convert number in string to Persian string

numToPersianString.js

const numberToString = (str:string):string=>{
    let firstNum = (str.match(/\d+/) ?? [undefined])[0]

    // couldn't find number
    if(isNaN(+firstNum)) return str;

    const num = parseInt(firstNum)

    const stringNum = (num)=>{
        switch (num) {
            case 0: return "صفر";
            case 1: return "یک";
            case 2: return "دو";
            case 3: return "سه";
            case 4: return "چهار";
            case 5: return "پنج";
            case 6: return "شش";
            case 7: return "هفت";
            case 8: return "هشت";
            case 9: return "نه";
            case 10: return "ده";
            case 11: return "یازده";
            case 12: return "دوازده";
            case 13: return "سیزده";
            case 14: return "چهارده";
            case 15: return "پانزده";
            case 16: return "شانزده";
            case 17: return "هفده";
            case 18: return "هجده";
            case 19: return "نانزده";
            case 20: return "بیست";
            case 30: return "سی";
            case 40: return "چهل";
            case 50: return "پنجاه";
            case 60: return "شصت";
            case 70: return "هفتاد";
            case 80: return "هشتاد";
            case 90: return "نود";
        }
    }



    if(num < 20){
        return str.replace(num.toString(),stringNum(num))
    }

    if(num < 100){
        const unity = parseInt(num.toString()[1]);
        const decimal = parseInt(num.toString()[0])*10;
        return str.replace(num.toString(),stringNum(decimal) + " و " + stringNum(unity))
    }
}