Accept a JS callback

callbacks.lucent.ts

/** Synchronous: the callback runs while JavaScript waits, and may return a value. */
export function sortBy(words: string[], key: (word: string) => number): string[] {
  return [...words].sort((a, b) => key(a) - key(b));
}

/** From async code: the callback is posted to the JS thread, so it returns void. */
export async function countTo(n: number, onStep: (step: number) => void): Promise<void> {
  for (let i = 1; i <= n; i++) onStep(i);
}

/** Returning a Promise lets async code wait for JavaScript's answer. */
export async function askTwice(ask: (question: string) => Promise<number>): Promise<number> {
  const a = await ask("first");
  const b = await ask("second");
  return a + b;
}

Called from	The callback	It may return
a synchronous export	runs at once, on the JS thread	any value that can cross; it's checked
async code	is posted to the JS thread	void, or a Promise that Lucent can await

• A callback that throws, or whose promise rejects, throws in Lucent too: catch it there.
• Lucent keeps a callback while it holds a reference to it, and releases it on the JS thread after.
• After a reload, a stale void callback does nothing; one that returns a value throws.