JavaScript-text input validation not work

I have a form

<form action="" method="post" onsubmit="return validate()" >
    <div class="form-group ">
        <div class="col-xs-3">
            <input type="text" class="form-control input-sm " name="s">
        </div>
    </div>
    <div class="container" id="btn">
        <button type="submit" class="btn btn-primary btn-default ">submit</button>
        <a class="btn btn-default btn-lg" href="">back</a>
    </div>
</form>

i try to do validation with JavaScript,but my JavaScript is not work. The form will submit without validation.

  <script type="text/javascript">
        function validate() {
            var x = document.forms["input"]["s"].value;
            if(x == null || x == "") {
                alert('Please enter something!');
                return false;
            }
            else 
                return confirm('confirm submit?');
        }
 </script>

How can i fix the problem?

jsfiddle