vitest-dev/vitest · error · SyntaxError

`bench.compare()` expects every argument to be the return…

Error message

`bench.compare()` expects every argument to be the return value of `bench` or `bench.from`.

What it means

Every argument to `bench.compare(...)` (except an optional trailing options object) must be the return value of `bench()` or `bench.from()` — objects carrying the internal `kRegistration` symbol. This `SyntaxError` fires when any argument is null, a primitive, or a plain object that is not a registration.

Solutions

1. Pass only values returned by `bench(...)` or `bench.from(...)`.
2. Filter out falsy values before spreading: `bench.compare(...regs.filter(Boolean))`.
3. If you intended to compare against a baseline, wrap it: `bench.from('name', () => baseline)` then pass the result.

Example fix

// before
bench.compare(regA, baselineData)

// after
const regB = bench.from('baseline', () => baselineData)
bench.compare(regA, regB)

if (!args.every(a => a != null && typeof a === 'object' && kRegistration in a)) throw new Error('all args must be bench()/bench.from() results')

function isRegistration(v: unknown): v is BenchRegistration<any> { return v != null && typeof v === 'object' && typeof (v as any)?.[kRegistration] === 'boolean' }

null

When it happens

Trigger: Calling `bench.compare(reg, 'not-a-reg')`, `bench.compare(reg, null)`, `bench.compare(reg, { name: 'x' })` (plain object without the symbol), or `bench.compare(reg, undefined)`. The loop at benchmark.ts:477-481 checks each arg for `kRegistration in reg`.

Common situations: Passing a benchmark function instead of its registration; passing the result of `.run()` (a promise/result) instead of the registration; conditional spreads that include `undefined`; passing a baseline data object instead of a `bench.from()` registration.

Related errors

• `bench.compare()` requires at least 2 benchmarks, received
• task " " was not defined
• `bench()` expects a benchmark function. Call `bench(name…
• `bench.from()` expects a string path or a function…
• `bench.from()` requires a name (string or named function)…

AI-assisted analysis of vitest-dev/vitest@1fa9837ec2 (2026-08-11). Data as JSON: /api/errors/250a7657136d639c. Report an issue: GitHub.

Appendix: source

Thrown at packages/vitest/src/runtime/benchmark.ts:479

    const isOptions = lastArg != null && typeof lastArg === 'object' && !(kRegistration in lastArg)
    const benchOptions = isOptions ? args.pop() as BenchRunOptions : undefined
    const registrations = args as BenchRegistration<any>[]

    // Mark every passed-in registration as consumed before validation so a
    // throwing `bench.compare()` (wrong arity, wrong shape) doesn't also
    // trigger the unrun-bench warning — the user's intent was to consume them.
    for (const reg of registrations) {
      if (reg != null && typeof reg === 'object' && kRegistration in reg) {
        pending.delete(reg)
      }
    }

    if (registrations.length < 2) {
      throw new SyntaxError(`\`bench.compare()\` requires at least 2 benchmarks, received ${registrations.length} instead. ${registrations.length === 1 ? 'Consider calling `bench().run()`. ' : 'Define benchmarks by calling `bench()`. '}See https://vitest.dev/guide/benchmarking#comparing-benchmarks`)
    }
    for (const reg of registrations) {
      if (reg == null || typeof reg !== 'object' || !(kRegistration in reg)) {
        throw new SyntaxError('`bench.compare()` expects every argument to be the return value of `bench` or `bench.from`.')
      }
    }

    const runnable: RunnableRegistration<any>[] = []
    const fromEntries: FromRegistration<any>[] = []
    for (const reg of registrations) {
      if (isFromRegistration(reg)) {
        fromEntries.push(reg)
      }
      else {
        runnable.push(reg as RunnableRegistration<any>)
      }
    }

    const taskMeta = new Map<string, TaskMeta>()
    for (const reg of runnable) {
      if (reg[kPerProject]) {
        taskMeta.set(reg.name, { perProject: true })

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