react 19.0.0: createElement

# Notes

## Goal
Verify `react.createElement` behavior in `react@19.0.0`.

## Verified Behaviors
1. `createElement` generates transitional elements with `$$typeof: Symbol.for('react.transitional.element')`.
2. Passed `ref` is attached directly to `element.props.ref` in React 19 rather than as a top-level property.
3. Supports HTML elements, custom functional components, and `React.Fragment`.
4. Handles single and multiple trailing children appropriately (scalar child or child array).
5. `isValidElement` validates elements created via `createElement` and rejects plain objects or primitives.
6. `cloneElement` overrides props and `ref` while leaving original element immutable.

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify react.createElement in pkg:npm/react@19.0.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:npm/react@19.0.0
Demonstrate these symbols/APIs:
  - react.createElement
Constraints:
  - executionContext: node
Required runtime conditions:
  - ecosystem: npm
  - language: javascript
  - moduleSystem: cjs
  - packageManager: npm@10.9.8
  - runtime: node@22.23.2

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:ce658b5fc549b6fd36b7151e9fc68d6b321e76dee628a89e4dd33765fd57ece9","constraints":{"executionContext":"node"},"contract":["React 19 createElement produces an element with Symbol.for('react.transitional.element') $$typeof and sets type, key, and props.","React 19 createElement places passed ref directly on element.props.ref rather than element.ref.","React 19 createElement supports string tags, custom component functions, and React.Fragment.","React 19 createElement passes a single child directly as props.children and multiple trailing children as an array.","React.isValidElement returns true for objects created with createElement and false for plain objects, functions, primitives, or null.","React.cloneElement creates a new element with merged props, overrides ref on props.ref, and leaves the original element unmodified."],"goal":"verify react.createElement in pkg:npm/react@19.0.0","kind":"HOW","packages":["pkg:npm/react@19.0.0"],"schemaVersion":1,"symbols":["react.createElement"]},"contractCommand":["node","test/contract.mjs"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"npm","executionContext":"node","language":"javascript","libc":"glibc","libcVersion":"2.39","moduleSystem":"esm","os":"linux","osVersionBucket":"24","packageManager":"npm","packageManagerVersion":"10.9.8","runtime":"node","runtimeVersion":"22.23.2","schemaVersion":1},"license":"MIT-0","packages":["pkg:npm/react@19.0.0"],"schemaVersion":1,"subject":"pkg:npm/react@19.0.0","symbols":["react.createElement"],"verifierAdapter":"node-typescript@1"}

{
  "name": "react-create-element",
  "version": "1.0.0",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "": {
      "name": "react-create-element",
      "version": "1.0.0",
      "dependencies": {
        "react": "19.0.0"
      }
    },
    "node_modules/react": {
      "version": "19.0.0",
      "resolved": "https://registry.npmjs.org/react/-/react-19.0.0.tgz",
      "integrity": "sha512-V8AVnmPIICiWpGfm6GLzCR/W5FXLchHop40W4nXBmdlEceh16rCN8O8LNWm5bh5XUX91fh7KpA+W0TgMKmgTpQ==",
      "license": "MIT",
      "engines": {
        "node": ">=0.10.0"
      }
    }
  }
}

{
  "name": "react-create-element",
  "version": "1.0.0",
  "type": "module",
  "private": true,
  "dependencies": {
    "react": "19.0.0"
  }
}

{
  "schemaVersion": 1,
  "goal": "verify react.createElement in pkg:npm/react@19.0.0",
  "kind": "HOW",
  "packages": [
    "pkg:npm/react@19.0.0"
  ],
  "symbols": [
    "react.createElement"
  ],
  "constraints": {
    "executionContext": "node"
  },
  "runtimeConditions": {
    "ecosystem": "npm",
    "language": "javascript",
    "moduleSystem": "cjs",
    "packageManager": "npm@10.9.8",
    "runtime": "node@22.23.2"
  }
}

import assert from 'node:assert/strict';
import React, { createElement, cloneElement, isValidElement, Fragment, version } from 'react';

// Verify version
assert.strictEqual(version, '19.0.0');

// 1. Element construction, symbol typing, key and props
const child1 = createElement('span', { key: 'c1' }, 'First');
const child2 = createElement('span', { key: 'c2' }, 'Second');
const element = createElement('div', { id: 'test-id', className: 'main-box', ref: 'boxRef' }, child1, child2);

assert.strictEqual(element.$$typeof, Symbol.for('react.transitional.element'));
assert.strictEqual(element.type, 'div');
assert.strictEqual(element.key, null);
assert.strictEqual(element.props.id, 'test-id');
assert.strictEqual(element.props.className, 'main-box');

// 2. Ref placement in React 19 (stored on props.ref)
assert.strictEqual(element.props.ref, 'boxRef');

// 3. Trailing children handling (array for multiple children, single value for single child)
assert.strictEqual(Array.isArray(element.props.children), true);
assert.strictEqual(element.props.children.length, 2);
assert.strictEqual(element.props.children[0], child1);
assert.strictEqual(element.props.children[1], child2);

const singleChildElement = createElement('p', null, 'Single text child');
assert.strictEqual(singleChildElement.props.children, 'Single text child');

// 4. Component functions and Fragment
function CustomComponent(props) {
  return createElement('section', props, props.title);
}

const componentElement = createElement(CustomComponent, { title: 'Heading', ref: 'compRef' });
assert.strictEqual(componentElement.$$typeof, Symbol.for('react.transitional.element'));
assert.strictEqual(componentElement.type, CustomComponent);
assert.strictEqual(componentElement.props.title, 'Heading');
assert.strictEqual(componentElement.props.ref, 'compRef');

const fragmentElement = createElement(Fragment, null, child1, child2);
assert.strictEqual(fragmentElement.type, Fragment);
assert.strictEqual(fragmentElement.props.children.length, 2);

// 5. isValidElement verification
assert.strictEqual(isValidElement(element), true);
assert.strictEqual(isValidElement(componentElement), true);
assert.strictEqual(isValidElement(fragmentElement), true);
assert.strictEqual(isValidElement({ $$typeof: Symbol.for('react.transitional.element') }), true);
assert.strictEqual(isValidElement({ $$typeof: Symbol.for('react.element') }), false);
assert.strictEqual(isValidElement(null), false);
assert.strictEqual(isValidElement(undefined), false);
assert.strictEqual(isValidElement({}), false);
assert.strictEqual(isValidElement(CustomComponent), false);
assert.strictEqual(isValidElement('plain string'), false);
assert.strictEqual(isValidElement(42), false);

// 6. cloneElement prop merging, ref overriding, and immutability
const cloned = cloneElement(element, { className: 'updated-box', ref: 'newRef' }, 'Replaced Child');
assert.strictEqual(cloned.type, 'div');
assert.strictEqual(cloned.props.id, 'test-id');
assert.strictEqual(cloned.props.className, 'updated-box');
assert.strictEqual(cloned.props.ref, 'newRef');
assert.strictEqual(cloned.props.children, 'Replaced Child');

// Immutability of original element
assert.strictEqual(element.props.className, 'main-box');
assert.strictEqual(element.props.ref, 'boxRef');
assert.strictEqual(element.props.children.length, 2);