ricardobeat/boomkat
A small, fast and modern strict-only JS engine. Contribute to ricardobeat/boomkat development by creating an account on GitHub.
3 stars · 0 forks
README
boomkat A modern, fully ES compatible, strict-only javascript engine
boomkat is the dutch name for the Margay, a small wild cat native to South America; translates to 'tree cat'. It fits with the tradition of animal-themed names for JS engines and sounds cool :)
About Boomkat is a strict-only Javascript engine developed from scratch. It is written in C3, using Duktape and QuickJS as architectural references. The goal was to match Duktape's performance, but ended up matching (even slightly surpassing) QuickJS in runtime performance and memory usage. 👉 This is not production-ready code. Surprisingly, 100% test262 conformance is not nearly enough to validate the engine. There are still bugs lurking. In particular, the source code has not been scanned for security vulnerabilities. Use at your own peril. Features and compatibility
Runs everything in strict mode only Passes 50k tests from the test262 suite. 100% pass rate on the targeted subset (skips sloppy mode, legacy and proposal-stage features) Supports all modern JS features like Map/Set, ArrayBuffer and TypedArray, Proxy, async/await, private fields, template literals, optional chaining Built-in support for ES modules Natively runs TypeScript code (including module support), including the TypeScript compiler itself
A full ECMAScript feature-by-feature breakdown against QuickJS, quickjs-ng, and Duktape can be found in FEATURES.md.
Note: boomkat is not a runtime; it does not offer file or network APIs and is not a replacement for Node/Bun - and is very far from matching V8/SpiderMonkey performance. It is meant to be embedded into a host application as a scripting language, like QuickJS.
Why? This started as a simple attempt at converting Duktape from C to C3. I was bored, excited about doing something in the C3 lang, and really wanted to see what could be done using the latest generation of open-weight LLMs. As development went on I dropped the idea of a 1:1 port and started looking at other engines for new ideas. This project took about 12 weeks of nearly non-stop AI agents, working day and night. An estimated ~$300 was spent across Minimax, DeepSeek, Xiaomi Mimo, Claude Code and a dozen other LLMs. A full write-up on the development process, timeline and cost breakdown is in the works. Design
Strict-only. There is no sloppy mode. Non-strict and Annex B features are rejected at parse time; "use strict" is accepted and ignored. One-pass compiler. No AST, parser emits bytecode. Register VM. 32-bit instructions, inline caches for property and variable access, hidden classes / object shapes, peephole optimizations and fused opcodes on the hot paths. Threaded dispatch. Direct-threaded dispatch achieved through tail-call optimization in LLVM as C3 does not have computed-goto. NaN-boxed values. Uses 8-byte packed TVal structs; A plain tagged union mode is available with -D NONANBOX for debugging and portability Hybrid collector. Refcounting reclaims most values; a mark-and-sweep GC collects the rest
Real-world validation Apparently fifty thousand tests are not nearly enough to verify that the engine actually works. A trillion bugs were uncovered by actually trying to run code. Javascript: we run unmodified npm sources against the engine. The python3 scripts/verify_libraries.py --api-checks script downloads and tries to execute each library, then runs a few tests against each, and compare the results with QuickJS:
Library Version Loading API checks
lodash 4.17.21 ✅ 17
underscore 1.13.6 ✅ 8
moment * ✅ 6
marked 4.3.0 ✅ 5
handlebars 4.7.8 ✅ 5
immutable * ✅ 6
acorn * ✅ 4
bluebird 3.7.2 ✅ 5
decimal.js * ✅ 5
bignumber.js * ✅ 7
mathjs * ✅ 5
jszip 3.10.1 ✅ 4
papaparse * ✅ 4
crypto-js * ✅ 4
protobufjs 7.4.0 ✅ 2
chance * ✅ 5
he * ✅ 4
nearley * ✅ 2
d3-array * ✅ 7
uuid 8.3.2 ✅ 2
typescript 5.4.5 ✅ 4
@babel/standalone 7.24.7 ✅ 3
TypeScript: python3 scripts/verify_ts_libraries.py downloads sources and runs them unmodified against the engine. The output must match the result of running the same source stripped by tsc.
Source Version What is checked Pass
microdiff 1.4.0 single-file diff library ✅
zustand 5.0.3 vanilla store (src/vanilla.ts) ✅
valtio 2.1.3 vanilla store + vendored proxy-compare 3.0.1 ✅
@preact/signals-core 1.14.4 signal primitives (src/index.ts) ✅
jotai 2.20.2 vanilla package tree (src/vanilla/) ✅
fp-ts 2.16.9 full source tree (123 modules), bare self-imports, import X = Y, overload signatures ✅
zod 4.4.3 lib tree (107 modules), NodeNext .js specifiers, type modifiers in re-export lists ✅
Benchmarks Run just bench to benchmark against Duktape and QuickJS:
Benchmark C3 Duktape QuickJS
arithmetic 350ms 3,491ms (0.1x) 260ms (1.3x)
array 14ms 42ms (0.3x) 10ms (1.4x)
date 78ms 3,639ms (0.0x) 55ms (1.4x)
function_call 226ms 1,322ms (0.2x) 162ms (1.4x)
ic_monomorphic 85ms 293ms (0.3x) 96ms (0.9x)
ic_proto 118ms 465ms (0.3x) 118ms (1.0x)
loop 118ms 1,405ms (0.1x) 126ms (0.9x)
memory_heavy 82ms 182ms (0.5x) 53ms (1.5x)
object 371ms 1,719ms (0.2x) 220ms (1.7x)
property_lookup 254ms 1,845ms (0.1x) 166ms (1.5x)
recursion_deep 875ms 2,034ms (0.4x) 528ms (1.7x)
recursion 210ms 485ms (0.4x) 130ms (1.6x)
regexp 558ms 697ms (0.8x) 271ms (2.1x)
shape_no_call 8ms 9ms (0.9x) 6ms (1.3x)
shape_stress 9ms 9ms (1.0x) 7ms (1.3x)
string 9ms 19ms (0.5x) 6ms (1.5x)
valstack_copy 11ms 14ms (0.8x) 10ms (1.1x)
Startup time
Runtime Median startup
boomkat 2.7 ms
QuickJS 2.4 ms
Bun 1.3.13 8.4 ms
Node 24.13.0 19.0 ms
(not an entirely fair comparison as bun/node do a lot more, but useful for a baseline) Build and run Requirements
C3 compiler: brew install c3c Just: brew install just Python 3
Task Command
Build a target just build <target> (e.g. boomkat, boomkat_debug, test262_runner)
Run one JS file just run <file>
Run one JS file as ESM just run-module <file>
Inspect bytecode just build-trace, then ./out/boomkat_debug -c <file>
Local suite just test-local
Rosetta suite just rosetta
One test262 phase just test262-phase <n>
Full test262 just test262
ASAN test262 runner just build-asan
lldb on a crash just lldb <file>
Embedding / bindings The engine ships a jse_ C ABI (include/jse.h, static libjse.a and a shared library). See docs/embedding.md. Bindings in C3, Rust, Python, Ruby, and Zig (bindings/ and examples/) are *work in progress. Build flags
-D NONANBOX: disable NaN-boxing, 16-byte tagged union TVal (just build-nonanbox) -D NOSHAPECACHE: drop the per-object shape pointer cache (just build-noshape) -D HEAP_VERIFY: validate GC roots at yield/resume (just build-verify) -D GC_STRESS: pin the collector trigger for stress runs -D ENV_STRICT: compile-time environment-handling checks (boomkat_envstrict target)
Support this project If you'd like to support the project, you can sponsor me on Github.
License Use of this software is governed by the Business Source License included in the LICENSE file and at www.mariadb.com/bsl11. Can be used without restrictions for open-source work, and commercial projects up to $2M/year revenue. Source becomes MIT-licensed after four years.