Map Concurrently

mapConcurrently.js

function mapConcurrently(input, mapper, concurrency) {
    let i = 0;
    const result = [];
    let pending = 0;
    let done = false;
    const promise = new Promise(function (resolve, reject) {
        function startOne() {
            if (done) return;
            if (i == input.length) {
                if (pending == 0) {
                    resolve(result);
                }
                return;
            }
            const item = input[i];
            mapper(item).then(function (value) {
                result.push(value);
                pending--;
                startOne();
            }, function (error) {
                done = true;
                reject(error);
            });
            i++;
            pending++;
            if (pending < concurrency) {
                startOne();
            }
        }
        startOne();
    });
    promise.cancel = function () {
        done = true;
    }
    return promise;
}