mx-js-member — Unsupported Builtin Member Access

Severity: error | Blocks morph build: yes

Status: documented rule — morph check does not report this code yet. The underlying problem still breaks your build or your UI; the cause and fix below apply when you hit it.

What this error means

You access a member of a JS builtin object Morph does not implement — Math.*, JSON.*, Date.*, RegExp.*, or console.* beyond log/warn/error/info:

error : mx-js-member : Unsupported member `JSON.parse` — no native counterpart
  hint: Parse with explicit conversion code or a `.cpp` helper
  Learn more: https://morph.levizr.com/docs/errors/mx-js-member

Why Morph raises it

Morph's runtime types (JsNumber, JsString, JsArray, JsObject — see runtime types) implement a curated subset of JavaScript semantics with zero-overhead native backing. Members outside that subset (Math.hypot, JSON.parse, Date.now, RegExp, console.table) have no implementation to call into. Rather than stubbing them with wrong semantics, Morph rejects the access and points at the replacement.

Example that triggers it

// ❌ JSON.parse is not implemented — mx-js-member
export default function App() {
  const data = JSON.parse('{"n": 1}');
  return <div>Hello</div>;
}

// ❌ console.table is not implemented (log/warn/error/info are) — mx-js-member
export default function App() {
  console.table([1, 2, 3]);
  return <div>Hello</div>;
}

How to fix

// ✅ console.log/warn/error/info are supported
export default function App() {
  console.log("rendered");
  return <div>Hello</div>;
}

For the rest, options in order of preference:

1. Supported equivalent — many common operations exist on the runtime types or as operators; check runtime types and native types.
2. Explicit code — date math with numbers, manual parsing for simple shapes.
3. .cpp helper — implement it natively and import it (see native C++).

// ✅ Native helper for unsupported builtins
import { parseCount } from './helpers.cpp';

export default function App() {
  return <div>{parseCount("3")}</div>;
}

Steps:

1. Check whether a supported equivalent exists in the runtime types docs.
2. Otherwise extract the operation into a .cpp function and import it.
3. Re-run morph check.

Tuning this rule

Do not disable this rule. Unimplemented members have no code to call.

See also

• mx-js-method — unsupported methods on values
• mx-js-global — unsupported globals
• JavaScript Runtime Types