Benchmarking Types with @ark/attest

Measure TypeScript type instantiations to optimize type-level performance.

Critical Concepts

Baseline Expression

Include a "baseline expression" at the top of benchmark files to exclude API setup overhead.

• The baseline warms up the type evaluation machinery and caches common type computations
• CRITICAL: The baseline expression must be different from any benchmark expression
• If the baseline is identical to a benchmark, that benchmark will reuse cached types → 0 instantiations (false result)

Type Caching

TypeScript caches type evaluations per exact type:

• Simplify.All<Map<1, 2>> and Simplify.All<Map<3, 4>> are different → no caching benefit
• If baseline uses Simplify.All<Map<0, 0>> and benchmark uses Simplify.All<Map<0, 0>> → cached (low instantiations)
• Use similar but distinct types in baseline vs benchmarks

Instantiation Costs

• Each benchmark measures instantiations for that specific expression in isolation
• Costs are not cumulative - every expression re-evaluates from scratch (except cached types)
• Complex pattern matches (e.g., Map<K, V> extends Map<infer K2, infer V2>) can be expensive (~1800+ inst)

Example

import { bench } from '@ark/attest'
import { type } from 'arktype'

// Baseline expression - similar to benchmarks but not identical
type('boolean')

bench('single-quoted', () => {
  const _ = type("'nineteen characters'")
  // Would be 2697 without baseline, now 610
}).types([610, 'instantiations'])

bench('keyword', () => {
  const _ = type('string')
  // Would be 2507 without baseline, now 356
}).types([356, 'instantiations'])

Best Practices

• Always include a baseline that exercises your API but with different inputs than benchmarks
• Use .bench-d.ts suffix for type benchmark files (convention)
• Benchmark files are type-only - no runtime execution
• Update baseline values when intentionally changing performance characteristics
• Beware: Complex built-in types (Map, Set) have inherent pattern-matching costs in TypeScript

Resources

• arktype attest documentation