<input maxlength="5"> <input type="button" value="Show input"> <script> var inputs = document.getElementsByTagName('input') inputs[1].onclick = function(){ alert(inputs[0].value) } </script>
<input maxlength="5"> <script> var input = document.getElementsByTagName('input') input[0].onkeyup = function(){ if(this.value.length == 5) alert(this.value) } </script>