Configuration

as-bench.config.json is auto-discovered in the current directory; point elsewhere with --config <path>. A JSON schema ships at node_modules/as-bench/as-bench.config.schema.json for editor autocomplete.

{
  "$schema": "./node_modules/as-bench/as-bench.config.schema.json",
  "input": ["assembly/__benches__/**/*.ts"],
  "outDir": ".as-bench/build",
  "baselineDir": ".as-bench/baselines",
  "runtime": "node",
  "verbose": false,
  "deterministic": false,
  "settings": {
    "warmupTime": 3000,
    "measurementTime": 3000,
    "sampleSize": 0,
    "numResamples": 100000
  },
  "render": { "significanceLevel": 0.05, "noiseThreshold": 0.01 },
  "buildOptions": { "optimize": true, "debug": false, "args": [] },
  "profile": { "top": 10, "all": false, "iters": 10, "minInstrs": 4 },
  "modes": {
    "quick": { "settings": { "warmupTime": 250, "measurementTime": 500, "numResamples": 20000 } }
  }
}

Precedence: defaults < config < mode < CLI flags. See Modes.

Top-level keys

Key	Type	Description
input	string[]	Glob(s) for bench files (default assembly/__benches__/**/*.ts).
outDir	string	Build output directory.
baselineDir	string	Where --save-baseline writes and --baseline / compare read.
runtime	string | string[]	Shorthand runtime(s); see runOptions for full control.
verbose	boolean	Print all estimates by default.
deterministic	boolean	Enable deterministic mode.
settings	object	Engine tunables (below).
render	object	Verdict thresholds (below).
buildOptions	object	Compiler options (below).
profile	object	Profiler defaults (below).
runOptions	object	Runtime command(s) (below).
modes	object	Named overlays applied with --mode.

settings

Engine tunables — every field maps to a Settings property and a run CLI flag.

Field	Default	Bound
warmupTime	3000	≥ 0
warmupMinTime	100	≥ 0
warmupTolerance	0.02	≥ 0
measurementTime	3000	≥ 1
sampleSize	0 (auto)	≥ 10 when set
numResamples	100000	≥ 1
samplingMode	auto	auto | linear | flat
confidenceLevel	0.95	(0, 1)

sampleSize: 0 auto-sizes from warmup to ~10 ms/sample, clamped to [10, 500]. See Statistics & Output.

render

Field	Default	Meaning
significanceLevel	0.05	p-value below which a change counts as significant.
noiseThreshold	0.01	Changes whose CI lies within ±this read as "no change".

buildOptions

Field	Default	Meaning
optimize	true	Compile with optimizations.
debug	false	Keep debug info / the name section.
args	[]	Extra asc arguments (e.g. ["--transform", "json-as/transform"]).

profile

Field	Default	Meaning
top	10	Rows per bench.
all	false	Include internal rows.
iters	10	Iterations per bench (--time).
minInstrs	4	Skip wrapping functions under this many static instructions (--time).

runOptions

For more than the top-level runtime shorthand, use runOptions.runtime with a {cmd, name} object or a list:

{
  "runOptions": {
    "runtime": [
      "node",
      "wasmtime",
      { "cmd": "wazero run <env:-env> <file>", "name": "wazero" }
    ]
  }
}

• <file> → the bench wasm path (appended last if omitted).
• <env:PREFIX> → expands AS_BENCH_TUNE_* settings as runtime env flags; a trailing = fuses (<env:--env=> → --env=K=V).

A list runs every bench under each runtime, rendered as a comparison table.

Modes

modes holds named partial configs applied with --mode <name>. Objects merge one level deep; scalars and arrays replace. A missing mode name errors with the list of available modes.

{
  "modes": {
    "full": { "settings": { "warmupTime": 3000, "measurementTime": 5000 } },
    "wasmtime": { "runtime": "wasmtime" }
  }
}