through your logger
const custom = createTracePerf({ reporter: (root) => console.log(formatTree(root)) });
custom.track(() => 1, { label: 'viaCustomReporter' });

Records (ExecutionRecord) contain label, duration, selfDuration, syncDuration, async, memoryDelta, error, args, startEpochMs and children, all in milliseconds. A plugin that throws is reported once and never breaks your code.

Frameworks

One tree per request, labelled by route pattern, with everything tracked during the request nested under it. Responses with status ≥ 500 and thrown errors mark the request as failed.

const { createTracePerf, expressMiddleware, fastifyPlugin, honoMiddleware, koaMiddleware, nestInterceptor, withTracing } = require('traceperf');
const tp = createTracePerf({ threshold: 100 }); // print only slow requests

app.use(expressMiddleware(tp));                      // Express / Connect
await fastify.register(fastifyPlugin(tp));           // Fastify
app.use('*', honoMiddleware(tp));                    // Hono
app.use(koaMiddleware(tp));                          // Koa
nestApp.useGlobalInterceptors(nestInterceptor(tp));  // NestJS
export const GET = withTracing(tp, async (req) => {  // Next.js route handlers, any fetch-style handler
  return Response.json(await loadUsers());
});

└─ GET /users/:id: 14.218 ms
  ├─ db.findUser: 6.021 ms
  └─ db.findPosts: 7.902 ms

Production

const { createTracePerf, TrackingMode } = require('traceperf');

const tp = createTracePerf(process.env.NODE_ENV === 'production'
  ? { trackingMode: TrackingMode.PERFORMANCE, sampleRate: 0.1 } // stats and exporters, no console
  : { trackingMode: TrackingMode.DETAILED });

tp.disable(); // kill switch: tracked functions become plain calls
tp.enable();

Overhead per call of a trivial function on Node 24 (npm run bench). CI fails the build if any of these regress past generous budgets. For a comparison with performance.now(), timerify and OpenTelemetry spans, see docs/benchmarks.md.

Scenario	Overhead
enabled: false / sampled out	~40–60 ns
PERFORMANCE mode without nesting	~0 (the function is returned as is)
tracked, no memory sampling, no nesting	~0.8 µs
tracked, no memory sampling, with nesting	~1.4 µs
tracked with memory sampling and nesting (default)	~3.5–4.5 µs

That's fine for request handlers, database calls and anything else that takes milliseconds. For tight loops called millions of times, track the loop rather than each iteration, or use sampleRate.

Safe settings, what each feature costs and how to roll traceperf out to a service are in docs/production.md.

Analyzing saved data: traceperf report

npx traceperf report trace.json          # from startTrace().stop()
npx traceperf report calls.jsonl --top 5 --sort p95   # from jsonLinesExporter()

120 calls, 3 labels (sorted by total)

label         calls  errors  total ms   mean    p50    p95    p99    max  self mean
...
Slowest 5 calls:
  95.212 ms  GET /users > db.query

Add --json for machine-readable output. A file of - reads stdin.

Comparing runs: traceperf compare

Record a JSON-lines log before and after a change (or on main and on a PR), then:

npx traceperf compare before.jsonl after.jsonl --metric p95 --threshold 10 --fail

label       before p95  after p95  change  status
----------  ----------  ---------  ------  ---------
db.query         4.741      8.795  +85.5%  regressed
render           1.886      1.966   +4.2%  unchanged

--fail exits with code 1 when anything regressed, so it can gate CI. Changes under --min-delta (0.5 ms by default) are ignored, so tiny functions don't fail builds on noise.

Reports and live view

npx traceperf report trace.json --html report.html   # one self-contained file: flame chart, stats, slowest calls
npx traceperf watch calls.jsonl                      # live top-style view while the app writes the log

Allocation profiling

The 📊 heap delta is a hint. For real numbers, profile the allocations of a function:

const { profileAllocations } = require('traceperf');

const { allocatedBytes, sites } = await profileAllocations(() => {
  const rows = [];
  for (let i = 0; i < 10000; i++) rows.push({ id: i, label: `row ${i}` });
  return rows.length;
});
console.log(allocatedBytes > 0, typeof sites[0].functionName); // allocated bytes and the top allocation sites

true string

This uses V8's sampling heap profiler through node:inspector and counts objects that were collected again (on Node 20+, see countsCollected).

CPU hotspots for slow calls

Tracking shows which call was slow; profileSlowCalls shows which lines were busy during it, including code you never wrapped. It runs the V8 sampling profiler and, for every top-level call over the threshold, reports the functions that used the CPU:

const { createTracePerf, profileSlowCalls, formatHotspots } = require('traceperf');
const tp = createTracePerf();
tp.use(profileSlowCalls({
  threshold: 200,                                      // ms
  onProfile: (call, profile) => console.log(`${call.label}\n${formatHotspots(profile)}`),
}));

POST /import: 438 ms
    315.0 ms  parseRows  /app/src/csv.js:18
    121.1 ms  hashRow  /app/src/hash.js:4
  cpu 436.0 ms, idle 0.0 ms, gc 2.4 ms

idle is time spent waiting for I/O or timers, so a slow call with a high idle time is waiting on something rather than computing. Results arrive when the profiler window closes (window, default 2 s; flush() forces it). Samples are process-wide, so concurrent work during the call shows up too. Sampling costs a few percent of CPU: turn it on while investigating, or on some instances only. Node.js only; profiler.available is false elsewhere.

Event-loop delay

const { monitorEventLoop } = require('traceperf');

const loop = monitorEventLoop();
setTimeout(() => {
  console.log(loop.stats()); // { min, max, mean, stddev, p50, p90, p99 } in ms
  loop.stop();
}, 100);

A tracked call tells you how long it took. Event-loop delay tells you whether something blocked everything else. This is Node.js only; elsewhere supported is false.

Browser

import { createTracePerf } from 'traceperf/browser';

const tracker = createTracePerf({ threshold: 16 }); // only calls longer than a frame

tracker.track(() => {
  document.getElementById('output').textContent = 'Updated';
}, { label: 'updateDOM' });

The browser entry shares the core with Node.js, so options, nesting, statistics, hooks and exporters all work the same. It adds info/warn/error/debug/group logging methods and prints in a browser-friendly format:

Execution Flow:
→ updateDOM (21.14ms)

Without a bundler, use the script-tag build (dist/traceperf.browser.min.js, about 9 KB gzipped), which defines a global TracePerf:

<script src="https://unpkg.com/traceperf/dist/traceperf.browser.min.js"></script>
<script>
  const { createTracePerf, TrackingMode } = TracePerf;
  const browserTracker = createTracePerf({ trackingMode: TrackingMode.BALANCED });

  browserTracker.track(() => {
    document.getElementById('output').textContent = 'Updated';
  }, { label: 'domOperation' });
</script>

Memory deltas are shown only where the browser exposes performance.memory (Chromium).

In Chrome DevTools

import { createTracePerf, performanceTimeline } from 'traceperf/browser';

const tp = createTracePerf({ silent: true, plugins: [performanceTimeline()] });
tp.track(() => [3, 1, 2].sort(), { label: 'sortItems' });

Every tracked call becomes a performance.measure() entry. In Chrome 129+ these appear on a traceperf track in the Performance panel, with failed calls in red and duration, self and sync time in the tooltip. Elsewhere (including Node.js) they appear as User Timing entries. Entries are cleared after emitting, so long-running pages don't accumulate them; DevTools recordings still capture them.

Edge runtimes

Cloudflare Workers and Vercel Edge work as-is. Nesting across await uses AsyncLocalStorage, which traceperf detects automatically where it is a global (Next.js Edge, Workers with nodejs_compat). Anywhere else, pass it in:

import { AsyncLocalStorage } from 'node:async_hooks';
const tp = createTracePerf({ asyncLocalStorage: AsyncLocalStorage });

Workers deliberately coarsen timers (time only advances on I/O), so durations there are only approximate.

How it compares

	Call tree	Async-aware nesting	Per-label stats	Production use	Export	Setup
console.time	no	no	no	no	no	none
performance.timerify (Node)	no	no	via observer	yes	no	some
clinic.js / 0x	sampled flame graph	n/a	no	no (offline profiling)	own UI	run app under the tool
OpenTelemetry SDK	yes (spans)	yes	via backend	yes	everywhere	SDK + exporter + collector
traceperf	yes	yes (Node)	yes	yes (sampling, kill switch)	Perfetto, OTel, Prometheus, JSON	none (--require traceperf/auto) or one import

Use a sampling profiler (clinic.js, --cpu-prof) when you don't know where to look, or let profileSlowCalls run one only around slow calls. Use traceperf when you want to measure specific functions, requests or spans continuously, in development and in production. If you already run OpenTelemetry, traceperf can feed it. A longer, honest comparison is in docs/comparison.md.

Compatibility

• Node.js 14+ (tested on 14, 16, 18, 20, 22 and 24), Bun, Deno, Cloudflare Workers (workerd) and Vercel Edge (edge-runtime). Nesting across await works on all of them. Also tested in current Chromium, Firefox and WebKit.
• Module formats: CommonJS (require), ESM (import), TypeScript (node10, node16/nodenext and bundler resolution), bundlers (esbuild, webpack, Vite, ...), and <script>.
• Runtime dependencies: none.

Limitations

• In browsers (and ESM on Node < 20.16), nesting doesn't survive await, because there is no AsyncLocalStorage. On those Node versions, starting with --import traceperf/auto fixes it.
• The heap delta is the V8 heap difference across the call. Other concurrent work and GC affect it, so treat it as a hint, not an allocation profile.
• traceperf/auto tracks Node's http/https servers (which Express, Fastify, Koa, Nest and most frameworks use), not http2 or uWebSockets.js servers. AWS SDK clients still need instrumentAwsClient(tp, client).
• explain() only sees tracked calls: it can say a request was delayed by blocking code, but names the function only if that function is tracked. profileSlowCalls finds untracked hot code.
• In the default OpenTelemetry mode, spans are created when the top-level call finishes. Use { live: true } if other instrumentation's spans should nest inside traceperf's.
• fetch instrumentation times the request until the response headers arrive, not the time spent reading the body.

Examples

• examples/slow-apps/: five small apps, one planted performance problem each; try npx traceperf run examples/slow-apps/n-plus-one/server.js --path /orders.
• examples/playground/: an Express app with a hidden N+1 query problem.
• examples/optimized-tracking-example.js: the basics.
• examples/stats-and-export.js: statistics, spans, a Perfetto trace and JSON lines in a production-style setup.
• examples/browser-example.html / .js: browser usage.
• examples/test/edge-case-tests.js: deep async nesting, memory churn, sampling, error propagation.
• examples/test/final-benchmark.js: overhead by tracking mode.

Contributing

npm ci && npm run build:all && npm test && npm run lint covers the basics. CI also runs:

• npm run verify:package: installs the packed tarball into fresh CJS, ESM, TypeScript and bundler projects.
• npm run verify:docs: runs every example in this README and checks its output.
• npm run test:browser: Playwright tests in Chromium, Firefox and WebKit.
• npm run bench:check: overhead and bundle-size budgets.
• npm run test:runtimes: smoke tests on Bun and Deno.
• npm run docs:api: the API reference (TypeDoc).

What's planned is in ROADMAP.md.

See CONTRIBUTING.md.

License

MIT