mx-js-method — Unimplemented Method

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 call a method the runtime types do not implement — for example .map() / .split() / .toUpperCase() in a position or on a value kind without support:

error : mx-js-method : Method `.toUpperCase()` is not implemented for this value
  hint: Check the runtime type docs for supported methods, or use a `.cpp` helper
  Learn more: https://morph.levizr.com/docs/errors/mx-js-method

Why Morph raises it

String, array, and object operations in Morph lower to morph::str helpers and container methods on the native types. Coverage is broad but not total (see translator coverage): exotic or highly dynamic methods have no helper to call. Emitting a guessed equivalent would change string semantics silently — the worst kind of bug in UI text — so the call is rejected with the exact method named.

Example that triggers it

import { morphState } from 'morph';

export default function App() {
  const [name, setName] = morphState("ada");
  // ❌ If the method/position lacks support — mx-js-method
  return <div>{name.toFancyCase()}</div>;
}

How to fix

1. Check the runtime types and comparisons docs — the method you need may exist under a supported form.
2. Rewrite with supported operations (concatenation, comparisons, array helpers that exist).
3. Otherwise implement it in C++ and import it:

// ✅ Native helper for the missing method
import { shout } from './helpers.cpp';

import { morphState } from 'morph';

export default function App() {
  const [name, setName] = morphState("ada");
  return <div>{shout(name)}</div>;
}

Steps:

1. Confirm the exact method in the error message.
2. Look for a supported equivalent; if none, write the .cpp helper.
3. Re-run morph check.

Tuning this rule

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

See also

• mx-js-member — unsupported builtin members
• mx-transpile — the general untranslatable-code rule
• JavaScript Runtime Types
• How to Call C++ from Morph