Create month calendar grid in JavaScript

calendar_grid.js

import moment from 'moment';

const getCalendarGrid = (year, month) => {

    let ret = [];

    const firstDayOfMonth = moment([year, month - 1, 1]);
    const firstDayOfMonthIndex = firstDayOfMonth.day();

    // grid row
    let row = [];

    // left padding of the first row with nulls
    for (let i = 1; i < firstDayOfMonthIndex; i++) {
        row.push(null);
    }

    let date = firstDayOfMonth;
    // while we stay in the same month
    while (date.month() === month - 1) {
        // check if this row is already full
        if (row.length === 7) {
            ret.push(row);
            // start new row
            row = [];
        }

        // add current date to the row
        row.push(date.date());

        // next date
        date = date.add(1, 'd')
    }

    // right padding of last row with nulls
    for (let i = row.length; i < 7; i++) {
        row.push(null)
    }
    ret.push(row);

    //
    return ret
};

export default getCalendarGrid;