bracers.js

const braces = '(())';

const checkConsistance = (str) => {
  const res = str
    .split('')
    .reduce(
      (acc, current) => ({
        '(': () => { ++acc['(']; return acc; },
        ')': () => { ++acc[')']; return acc; }
      })[current](), {
        '(': 0,
        ')': 0
      }
    );
  return res['('] === res[')'];
}

const isValid = (str) => {
  if (str[0] === ')') return false;
  let res = str
    .split('')
    .reduce(
      (acc, current) => ({
        '(': () => { if (acc.multiple) acc.appendedTwice = true; acc.stack.push('0'); return acc; },
        ')': () => { if (acc.stack.length === 1) acc.multiple = true; acc.stack.pop(); return acc; }
      })[current](), {
        stack: [],
        multiple: false,
        appendedTwice: false
      }
    );

  return (res.stack.length === 0) && !res.appendedTwice;
}

console.log(checkConsistance(braces));
console.log(isValid(braces));