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once ​

Problem ​

You need a function to execute at most once — for example initializing a singleton, registering a global listener, or running a migration.

Solution ​

Use once(fn) to wrap a function so it runs only on the first call. Subsequent calls return the same result. The returned function exposes .reset() to allow re-invocation.

ts
import { once } from '@vielzeug/arsenal';

const init = once(() => {
  console.log('initialized');
  return 42;
});

init(); // 'initialized' logged, returns 42
init(); // returns 42, no log
init(); // returns 42, no log

init.reset(); // allow re-invocation
init(); // 'initialized' logged again, returns 42

Pitfalls ​

  • The result is memoized after the first call — if fn throws, the error is not cached and subsequent calls re-throw.