flutter_qjs_next

Flutter / Dart bindings for QuickJS via dart:ffi.

Embedded QuickJS 2026-06-04
Platforms: Android, iOS, macOS, Linux, Windows (no Web — native FFI only)
API style compatible with flutter_js (JavascriptRuntime, getJavascriptRuntime())

Install

flutter pub add flutter_qjs_next

import 'package:flutter_qjs_next/flutter_qjs.dart';

Quick start

import 'package:flutter_qjs_next/flutter_qjs.dart';

void main() {
  final js = getJavascriptRuntime(
    timeout: 5000, // recommended for untrusted scripts; null/0 = off
    // memoryLimit defaults to 64 MiB; use 0 for unlimited
  );

  final r = js.evaluate('Math.trunc(Math.random() * 100).toString()');
  print(r.stringResult);

  // Promise / setTimeout need the host event loop:
  // either call js.dispatch() after async work, or use evaluateAsync + handlePromises.
  js.dispose(); // always dispose — free native heap + channel maps
}

Limits

Parameter	Meaning	Default
`stackSize`	JS stack size in bytes	1 MiB
`timeout`	Interrupt after this many ms of wall-clock JS work (`null`/`0` = off)	off
`memoryLimit`	Heap limit in bytes (`0` = unlimited)	64 MiB

Each getJavascriptRuntime() is a separate QuickJS engine (own heap, channels, ReceivePort). Create many only when you need isolation; otherwise reuse one runtime or use a pool.

Multi-engine pool

final pool = JsEnginePool(
  maxSize: 4,
  config: JsEnginePoolConfig(
    timeout: 3000,
    // default: resetMode none (warm reuse). soft/hard or resetOnRelease:true between tenants
  ),
);
final out = await pool.withEngine((js) async {
  return js.evaluate('1 + 1').stringResult;
});
pool.dispose();

Engine ids include the isolate hash (qjs-<isolate>-<serial>-<us>) so parallel isolates do not collide in channel maps.

forceJavascriptCoreOnAndroid and xhr are accepted for API compatibility with flutter_js but are not implemented (always QuickJS; no built-in XHR/fetch polyfill).

Dart ↔ JS bridge

js.onMessage('log', (args) {
  print(args); // typically a List from JSON
});

sendMessage('log', JSON.stringify([1, 2, 3]));

Prefer setupBridge / newer channel APIs when available; channel names and payloads should be treated as untrusted if they come from user scripts.

TypedArray / binary

TypedData (e.g. Uint8List) maps to JS TypedArrays via a bulk buffer path.
ByteBuffer maps to ArrayBuffer.
Use evaluateJson when you only need a Dart JSON-like tree (often faster for large objects).

Event loop / Promises

QuickJS jobs and setTimeout are drained through the runtime’s ReceivePort. After scheduling async JS, call dispatch() (or rely on paths that already pump the port, e.g. promise helpers in handle_promises.dart).

// Drain Promise microtasks / jobs in a tight loop:
js.executePendingJobs(); // or executePendingJob() once per job

// QuickJsRuntime2 only:
// (js as QuickJsRuntime2).hasPendingJobs

`autoExecutePendingJobs` (QuickJsRuntime2)

Default is true: after evaluate, evaluateJson, evaluateBytecode, and callFunction, the runtime automatically drains the QuickJS job queue via executePendingJobs() (Promise reactions / microtasks).

Implications:

Synchronous scripts that schedule Promises often settle without an extra manual drain.
Call order / timing can differ from engines that only run jobs when you call dispatch() / executePendingJobs() yourself.
There is a small cost after each of those entry points when jobs are pending.

Disable when you need explicit control or minimal post-call work:

final js = QuickJsRuntime2(autoExecutePendingJobs: false);
// or:
(js as QuickJsRuntime2).autoExecutePendingJobs = false;

With autoExecutePendingJobs: false, drain jobs yourself (executePendingJobs(), dispatch(), or handlePromises) after async JS.

Memory / GC

js.runGC();
final m = js.getMemoryUsage(); // JsMemoryUsage? (malloc / JS heap sizes)

Default heap limit is 64 MiB (memoryLimit: 0 for unlimited). Always call dispose() so native JSRuntime / JSContext, ReceivePort, and channel maps are released.

Logging

FlutterQjsLogger.level = FlutterQjsLogLevel.debug;
FlutterQjsLogger.handler = (level, message, error) { /* ... */ };

Documentation

Full package wiki: docs/wiki/


Start here	Getting started · Concepts
API	API reference · Runtime · Bridge
Guides	Security · Performance · Migration
Recipes	Hello evaluate · Async bridge · Pool
Ops	FAQ · Troubleshooting · Testing

Example app

See example/ for a full Flutter demo (AJV, typed arrays, etc.).

cd example && flutter run

Benchmark

Use Flutter’s test harness (loads the plugin native library correctly). Do not use bare dart run on this package: on recent Dart SDKs, standalone compilation of dart:ffi callbacks (Pointer.fromFunction) can crash the compiler before any code runs.

cd example
# default: 8 RNG seeds → mean ± σ (us/op; buffers also MiB/s)
flutter test test/benchmark_test.dart

# adjust repeats (1 = single seed, faster smoke)
flutter test test/benchmark_test.dart --dart-define=BENCH_RUNS=3
flutter test test/benchmark_test.dart --dart-define=BENCH_RUNS=1

# base seed (used when BENCH_RUNS=1, or to extend seeds when runs>8)
flutter test test/benchmark_test.dart --dart-define=BENCH_SEED=464037

Shared logic: example/lib/benchmark_runner.dart (runFlutterQjsBenchmarkSuite / runFlutterQjsBenchmarks).

Coverage:

Hot path: tiny evaluate and cached JSInvokable.invoke
String / small Map Dart↔JS identity round-trips
Buffer size ladder (1 KiB / 64 KiB / 1 MiB / 16 MiB) for owned TypedArray paths
TypedArray variants and non-zero-offset views in both directions
evaluateJson vs full evaluate (deep jsToDart) on array and object payloads
Multi-seed mean ± σ (kBenchmarkDefaultSeeds, default 8 runs) plus OS / CPU / executable banner for cross-commit comparison

Interactive option:

cd example && flutter run
# tap "Run Benchmarks" (same 8-run suite as the test default)

benchmark/flutter_qjs_benchmark.dart only prints these instructions if invoked with dart run by mistake.

Performance vs pre-optimization baseline

Compared the current revision to git commit 3261eb4 (pre bulk-buffer / marshalling work) on the same machine and seeds with BENCH_RUNS=32. The old commit uses a compatibility runner: its 16 MiB JS→Dart case is omitted because it cannot complete a practical multi-seed run, and its 1 MiB JS→Dart steady cases use two iterations instead of twenty. Large-path ratios are therefore directional, not exact apples-to-apples results.

Scenario	3261eb4 (us/op)	current (us/op)	Speedup
evaluate tiny (`1+1`)	4.1	3.5	1.2×
host invoke `(a,b)=>a+b`	2.4	2.2	1.1×
string Dart→JS→Dart	3.2	3.1	1.0×
small Map Dart→JS→Dart	21.1	18.1	1.2×
Dart `Uint8List`→JS (1 MiB)	141.6	49.3	2.9×
Dart `Float64List`→JS (10k)	1863.5	4.6	405×
evaluate large array (full jsToDart)	3004.1	1877.5	1.6×
evaluate large object (full jsToDart)	5934.5	4916.3	1.2×
JS `Uint8Array`→Dart (1 KiB)	602.7	5.9	102×
JS `Uint8Array`→Dart (64 KiB)	47392.2	21.9	2164×
JS `Uint8Array`→Dart (1 MiB)	975550.7	421.5	2314×
JS `Float64Array`→Dart (10k)	7844.7	623.1	12.6×
JS `Uint8ClampedArray`→Dart (offset)	22206.8	9.3	2388×

What improved most

TypedArray / buffer paths — bulk copy instead of per-element marshalling (largest wins on JS→Dart and non-byte TypedArrays Dart→JS).
String / host invoke — leaner FFI and value conversion.
Large array evaluate — faster recursive jsToDart; use evaluateJson when you only need a JSON-like tree (avoids deep object graph conversion).

Raw logs (full suite output + aggregated means):
benchmark_results/3261eb4/3261eb4_BENCH_RUNS32.txt, benchmark_results/1c9561d/optimized_BENCH_RUNS32_clamped.txt.

Numbers are single-host microbenchmarks (Linux flutter_tester); treat them as relative, not absolute product SLOs.

Soak / stress (long-haul)

Separate from micro-benchmarks: high-concurrency burn-in, RSS/metrics, fail-fast dump (not us/op). Shared logic: example/lib/soak_stress_runner.dart.

cd example
# short smoke (default ~30s)
flutter test test/soak_stress_test.dart

# ≥1h burn-in example
flutter test test/soak_stress_test.dart \
  --dart-define=SOAK_DURATION_SEC=3600 \
  --dart-define=SOAK_POOL_SIZE=8 \
  --dart-define=SOAK_WORKERS=32

On failure, a dump is written under soak_dumps/ (config, pool stats, RSS, recent ops, sample getMemoryUsage). Native core dump is optional/external (gcore / ulimit -c).

Architecture (short)

lib/quickjs/* — Dart FFI bindings and marshalling
cxx/ffi.cpp — stable C ABI around QuickJS (JSValue* on the heap)
cxx/quickjs/ — embedded engine (same tree used on Windows via cxx-windows/)

Limitations / security

Scripts run with full engine capability; do not eval untrusted code without your own sandbox policy.
Prefer timeout (wall-clock interrupt) and the default memoryLimit (64 MiB) for untrusted scripts.
No Web platform; no shipping XHR implementation in this package.
Dispose runtimes you create (dispose()) to free native resources.
Module sources from moduleHandler are copied into QuickJS then free’d in native (no Dart microtask free race).
JSValue* returned across the FFI boundary is heap-allocated; callers must free via the package APIs (jsFreeValue / Dart wrappers) — double-free of the same handle after dispose is undefined.

flutter_qjs_next

Install

Quick start

Limits

Multi-engine pool

Dart ↔ JS bridge

TypedArray / binary

Event loop / Promises

`autoExecutePendingJobs` (QuickJsRuntime2)

Memory / GC

Logging

Documentation

Example app

Benchmark

Performance vs pre-optimization baseline

Soak / stress (long-haul)

Architecture (short)

Limitations / security

References

Libraries

flutter_qjs_next package

flutter_qjs_next

Install

Quick start

Limits

Multi-engine pool

Dart ↔ JS bridge

TypedArray / binary

Event loop / Promises

autoExecutePendingJobs (QuickJsRuntime2)

Memory / GC

Logging

Documentation

Example app

Benchmark

Performance vs pre-optimization baseline

Soak / stress (long-haul)

Architecture (short)

Limitations / security

References

Libraries

flutter_qjs_next package

`autoExecutePendingJobs` (QuickJsRuntime2)