isPromise

Problem

You need to check whether a value is a thenable (Promise-like) — for example handling functions that may return either a value or a Promise.

Solution

Use isPromise(value) to narrow to Promise<unknown>.

import { isPromise } from '@vielzeug/arsenal';

function normalize<T>(valueOrPromise: T | Promise<T>): Promise<T> {
  return isPromise(valueOrPromise) ? valueOrPromise : Promise.resolve(valueOrPromise);
}

Pitfalls

• Checks for a .then method — matches any thenable, not just native Promise instances.

Related

• isFunction
• attempt