polymerUnits.js

const polymer = (string) => {
  const arr = string.split('');
  const stack = [];
  for (let i = 0; i < arr.length; i++) {
    let lastStackVal = stack[stack.length - 1];
    let currOppositeCase = arr[i] === arr[i].toUpperCase() ? arr[i].toLowerCase() : arr[i].toUpperCase();
    if (lastStackVal === currOppositeCase) {
      stack.pop();
    } else {
      stack.push(arr[i]);
    }
  }
  return stack.length;
};

let str = 'dabAcCaCBAcCcaDA'

polymer(str); // 10