Haxe Compiler and JavaScript Target: ES2022 Syntax Compatibility

Enforcement mechanism

Confirmed fact: the Haxe JavaScript target is capped with the compiler flag -js-es <version>, also exposed as --js-es-version. Accepted values include es5, es6, es2015-es2024, esnext. Setting -js-es es2020 tells the compiler to restrict emitted syntax to the ES2020 ceiling.

Likely explanation for silent ES2022 output: recent Haxe releases default toward esnext when the flag is omitted. The -js-modern flag controls use of const/let, arrow functions, template literals and destructuring, but it does not override the -js-es ceiling. The compiler does not emit a warning when a Haxe construct would require newer JavaScript syntax; it either lowers the construct if possible or emits the modern syntax anyway.

Minimum steps for this case

1. Pin the ceiling in the build. In hxml:

   -js-es es2020
   -js output.js
   -main Main

   In npm scripts: haxe -js-es es2020 -js build/out.js Main.hx
2. Verify the Haxe version is known. Run haxe --version. The ES version support matrix changed across 4.2, 4.3 and 5.0, so the exact version changes the recommendation.
3. Inspect generated output for ES2021+ constructs. Quick manual checks:

   grep -n '??' build/out.js
   grep -n '?.' build/out.js
   grep -n '??=\|||=\|&&=' build/out.js
   grep -n '#[a-zA-Z]' build/out.js

4. Add a CI gate for syntax. Example eslint check limited to generated files:

   npx eslint --parser-options=ecmaVersion:2020 --no-eslintrc --rule 'no-restricted-syntax: [error, "ChainExpression", "LogicalAssignmentExpression"]' build/*.js

5. Syntax check on target runtime:

   node --check build/out.js

   This catches parse errors early without executing code.

Assumptions and uncertainty

This assumes Haxe 4.3+ where -js-es es2020 avoids nullish coalescing ??, optional chaining ?., logical assignment ??= ||= &&=, class fields, private fields #, and top-level await. Behavior with async/await lowering and with third-party haxelibs that use @:jsRequire or @:native metadata can inject modern syntax regardless of the flag. The flag does not polyfill missing globals such as Promise or Symbol.

One missing diagnostic detail that changes the recommendation: which Haxe version and build file type you use, hxml vs. hxargs vs. build tool. Confirming that lets me give the exact flag placement for your setup.