René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"ffi: bool changes from uint8 to Boolean after Fast API optimization","articleBody":"### Version\n\nmain\n\n### Platform\n\n```text\nmacOS 26.5.2\n```\n\n### Subsystem\n\nffi\n\n### What steps will reproduce the bug?\n\n`add.c`\n```c\n#include \u003cstdint.h>\n\nuint8_t add_u8(uint8_t a, uint8_t b) {\n  return a + b;\n}\n```\n\nBuild the file\n```sh\n$ cc -dynamiclib add.c -o libadd.dylib\n```\n\nUse in `repro.js`\n```js\nimport { dlopen } from 'node:ffi';\n\nconst handle = dlopen(`./libadd.dylib`, {\n  add_u8: {\n    arguments: ['bool', 'bool'],\n    return: 'bool',\n  },\n});\n\nconst native = handle.functions.add_u8;\n\nfunction fn(a, b) {\n  return native(a, b);\n}\n\neval('%PrepareFunctionForOptimization(fn)');\n\nlet result = fn(1, 0);\nconsole.log('unoptimized:', result, typeof result);\n\neval('%OptimizeFunctionOnNextCall(fn)');\n\nresult = fn(1, 0);\nconsole.log('optimized:', result, typeof result);\n\nresult = fn(true, false);\nconsole.log('booleans:', result, typeof result);\n```\n\nRun\n```console\n$ node --experimental-ffi --allow-natives-syntax --no-warnings repro.js\n```\n\n\n### How often does it reproduce? Is there a required condition?\n\nAlways\n\n### What is the expected behavior? Why is that the expected behavior?\n\n```console\nunoptimized: 1 number\noptimized: 1 number\nTypeError: Argument 0 must be a uint8\n```\n\nThe optimized numeric call should behave exactly like the unoptimized call.\nThe final `fn(true, false)` call should throw `ERR_INVALID_ARG_VALUE`.\n\n### What do you see instead?\n\n```console\nunoptimized: 1 number\noptimized: true boolean\nbooleans: true boolean\n```\n\nAfter JIT optimization, the return type changes from numeric `uint8` to Boolean, and Boolean arguments become accepted\n\n### Additional information\n\n_No response_","author":{"url":"https://github.com/trivikr","@type":"Person","name":"trivikr"},"datePublished":"2026-07-16T03:24:14Z","url":"https://github.com/nodejs/node/issues/64526"}