Finding longest substring between two same characters JavaScript

We are required to write a JavaScript function that takes in a string. The function should find and return the length of the longest substring that is sandwiched between two same letters.

For example − 

If the input string is −

const str = 'avbghvh';

Then the output should be −

const output = 3;

because the desired longest substring is 'bgh' between two v's.

Example

const str = 'avbghvh';
const longestSub = (str = '') => {
   const map = new Map();
   let max = -1;
   for(let i = 0; i < str.length; i++){
      if(map.has(str.charAt(i))){
         max = Math.max(max, i - map.get(str.charAt(i)) - 1);
      }else{
         map.set(str.charAt(i), i);
      };
   };
   return max;
};
console.log(longestSub(str));

Output

This will produce the following output −

3