Find Index of First Repeating Character in a String in JavaScript

We are required to write a JavaScript function that takes in a string and returns the index of the first character that appears twice in the string. If there is no such character then we should return -1.

Let’s say the following is our string −

const str = 'Hello world, how are you';

We need to find the index of the first repeating character.

Example

The code for this will be −

const str = 'Hello world, how are you';
const firstRepeating = str => {
   const map = new Map();
   for(let i = 0; i < str.length; i++){
      if(map.has(str[i])){
         return map.get(str[i]);
      };
      map.set(str[i], i);
   };
   return -1;
};
console.log(firstRepeating(str));

Output

The output in the console will be −

2