Valid Parentheses

function Stack(tArr) {
  var arr = tArr || [];
  this.push = function(n) {
    arr.push(n);
    return this;
  };
  
  this.pop = function() {
    arr.pop();
    return this;
  };
  
  this.empty = function() {
    return arr.length==0;
  }
  
  this.top = function() {
    return arr[arr.length-1];
  }
}
function validParentheses(parens){
  var stack = new Stack();
  parens.split('').forEach(function(el) {
    if(el==')' && stack.top() != '(') {
      stack.push(el);
    } else if(el=='(') {
      stack.push(el);
    } else if(el==')') {
      stack.pop();
    }
  })
  return stack.empty();
}
Test.assertEquals(validParentheses( "()" ), true);
Test.assertEquals(validParentheses( "())" ), false);