source: trip-planner-front/node_modules/p-map/index.js

'use strict';
const AggregateError = require('aggregate-error');

module.exports = async (
        iterable,
        mapper,
        {
                concurrency = Infinity,
                stopOnError = true
        } = {}
) => {
        return new Promise((resolve, reject) => {
                if (typeof mapper !== 'function') {
                        throw new TypeError('Mapper function is required');
                }

                if (!((Number.isSafeInteger(concurrency) || concurrency === Infinity) && concurrency >= 1)) {
                        throw new TypeError(`Expected \`concurrency\` to be an integer from 1 and up or \`Infinity\`, got \`${concurrency}\` (${typeof concurrency})`);
                }

                const result = [];
                const errors = [];
                const iterator = iterable[Symbol.iterator]();
                let isRejected = false;
                let isIterableDone = false;
                let resolvingCount = 0;
                let currentIndex = 0;

                const next = () => {
                        if (isRejected) {
                                return;
                        }

                        const nextItem = iterator.next();
                        const index = currentIndex;
                        currentIndex++;

                        if (nextItem.done) {
                                isIterableDone = true;

                                if (resolvingCount === 0) {
                                        if (!stopOnError && errors.length !== 0) {
                                                reject(new AggregateError(errors));
                                        } else {
                                                resolve(result);
                                        }
                                }

                                return;
                        }

                        resolvingCount++;

                        (async () => {
                                try {
                                        const element = await nextItem.value;
                                        result[index] = await mapper(element, index);
                                        resolvingCount--;
                                        next();
                                } catch (error) {
                                        if (stopOnError) {
                                                isRejected = true;
                                                reject(error);
                                        } else {
                                                errors.push(error);
                                                resolvingCount--;
                                                next();
                                        }
                                }
                        })();
                };

                for (let i = 0; i < concurrency; i++) {
                        next();

                        if (isIterableDone) {
                                break;
                        }
                }
        });
};