I wanted to try solving parenthesis balance checking using map-reduce approach in JS :)

checkBalancedParsMapReduce.js

function balancedPars(str) {
  var result = 0;

  if(typeof(str) !== 'string') return false;

  if(str) {
    result = str.split('')
      .map(function(elem){
        return elem === '(' ? 1 : (elem === ')' ? -1 : 0);
      })
      .reduce(function(prev, curr, index, arr){
        return prev < 0 ? prev : prev + curr;
      });
  }

  return result === 0;
}

// Test it!
assert(balancedPars, true,  '()');
assert(balancedPars, false, ')(');
assert(balancedPars, true,  '');
assert(balancedPars, false, '(()');
assert(balancedPars, false, '())');
assert(balancedPars, true,  '(())()()');
assert(balancedPars, false, 123);
assert(balancedPars, true,  '(a + (b - c) * d)');

// Assert Help Function
function assert(fn, expected, args) {
  var args = [].slice.call(arguments, 2),
      fnName = fn.toString().replace('function ', '').split('(')[0],
      message = 'Test: ' + fnName + ' : ' + args.join(', ') + ' => ',
      result = fn.apply(this, args);

  if(result == expected) {
    console.log(message + 'OK');
  } else {
    console.error(message + 'Assertion error. Expected: ', expected, 'Got: ', result);
  }
}