Finding alphabet from ASCII value without using library functions in JavaScript

Problem

We are required to write a JavaScript function that takes in a number. Our function should return the corresponding ASCII alphabet for that number (if there exists an alphabet for that ASCII value), -1 otherwise.

The condition here is that we cannot use any inbuilt function that converts these values.

Example

Following is the code −

const num = 98;
const findChar = (num = 1) => {
   const alpha = 'abcdefghijklmnopqrstuvwxyz';
   if(num >= 97 && num <= 122){
      return alpha[num - 97];
   };
   if(num >= 65 && num <= 90){
      return alpha.toUpperCase()[num - 65];
   };
   return -1;
};
console.log(findChar(num));

Output

b