mx-js-op — Unemittable Operator

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

Your code uses an operator the C++ translator cannot emit: typeof, **, instanceof, in, delete, ??=, &&=, ||=, >>>, or void:

error : mx-js-op : Operator `typeof` cannot be emitted to C++
  hint: Track types explicitly with state or TypeScript annotations
  Learn more: https://morph.levizr.com/docs/errors/mx-js-op

Why Morph raises it

Each operator needs a native lowering with identical semantics, and these do not have one:

• typeof, instanceof, in need runtime type reflection Morph's zero-overhead types deliberately omit (see intent-based codegen).
• delete assumes garbage-collected object graphs; Morph uses explicit ownership.
• **, >>>, ??=, &&=, ||= have no single C++ token with matching edge semantics — emitting an approximation would compute different values than the same code in a browser.

Refusing is safer than approximating: numeric and identity bugs from approximated operators would be nearly impossible to trace.

Example that triggers it

// ❌ typeof has no native lowering — mx-js-op
export default function App() {
  const kind = typeof 42;
  return <div>{kind}</div>;
}

// ❌ ** has no direct C++ token with matching semantics — mx-js-op
export default function App() {
  const area = 3 ** 2;
  return <div>{area}</div>;
}

How to fix

Instead of	Write
typeof x	track the type explicitly (state, union flag, TS annotation)
a ** b	repeated multiplication or a .cpp pow helper
x instanceof Y	a discriminant field / explicit kind flag
k in obj	explicit key check against known keys
delete obj.k	set to a default / restructure state
??=, &&=, ||=	explicit if assignment
>>>	explicit unsigned arithmetic in .cpp
void expr	drop the void — expression statements need no coercion

// ✅ Explicit exponentiation
export default function App() {
  const area = 3 * 3;
  return <div>{area}</div>;
}

// ✅ Logical-assignment spelled out
import { morphState } from 'morph';

export default function App() {
  const [name, setName] = morphState("");
  const ensure = () => {
    if (!name) {
      setName("Ada");
    }
  };
  return <button onClick={ensure}>{name}</button>;
}

Steps:

1. Find the flagged operator and pick the rewrite from the table.
2. For math-heavy needs, put the operation in a .cpp helper.
3. Re-run morph check.

Tuning this rule

Do not disable this rule. These operators have no correct native emission.

See also

• mx-js-syntax — unsupported syntax constructs
• mx-transpile — the general rule
• Intent-Based Codegen