test.js

'use strict';

const rowProducer = (num) => {
    return new Promise((resolve) => {
        setTimeout(() => {
            let rows = [];
            for (let i = 0; i < num; i++) {
                rows.push([['a' + i, 'b' + i], ['c' + i, 'd' + i]]);
            }
            return resolve(rows);
        }, 100);
    });
};

const results = [];
rowProducer(10)
    .then((rows) => {
        rows.forEach((row, i) => {
            row.forEach((col) => {
                col.push(`processed at row ${i}`);
                results.push(col);
            });
        });
        console.log(results);
    })
    .catch((err) => {
        console.log(err);
    });