Sequentially map an action (function that returns a promise) to an array of items #javascript

promises.ts

// I've deliberately named this `forP_` becaus it matches Haskell's `forP_` too

function forP_<T, P extends PromiseLike<void> = Promise<void>>(
  items: readonly T[],
  f: (item: T) => PromiseLike<void>,
  seed?: P
): P {
  return items.reduce((pChain, item) => {
    return pChain.then(() => f(item));
  }, seed ?? Promise.resolve()) as P;
}

Basically this is the equivalent of doing:

for (const x of somearr) {
  await f(x);
}

But without using async await syntax.