Change every letter to next letter - JavaScript

We are required to write a JavaScript function that takes in a string and changes every letter of the string from the English alphabets to its succeeding element.

For example: If the string is −

const str = 'how are you';

Then the output should be −

const output = 'ipx bsf zpv'

Example

Following is the code −

const str = 'how are you';
const isAlpha = code => (code >= 65 && code <= 90) || (code >= 97 && code
<= 122);
const isLast = code => code === 90 || code === 122;
const nextLetterString = str => {
   const strArr = str.split('');
   return strArr.reduce((acc, val) => {
      const code = val.charCodeAt(0);
      if(!isAlpha(code)){
         return acc+val;
      };
      if(isLast(code)){
         return acc+String.fromCharCode(code-25);
      };
      return acc+String.fromCharCode(code+1);
   }, '');
};
console.log(nextLetterString(str));

Output

Following is the output in the console −

ipx bsf zpv