Look and say sequence

by Anton Bagayev

HTML

<p id="output"></p>

JavaScript

function nextNumber(number){
	numStr = number.toString();
  let index = 0;
  let currChar = numStr[index];
  let counter = 1;
  let result = "";
  while (index < numStr.length - 1){
  	index++;
    if (numStr[index] == currChar){
    	counter++;
    } else {
    	result += counter.toString() + currChar;
      currChar = numStr[index];
      counter = 1;
    }
  }
  result += counter.toString() + currChar;
  
  return result;
}

let currNum = 21;
document.getElementById("output").innerText = nextNumber(currNum);