Converts integer input to a numerical string (i.e. how you would write numbers on a check - e.g. "One hundred seventy-four");

integerToNumericalString.js

const ZERO = "0";
const ONE = "1";

const HUNDRED_AS_WORD = "hundred";
const ZERO_AS_WORD = "zero";

const digitMap = {
    "0": "",
    "1": "one",
    "2": "two",
    "3": "three",
    "4": "four",
    "5": "five",
    "6": "six",
    "7": "seven",
    "8": "eight",
    "9": "nine",
    "10": "ten",
    "11": "eleven",
    "12": "twelve",
    "13": "thirteen",
    "14": "fourteen",
    "15": "fifteen",
    "16": "sixteen",
    "17": "seventeen",
    "18": "eightteen",
    "19": "nineteen"
}

const tensPlaceMap = {
    "2": "twenty",
    "3": "thirty",
    "4": "fourty",
    "5": "fifty",
    "6": "sixty",
    "7": "seventy",
    "8": "eighty",
    "9": "ninety"
}

const placeValueMap = {
    "4": "thousand",
    "5": "thousand",
    "6": "thousand",
    "7": "million",
    "8": "million",
    "9": "million",
    "10": "billion",
    "11": "billion",
    "12": "billion",
    "13": "trillion",
    "14": "trillion",
    "15": "trillion",
    "16": "quadrillion",
    "17": "quadrillion",
    "18": "quadrillion"
}

digitNameLookup = (input) => {
    return digitMap[input];
}

tensPlaceLookup = (input) => {
    return tensPlaceMap[input];
}

getPlaceValue = (digits, index) => {
    if (index <= 3) {
        return "";
    } else {
        if (digitsToInt(digits) === 0) {
            return "";
        } else {
            const placeMapIndex = index;
            return placeValueMap[placeMapIndex];
        }
    }
}

integerToNumericalString = (input) => {
    let digits = input.toString().split("");
    console.log(input + ": " + digitsToString(digits));
};

digitsToInt = (digits) => {
    return Number(digits.slice(0).join(""));
}

digitsToString = (digits) => {
    let digitStrings = [];

    if (digits[0] === ZERO) {
        digitStrings[0] = ZERO_AS_WORD;
    } else {
        for (let i = digits.length; i > 0; i -= 3) {
            const startIndex = Math.max(i - 3, 0);
            const currentDigits = digits.slice(startIndex, i);

            let digitString = threeDigits(currentDigits);
            const placeValueString = getPlaceValue(currentDigits, digits.length - i + 1);

            if (placeValueString != "") {
                digitString = digitString + " " + placeValueString;
            }

            if (digitsToInt(currentDigits) !== 0) {
                digitStrings.push(digitString);
            }
        }
    }

    let result = "";
    digitStrings = digitStrings.reverse();
    
    for (let i = 0; i < digitStrings.length; i++) {
        result += digitStrings[i] + " ";
    }

    //Capitalize the first letter
    result = result[0].toUpperCase() + result.substring(1);
    //Remove trailing spaces
    result.trim();
    return result;

}

threeDigits = (input) => {
    if (input.length < 3) {
        return twoDigits(input);
    } else if (input[0] === ZERO) {
        return twoDigits(input.slice(1, 3));
    } else {
        return digitNameLookup(input[0]) + " " + HUNDRED_AS_WORD + " " + twoDigits(input.slice(1, 3));
    }
}

twoDigits = (input) => {
    const finalElement = input.length - 1;

    //0 - 9
    if (input.length < 2 || input[0] === ZERO) {
        return digitNameLookup(input[finalElement]);
    }

    //10 - 19 
    else if (input[0] === ONE) {
        return digitNameLookup(input[0] + input[1]);
    }

    //20 - 99
    else {
        if (input[finalElement] === ZERO) {
            return tensPlaceLookup(input[0]);
        } else {
            return tensPlaceLookup(input[0]) + "-" + digitNameLookup(input[1]);
        }
    }
}

const inputs = [
    0,
    1,
    9,
    18,
    23,
    99,
    877,
    1000,
    1001,
    1010,
    1101,
    1110,
    1111,
    12345678,
    10000000,
    10000000001,
    10000000101,
    10001111,
    98080800,
    111111111111111
];

for (let i in inputs) {
    integerToNumericalString(inputs[i]);
}

integerToNumericalString(Number.MAX_SAFE_INTEGER);