balanced.js

<script type="text/javascript">
  function balancedParens(expr){
    var tokens={
        "(":")", 
        "[":"]", 
        "{":"}" };
  
    var beginners=[];
    
    var finishers=[
        ")",
        "]",
        "}"];
    
    for (var i=0;i<expr.length;i++){
      var chr=expr[i];
      
      if (tokens[chr]){
        beginners.Add(chr);
      }
      else if (finishers.Has(chr))
      {
        if (tokens[beginners.GetLast()] != chr){
          return false;
        }
      }
    }
    
    return beginners.length == 0;
  }
  
  Array.prototype.Has=function(what){
    return this.indexOf(what) != -1;
    };
  Array.prototype.Add=function(what){
    return this.push(what);
    };
  Array.prototype.GetLast=function(what){
    return this.pop();
    };

  function assert(what, expected){
    if (what != expected){
      throw new Error(what + " is different from " + expected);
      alert("error!!");
    }
  }
  
  assert(balancedParens("("), false);
  assert(balancedParens("()"), true);
  assert(balancedParens(")("), false);
  assert(balancedParens("(())"), true);
  assert(balancedParens("[](){}"), true);
  assert(balancedParens("[({})]"), true);
  assert(balancedParens("[(]{)}"), false);
  assert(balancedParens("var supYo  = { hey: dog() }"), true);
  assert(balancedParens("var fiddle = function() { doggy.eat();"), false);

</script>