improve run-length-encoding algorithm

Question

I've got string interview question regarding run-length-encoding and my solution is O(n). Is there any way to improve it:

• input is AABBBCAAEE
• output suppose to be A2B3C1A2E2

const firstString=(str)=>{
  const arr = str.split('');
  let counter = 1;
  let result ='';
  for (let i=0; i<arr.length; i++){
    if(arr[i] === arr[i+1]){
      counter++;
    } else {
      result +=arr[i]+counter;
      counter = 1;
    }
  } return result
};
firstString('AABBBCAAEE');

Answer 1

Using this approach with regex: Regex to match/group repeating characters in a string

Regex explanation: /((.)\2*)/g

var str = 'AABBBCAAEE';
var result = str.replace(/((.)\2*)/g, function(match) {
  var [letter] = match;
  return `${letter}${match.length}`;
});

console.log(result);

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