//You can use the charCodeAt function to achieve this.
//example
//js file
function showKeyCode () {
var character = document.getElementById("character").value.substring(0, 1);
var code = document.getElementById("character").value.charCodeAt(0);
var msg = "The Key Code for the "" + character + "" character is " + code + ".";
alert(msg);
}
//html file
<input type="text" id="character" size="15">
<input type="button" value="Show Key Code" onclick="showKeyCode();">