<html>
<body>
<input type='button' value='F3' class="c2" id="btn_1">
<input type='button' value='F3' class="c3" id="btn_2">
<input type='button' value='F1' class="c2" id="btn_3">
<input type='submit' value='F2' class="c1" id="btn_4">
<input type='submit' value='F1' class="c3" id="btn_5">
<input type='submit' value='F2' class="c1" id="btn_6">
<br/>
<br/>
<button onclick="myFunction()">Try it</button>
<script>
function myFunction()
{
var arrFiltered = document.querySelectorAll('input');
arrFiltered.forEach(function (el)
{
var node = document.createElement("p");
node.innerHTML = el.getAttribute('id');
window.document.body.appendChild(node);
});
}
</script>
</body>
</html>