Selecting a single input value from an HTML file using only javascript

I am trying to do a form validation WITHOUT jquery. I have found this code, but I am not sure how to convert it from Jquery to plain Javascript.
I found this code here

$('.clickme').click(function() {

  alert($(this).prev().val())
})

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<input type="hidden" name="myvalue" class="form-control input_text" value="aaa" />
<input type="button" class="clickme" value="Get Value" />

<input type="hidden" name="myvalue" class="form-control input_text" value="bbb" />
<input type="button" class="clickme" value="Get Value" />

<input type="hidden" name="myvalue" class="form-control input_text" value="ccc" />
<input type="button" class="clickme" value="Get Value" />


<input type="hidden" name="myvalue" class="form-control input_text" value="ddd" />
<input type="button" class="clickme" value="Get Value" />

<input type="hidden" name="myvalue" class="form-control input_text" value="eee" />
<input type="button" class="clickme" value="Get Value" />

Any help with this would be amazing. I have multiple forms in one HTML file, all with the same input variables, and I need the functions to only run for when that specific input button is clicked.