brackets.js

// var input = '))';
var input = '(())))((';

var opening = '(';
var closing = ')';
var closingToEnd = function(array, pointer) {
  var newArr = array.slice(pointer);
  return newArr.filter(function(el) {
    return el === closing;
  }).length;
};

var openingSinceBeginning = function(array, pointer) {
  var newArr = array.slice(0, pointer);
  return newArr.filter(function(el) {
    return el === opening;
  }).length;
};

input = input.split('');

for (var i = 0; i < input.length+1; i++) {
  if (closingToEnd(input, i) === openingSinceBeginning(input, i)) {
    console.log('ELO! ', i);
    break;
  }
}