Benchmarking (v5)

In v5 the benchmark API was rewritten: bench is no longer a top-level import. It is a test-context fixture used inside a regular test(), available only in files matched by benchmark.include (default **/*.{bench,benchmark}.?(c|m)[jt]s?(x)). Benchmarks are powered by Tinybench.

Defining & Running

import { expect, test } from 'vitest'

test('parse performance', async ({ bench }) => {
  // bench() registers; .run() executes and returns the result
  const result = await bench('parse', () => {
    const data = JSON.parse('{"key":"value"}')
    use(data) // consume the result — engines may eliminate dead code
  }).run()

  expect(result.throughput.mean).toBeGreaterThan(10_000)
})

Run benchmarks:

vitest bench           # only benchmarks (implicitly enables them)
vitest bench parser    # filter by filename
vitest bench -t JSON   # filter by test name

Set benchmark: { enabled: true } to run them alongside regular tests in a separate isolated group.

Comparing Implementations

test('compare parsers', async ({ bench }) => {
  const result = await bench.compare(
    bench('JSON.parse', () => { JSON.parse(input) }),
    bench('custom', { beforeEach: () => reset() }, () => { customParse(input) }),
    { iterations: 100, time: 1000 }, // shared Tinybench options (last arg)
  )

  // Assertion matchers (delta avoids flaky failures)
  expect(result.get('JSON.parse')).toBeFasterThan(result.get('custom'), { delta: 0.1 })
  expect(result.get('custom')).toBeSlowerThan(result.get('JSON.parse'))
})

bench.compare interleaves iterations to reduce environmental bias and prints a comparison table after the test.

Storing & Replaying Baselines

test('compare against baseline', async ({ bench }) => {
  await bench.compare(
    bench('current', { writeResult: './benchmarks/parse.json' }, () => parse(input)),
    bench.from('previous', './benchmarks/parse.json'),       // reads a stored result, no run
    bench.from('remote', () => fetch(url).then(r => r.json())),
  )
})

• writeResult overwrites the JSON file on every successful run (no skip-when-cached).
• bench.from(name, source) reads a stored result without invoking any function.
• For multi-project workspaces, pass { perProject: true } and use ${projectName} in writeResult paths to collect a cross-project comparison table.

Stability Notes

• Benchmark files run sequentially and never in parallel; retry and the delta option reduce flakiness.
• Consume the result inside the bench fn — JS engines eliminate side-effect-free code.
• In Node mode every imported binding goes through Vite's module-runner getter; store hot references locally (const _parse = parse), benchmark the built package, or disable experimental.viteModuleRunner for the bench project.

v5 Migration

• bench top-level import → ({ bench }) from the test context
• bench.skip/only/todo removed → use test.skip/only/todo on the surrounding test
• benchmark.reporters/outputFile/compare/outputJson and --compare/--outputJson removed → use --reporter=json --outputFile (JSON now has a benchmarks field)

Key Points

• Benchmarks live in *.bench.ts files and run inside test() via { bench }
• Use bench.compare + toBeFasterThan/toBeSlowerThan (with delta) for relative perf
• Persist baselines with writeResult and replay with bench.from

<!-- Source references:

• https://vitest.dev/guide/benchmarking
• https://vitest.dev/guide/test-context#bench

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