Made of little things.

// Node module hooks so tests can import the TypeScript sim directly:
// map './x.js' to './x.ts' when only the .ts exists, and transpile .ts with the
// project's TypeScript (no type checking; `pnpm type-check` covers that).
import { existsSync } from 'node:fs';
import { readFile } from 'node:fs/promises';
import { fileURLToPath } from 'node:url';
import ts from 'typescript';

export async function resolve(specifier, context, nextResolve) {
  if (specifier.endsWith('.js') && context.parentURL?.startsWith('file:') && (specifier.startsWith('.') || specifier.startsWith('/'))) {
    const tsUrl = new URL(specifier.replace(/\.js$/, '.ts'), context.parentURL);
    if (existsSync(fileURLToPath(tsUrl))) return { url: tsUrl.href, shortCircuit: true };
  }
  return nextResolve(specifier, context);
}

export async function load(url, context, nextLoad) {
  if (url.endsWith('.ts')) {
    const source = await readFile(fileURLToPath(url), 'utf8');
    const { outputText } = ts.transpileModule(source, {
      compilerOptions: { module: ts.ModuleKind.ESNext, target: ts.ScriptTarget.ES2022, verbatimModuleSyntax: true },
      fileName: fileURLToPath(url),
    });
    return { format: 'module', source: outputText, shortCircuit: true };
  }
  return nextLoad(url, context);
}