Node.js v27.0.0 documentation

Benchmark runner#

The node:bench module supports defining and running JavaScript benchmarks in the current process. To access it:

import { bench, suite } from 'node:bench';
const { bench, suite } = require('node:bench');
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This module is only available under the node: scheme.

Example benchmark#

Save the following as benchmark.mjs:

import { bench, suite } from 'node:bench';

suite('URL', () => {
  const input = 'https://example.com/a?b=c';

  bench('construct', {
    samples: 30,
    params: { input: 'short' },
  }, (b) => {
    const operations = 10_000;

    b.start();
    for (let i = 0; i < operations; i++) {
      new URL(input);
    }
    b.end(operations);
  });
});
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Run the benchmark from the command line:

node --bench benchmark.mjs
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Benchmarks are executed serially in declaration order. Declared benchmarks are scheduled automatically. Call run() during the same turn as the declarations to consume the event stream or configure filtering. If an automatically scheduled run fails and run() was not called, the process exit code is set to 1.

Measurement model#

Each warmup and measured sample invokes the benchmark function once with a fresh {BenchContext}. The function must either call context.start() and context.end(operations) exactly once, or call context.record(sample) exactly once to provide an externally measured sample. Setup before start() and cleanup after end() are outside the measured region. Promise-returning functions are awaited.

By default, an event loop turn occurs between sample invocations. An embedded runner can disable this using yieldBetweenSamples. The runner executes benchmarks serially, but it does not provide process isolation. Other work in the process, JIT compilation, garbage collection, CPU frequency changes, and system load can all affect results. Keep raw samples when comparing results and investigate noisy or skewed distributions rather than treating a confidence interval as a pass/fail threshold.

Calling context.done() during a measured sample completes the benchmark after that sample. This allows a higher-level tool to treat samples as a maximum and implement a dynamic sampling policy.

Reusable runners#

The module-level declaration functions use a shared runner and schedule it automatically. Higher-level tools can create isolated, explicitly started runners instead:

import { createRunner } from 'node:bench';

const runner = createRunner({ yieldBetweenSamples: false });

runner.bench('example', { samples: 100 }, (b) => {
  const operations = chooseOperationCount();
  b.start();
  runOperations(operations);
  const sample = b.end(operations);

  if (hasEnoughData(sample)) b.done();
});

for await (const record of runner.run()) {
  // Consume structured benchmark records.
}
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Each runner has independent declarations, hooks, filtering, and output. Unlike the module-level declarations, creating a benchmark on an explicit runner does not schedule execution. This allows packages to collect declarations and start them later. Calling the explicit runner's run() function prevents additional declarations and a second call to run() is an error.

Command-line runner#

The --bench flag runs one or more explicit benchmark files or glob patterns:

node --bench benchmark.mjs
node --bench --bench-reporter=json 'benchmarks/**/*.js'
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Files are sorted and executed serially. The default --bench-isolation=process mode runs each file in a separate child process and emits one aggregate summary. Structured events are transferred to the parent without JSON conversion, preserving BigInt durations, errors, and parameter values. Child writes to stdout and stderr are emitted as diagnostic records so they do not corrupt reporter output.

--bench-isolation=none imports all files into the runner process. This mode has lower startup overhead, but module, heap, and process state carry between files, and user writes share stdout and stderr with reporters.

Benchmark files passed to --bench should declare benchmarks but must not call run(). The CLI supports --bench-name-pattern, --bench-samples, --bench-warmup, --bench-reporter, and --bench-reporter-destination. See the command-line options documentation for details.

Benchmark reporters#

The built-in reporters are available from the scheme-only node:bench/reporters module:

import { json, spec } from 'node:bench/reporters';
const { json, spec } = require('node:bench/reporters');
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Reporter values can be passed directly to stream.compose():

import { bench, run } from 'node:bench';
import { spec } from 'node:bench/reporters';
import process from 'node:process';

bench('example', (b) => {
  b.start();
  doWork();
  b.end(1);
});

run().compose(spec).pipe(process.stdout);
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The spec reporter buffers results and outputs a concise table containing the sample count, mean rate, 95% confidence interval for the mean, median rate, and warnings. A coefficient of variation above 5% is reported as noisy, and an absolute skewness above 1 is reported as skewed. The exact human-readable format is subject to change.

The json reporter emits every lifecycle record as newline-delimited JSON. BigInt values, including duration_ns, are encoded as decimal strings. Errors are represented using their name, message, stack, code, cause, and errors properties. As required by JSON, non-finite numbers are encoded as null.

Custom reporters use the same composition contract. They can be transforms or functions accepted by stream.compose(). The composed readable can be piped to any writable destination:

import { run } from 'node:bench';
import process from 'node:process';

async function* names(source) {
  for await (const { type, data } of source) {
    if (type === 'bench:complete') {
      yield `${data.name}\n`;
    }
  }
}

run().compose(names).pipe(process.stdout);
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createRunner([options])#

• options <Object>
  • yieldBetweenSamples <boolean> Schedule an event loop turn between sample callbacks. Disabling this also prevents timer-based abort signals from firing between synchronous callbacks. Benchmark timeouts continue to be checked against a monotonic deadline. Default: true.
• Returns: <Object> An isolated benchmark runner with bound after, afterEach, before, beforeEach, bench, describe, run, and suite functions.

Creates an explicitly started benchmark runner. Declarations made through one runner do not interact with declarations made through another runner or through the module-level functions. Call the returned run() function to start the runner and obtain its {BenchmarksStream}.

Each runner can be started once. Its run() function accepts the same options as the module-level run(). run({ yieldBetweenSamples }) overrides the value passed to createRunner().

bench([name][, options], fn)#

• name <string> The benchmark name. Default: The name property of fn, or '<anonymous>' when fn has no name.
• options <Object>
  • only <boolean> When any benchmark or containing suite has only set, benchmarks without only in their hierarchy are skipped. Default: false.
  • params <Object> String, finite number, or boolean metadata identifying this benchmark configuration. Parameter keys are sorted when constructing the stable benchmark identity. Default: An empty object.
  • samples <number> The maximum number of measured callback invocations. Must be a positive 32-bit unsigned integer. The benchmark may finish earlier by calling context.done(). Default: 30.
  • signal <AbortSignal> Allows aborting this benchmark.
  • skip <boolean> | <string> If truthy, the benchmark is skipped. A string is included in the result as the skip reason. Default: false.
  • tags <string>[] Labels associated with the benchmark. Tags are lowercased, deduplicated, and inherited from containing suites by union. Default: [].
  • timeout <number> The number of milliseconds after which the benchmark fails. Default: Infinity.
  • warmup <number> The number of unreported callback invocations before measured samples. Must be a 32-bit unsigned integer. Default: 0.
• fn <Function> | <AsyncFunction> The benchmark function. It receives a {BenchContext}.
• Returns: <Promise> Fulfilled with the benchmark result after a top-level benchmark finishes, or with undefined immediately when declared in a suite.

Warmup invocations use the same callback and timing contract as measured samples, but their samples are discarded. An exception, rejection, timeout, abort, missing timing call, or duplicate timing call stops the current benchmark. Later benchmarks continue to run.

A timeout or abort cannot interrupt synchronous JavaScript and does not forcibly cancel asynchronous work that ignores context.signal.

The stable benchId is based on the source file, hierarchical suite and benchmark names, and canonicalized parameters. Declaring the same identity more than once reports an error rather than merging the samples.

bench.skip([name][, options], fn)#

Shorthand for bench(name, { ...options, skip: true }, fn).

bench.only([name][, options], fn)#

Shorthand for bench(name, { ...options, only: true }, fn).

suite([name][, options], fn)#

• name <string> The suite name. Default: The name property of fn, or '<anonymous>' when fn has no name.
• options <Object>
  • only <boolean> Selects all benchmarks nested in this suite. Default: false.
  • skip <boolean> | <string> Skips all benchmarks nested in this suite. Default: false.
  • tags <string>[] Labels inherited by nested suites and benchmarks. Default: [].
• fn <Function> | <AsyncFunction> A function that declares nested suites, benchmarks, and hooks.
• Returns: <Promise> Fulfilled when a top-level suite finishes, or with undefined immediately when declared in another suite.

Suite functions run while declarations are collected. Promise-returning suite functions are awaited before benchmark execution begins.

describe([name][, options], fn)#

Alias for suite().

before(fn)#

• fn <Function> | <AsyncFunction> The hook function.

Registers a hook that runs once before the benchmarks in the current suite.

after(fn)#

• fn <Function> | <AsyncFunction> The hook function.

Registers a hook that runs once after the benchmarks in the current suite.

beforeEach(fn)#

• fn <Function> | <AsyncFunction> The hook function. It receives an object with the benchmark's name, params, and signal.

Registers a hook that runs once before each complete logical benchmark in the current suite. It does not run before every sample. Per-sample setup belongs in the benchmark function before context.start() or context.record().

afterEach(fn)#

• fn <Function> | <AsyncFunction> The hook function. It receives an object with the benchmark's name, params, and signal.

Registers a hook that runs once after each complete logical benchmark in the current suite. It does not run after every sample. Per-sample cleanup belongs in the benchmark function after context.end() or context.record().

run([options])#

• options <Object>
  • namePattern <string> | <RegExp> Only runs benchmarks whose full hierarchical name matches the pattern. String values are interpreted as JavaScript regular expressions.
  • samples <number> Overrides the maximum number of measured callback invocations for every benchmark. Must be a positive 32-bit unsigned integer.
  • signal <AbortSignal> Allows aborting in-progress benchmark execution.
  • warmup <number> Overrides the number of unreported warmup callback invocations for every benchmark. Must be a 32-bit unsigned integer.
  • yieldBetweenSamples <boolean> Schedule an event loop turn between sample callbacks. Default: true, or the value passed to createRunner() for an explicit runner.
• Returns: {BenchmarksStream}

Returns the object-mode event stream for the in-process benchmark run. Call run() during the same turn in which benchmarks are declared, before automatic execution begins. Calling run() is optional when the returned stream is not needed. An explicit runner created by createRunner() does not run automatically, so its run() function may be called later.

import { bench, run } from 'node:bench';

bench('example', { samples: 3 }, (b) => {
  b.start();
  doWork();
  b.end(1);
});

for await (const { type, data } of run()) {
  if (type === 'bench:complete' && data.error === undefined) {
    console.log(data.name, data.summary.mean);
  }
}
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Class: BenchContext#

An instance of BenchContext is passed to every benchmark invocation. A new instance is created for every warmup and measured sample.

context.index#

• <number>

The zero-based invocation index within the current context.phase. Warmup and measured samples have separate index sequences.

context.name#

• <string>

The benchmark name.

context.params#

• <Object>

The benchmark's canonicalized parameter metadata.

context.phase#

• <string>

The current sample phase. It is 'warmup' for an unreported warmup invocation and 'measurement' for a measured invocation.

context.signal#

• <AbortSignal>

An abort signal that is triggered when the benchmark is aborted, times out, or finishes.

context.start()#

Starts the measured region using process.hrtime.bigint(). Calling start() more than once is an error.

context.end(operations[, options])#

• operations <number> The number of completed operations. Must be a positive safe integer.
• options <Object>
  • detail <any> Additional structured-cloneable sample data. With CLI process isolation, it must also be supported by advanced child process serialization.
• Returns: <Object> The sample's operations, duration_ns, computed rate, and optional cloned detail.

Ends the measured region. The end timestamp is captured before operations is validated. Calling end() before start(), calling it more than once, or recording a zero-duration sample is an error. When provided, detail is cloned after the end timestamp is captured, so cloning time is outside the measured region.

context.record(sample)#

• sample <Object>
  • operations <number> The number of completed operations. Must be a positive safe integer.
  • duration_ns <bigint> An externally measured positive duration in nanoseconds no greater than Number.MAX_SAFE_INTEGER.
  • detail <any> Additional structured-cloneable sample data. With CLI process isolation, it must also be supported by advanced child process serialization.
• Returns: <Object> The normalized sample, including its computed rate and optional cloned detail.

Records a measurement made by another clock or execution environment. This is useful when a higher-level tool measures work in a worker and needs to exclude message transport from the duration. record() is mutually exclusive with start() and end() within one callback and must be called exactly once.

context.done()#

Requests successful benchmark completion after the current measured sample. The callback must still call either start() and end(), or record(). Calling done() during a warmup invocation is an error. The configured samples value remains the maximum number of measured invocations if done() is not called.

Class: BenchmarksStream#

BenchmarksStream is an object-mode <stream.Readable>. Each lifecycle record is both emitted as a named event and made available on the stream as { type, data }.

The events are emitted in execution order:

• 'bench:start'
• 'bench:sample'
• 'bench:complete'
• 'bench:diagnostic'
• 'bench:summary'

Every benchmark-scoped event contains benchId and parentId. 'bench:complete' data contains a benchmark result. A failed result has an additional error property and may contain samples recorded before the error. A skipped result has an additional skip property and an empty samples array. 'bench:diagnostic' reports suite and hook errors. 'bench:summary' contains overall success, counts, duration_ns, and file properties. The file is <string> | <null>; it is null when the summary aggregates multiple files.

Sample result#

Each measured sample has the following properties:

• operations <number> The positive operation count passed to context.end() or context.record().
• duration_ns <bigint> The measured duration in nanoseconds.
• rate <number> Operations per second.
• detail <any> The optional cloned sample detail.

Benchmark result#

A completed benchmark result contains:

• benchId <string> The stable benchmark identity.
• parentId <string> | <null> The stable containing suite identity.
• name <string> The benchmark name.
• file <string> The source file.
• line <number> The source line.
• column <number> The source column.
• tags <string>[] The inherited canonical tags.
• params <Object> The canonical parameter metadata.
• samples <Object>[] The exact measured samples.
• summary <Object>
  • mean <number> The arithmetic mean of per-sample rates.
  • median <number> The median per-sample rate.
  • min <number> The minimum per-sample rate.
  • max <number> The maximum per-sample rate.
  • stddev <number> The population standard deviation of rates.
  • coefficientOfVariation <number> stddev / mean.
  • confidenceInterval <Object> The 95% Student's t confidence interval for the mean rate, with lower and upper properties.
  • medianConfidenceInterval <Object> The 95% nonparametric confidence interval for the median rate, with lower and upper properties.
  • skewness <number> The skewness of the scaled rate histogram.