Early returnable Fold

fold.ts

class FoldBreak <T> {
  constructor(public value: T) {};
};

const foldBreak = <T>(value: T) => new FoldBreak(value);

export const fold = Object.assign(<X, Acc>(
    reducer: (acc: Acc, x: X) => Acc | FoldBreak<Acc>,
    init: Acc,
    xs: X[],
): Acc => {
  let acc: Acc | FoldBreak<Acc> = init;
  for (const x of xs) {
    acc = reducer(acc, x);
    if (acc instanceof FoldBreak) return acc.value;
  }
  return acc;
}, { break: foldBreak });

const folded = fold(
 (acc, item) => item > 3 ? fold.break(acc) : acc.concat(item),
 [] as number[],
 [1, 2, 3, 4, 5]);