function zeroPad(fullLen, numStr) {
    const numZeroes = fullLen - numStr.length;
    let outStr = '';
    for (let i = 0; i < numZeroes; i++) {
        outStr += '0';
    }
    outStr = outStr + numStr;
    return outStr;
}
Zero-pad a string to a given length
@private
@param  {number} fullLen Target length
@param  {string} numStr  String to zero-pad
@return {string}         Zero-padded string