Sequential version of Map in continuation passing style

sequential-map.ts

type MapHandler<T> = (item: T, index: number, callback: (result: T) => void) => void;
type MapDoneHandler<T> = (results: T[]) => void;

function map<T>(array: T[], handler: MapHandler<T>, done: MapDoneHandler<T>) {
    let index = 0;
    let results: T[] = [];

    step()

    function step() {
        if (index < array.length) {
            handler(array[index], index, (result) => {
                results.push(result);
                index++;
                step();
            })
        } else {
            done(results)
        }
    }
}

map(
    [1, 2, 3],
    (item, index, callback) => callback(item * (index + 4)),
    console.log
)