asyncForEach.js

    async asyncForEach(array, callback) {
        for (let index = 0; index < array.length; index++) {
            await callback(array[index], index, array);
        }
    }

    async asyncParallelForEach(array, callback, nParallelism) {
        nParallelism = Number(nParallelism);
        if(!nParallelism || isNaN(nParallelism)) {
            nParallelism = 4;
        }
        let newTasks = [];
        let currentTasks = [];
        let arrCopy = array.slice();
        let returnVals = [];
        let arrIdx = -1;
        do {
            while(newTasks.length + currentTasks.length < nParallelism && arrCopy.length > 0) {
                let thisIdx = ++arrIdx;
                let thisObj = arrCopy.shift();
                let promise = new Promise(async res=> {
                    let result = {};
                    result.target = thisObj;
                    result.idx = thisIdx;
                    try {
                        let ret = await callback(thisObj, thisIdx);
                        result.retv = ret;
                    } catch(err) {
                        result.err = err;
                    }
                    promise.fulfiled = true;
                    res(result);
                });
                newTasks.push(promise);
            }
            while(newTasks.length) currentTasks.push(newTasks.shift());
            let ret = await Promise.race(currentTasks);
            returnVals.push(ret);
            currentTasks = currentTasks.filter(t=> !t.fulfiled);
        } while(arrCopy.length > 0 || currentTasks.length > 0);

        returnVals = returnVals.sort((a,b)=> a.idx > b.idx ? 1 : b.idx > a.idx ? -1 : 0);

        return returnVals;
    }