// Examples: 
// '()()' - balanced
// '((()))' - balanced
// '((()' - not balanced
// ')(' - not balanced
// ')()(' - not balanced

function balancedParens(string) {
  // turn string into an array
  // apply reduce method
  // add 1 to the counter for '(' and subtract 1 from the counter for ')'
  // test for characters other than parens in the string
  // in the end if counter is 0, it is balanced.
  return !string.split('').reduce((prev, char) => {
    if (prev < 0) return prev;
    if (char === '(') return ++prev;
    if (char === ')') return --prev;
    return prev;
  }, 0);
}

console.log(balancedParens(')(')); // false
console.log(balancedParens('()()')); // true

Thanks to Stephen Grider.