JSFiddle - React, Tailwind, and code Playground

JavaScript

var str="999";
var result="";

var first =str.charAt(0);
switch (first){
case "9": 
result=result+"nine hundred "
break;
case "8": 
result=result+"eight hundred "
break;
case "7": 
result=result+"seven hundred "
break;
case "6": 
result=result+"six hundred "
break;
case "5": 
result=result+"five hundred "
break;
case "4": 
result=result+"four hundred "
break;
case "3": 
result=result+"three hundred "
break;
case "2": 
result=result+"two hundred "
break;
case "1": 
result=result+"one hundred "
break;        
default : result=result+"NaN"
}

var second =str.charAt(1);
switch (second){
case "9": 
result=result+"ninety "
break;
case "8": 
result=result+"eighty "
break;
case "7": 
result=result+"seventy "
break;
case "6": 
result=result+"sixty "
break;
case "5": 
result=result+"fifty "
break;
case "4": 
result=result+"fourty "
break;
case "3": 
result=result+"thirty "
break;
case "2": 
result=result+"twenty "
break;
case "1": 
result=result+"ten "
break;        
default : result=result+"NaN"
}

var third =str.charAt(2);
switch (third){
case "9": 
result=result+"nine"
break;
case "8": 
result=result+"eight"
break;
case "7": 
result=result+"seven"
break;
case "6": 
result=result+"six"
break;
case "5": 
result=result+"five"
break;
case "4": 
result=result+"four"
break;
case "3": 
result=result+"three"
break;
case "2": 
result=result+"two"
break;
case "1": 
result=result+"one"
break;        
default : result=result+"NaN"
}
alert(result);