Question: I have the following trivial page: 

```
<html>  
<head>  
<script type="text/javascript">  
    function check(){  
        var cb = document.getElementById('aaa');  
        if(!cb.checked()){  
            alert("test!!!");  
        }  
    }  
</script>  
</head>  
<body>  
<input id="aaa" type="checkbox" name="modify" onclick="check()">Modify</input>  
</body>  
</html>  

```

If I press the check box I see the following error in the javascript console: 

```
Uncaught TypeError: Property 'checked' of object #<HTMLInputElement> is not a function  
check   
onclick    

```

I also tried with `documents.getElementsByName('modify')` and the same problem.  

What am I doing wrong here?

Answer: 
`checked` isn't a function, as the error message states.

Use

```
if(!cb.checked){  

```

(without the parenthesis)

Note that you don't have to look for the element using `getElementById`, you can pass the element :

```
<input id="aaa" type="checkbox" name="modify" onclick="check(this)">Modify  

function check(cb){  
    if(!cb.checked()){  
        alert("test!!!");  
    }  
} 

```

Note also that the `input` element should not be closed. What you probably need here is the label `element`, which lets you click on the text to (un)check the box :

```
 <input id="aaa" type="checkbox" name="modify" onclick="check(this)">
 <label for=aaa>Modify</label> 

```
remove the brackets from cb.checked() - it is not a function.
This code should work:

```
<html>  
<head>  
<script type="text/javascript">  
    function check(){  
        var cb = document.getElementById('aaa');  
        if(!cb.checked){  
            alert("test!!!");  
        }  
    }  
</script>  
</head>  
<body>  
<input id="aaa" type="checkbox" name="modify" onclick="check()">Modify</input>  
</body>  
</html>  

```