Largest Substring Between Two Equal Characters in a string in JavaScript

We are required to write a JavaScript function that takes in a string of characters as the only argument.

The function should find that longest string which is sandwiched between two identical characters and return its length.

For example −

If the input string is −

const str = 'sadtrsewak';

Then the output should be −

const output = 6;

because between two 'a' we have the longest desired substring of length 6.

Example

Following is the code −

const str = 'sadtrsewak';
const longestSubstringBetween = (str = '') => {
   const map = {};
   let res = -1;
   for(let i = 0; i < str.length; i++){
      const el = str[i];
      if(map.hasOwnProperty(str[i])){
         res = Math.max(res, i - map[el] - 1);
      }else{
         map[el] = i;
      };
   };
   return res;
}
console.log(longestSubstringBetween(str));

Output

Following is the console output −

6