A function that convert integers to English words (short scale)

numToWords.js

function numToWord(num) {
    if (!Number.isSafeInteger(num) || num < 0) return null
    if (!num) return 'zero '
    const ntt = ['', 'one ', 'two ', 'three ', 'four ', 'five ', 'six ', 'seven ', 'eight ', 'nine ', 'ten ', 'eleven ', 'twelve ', 'thirteen ', 'fourteen ', 'fifteen ', 'sixteen ', 'seventeen ', 'eighteen ', 'nineteen ']
    let mstr = ''
    let nArr = num.toString().padStart(18, '0').match(/.{1,3}/g)
    for (let i = 5; i > -1; i--) {
        let wkv = nArr.shift()
        if (!+wkv) continue
        if (+wkv[0]) mstr += ntt[+wkv[0]] + 'hundred '
        if ((+wkv[0] || !i) && wkv % 100 && num > 100) mstr += 'and '
        wkv %= 100
        if (wkv > 19) {
            mstr += ['', 'ten ', 'twenty ', 'thirty ', 'fourty ', 'fifty ', 'sixty ', 'seventy ', 'eighty ', 'ninety '][~~(wkv / 10)]
            wkv %= 10
        } 
        mstr += ntt[wkv] + ['', 'thousand ', 'million ', 'billion ', 'trillion ', 'quadrillion '][i]
    }
    return mstr
}