JSFiddle - React, Tailwind, and code Playground

HTML

<label for="int"> Insert number </label>
    <input type="text" name="int" id="int"/>
    <br/>
    <button type="submit" id="success"> Go </button>
    <br/>
    <span id="result"></span>

JavaScript

window.onload = function(){
var input = document.getElementById("int").value,
    button = document.getElementById("success"),
    result = document.getElementById("result"),
    assocArr = [],
    assocArr[0] = new Array("", "один", "два", "три", "четыри", "пять", "шесть", "семь", "восемь", "девять"),
    assocArr["d"] = new Array("десять", "одинадцать", "двенадцать", "тринадцать", "четырнадцать", "пятнадцать", "шеснадцать", "семнадцать", "восемнадцать", "девятнадцать"),
    assocArr[1] = new Array("", "", "двадцать", "тридцать", "сорок", "пятьдесят", "шестьдесят", "семьдесят", "восемьдесят", "девяносто"),
    assocArr[2] = new Array("", "сто", "двести", "триста", "четыреста", "пятьсот", "шестьсот", "семьсот", "восемьсот", "девятьсот"),
    assocArr["s"] = new Array("", "одна", "две"),
    assocArr[3] = new Array("тысяч", "тысяча", "тысячи", "тысячи", "тысячи", "тысяч", "тысяч", "тысяч", "тысяч", "тысяч", "");

function numToWord(number) {
    var resp = "",
        numArr = [],
        flag = true;

    if (isNaN(number) || number < 1 || number > 9999) {
        return "Invalid input!";
    }

    for (; number != 0; number = Math.floor(number / 10)) {
        numArr.push(number % 10);
    }

    for (var i = numArr.length - 1; i >= 0 ; i--) {
        if (flag) {
            if (numArr[i] == 1 && i == 1 || numArr[i] == 1 && i == 4) {
                flag = false;
            } else {
                resp += digitToWord(i, numArr[i], 0);
            }
        } else {
            resp += digitToWord("d", numArr[i], i);
            flag = true;
        }
    }
    return resp.trim();
}

function digitToWord(digit, offset, char) {
    var resp = "";
    switch (digit) {
        case 3:
            resp += (offset == 1 || offset == 2 ? assocArr["s"][offset] : assocArr[0][offset]) + " ";
            break;
        case 4:
            digit = 1;
            break;
        case "d":
            resp += assocArr[digit][offset] + " ";
            digit = char;
         ...