How to set focus and set the text in input text using createEvent and dispatchEvent?

I'm trying to write a code in Javascript, which should set the cursor focus on the input textbox and should trigger a "keypress" event which should set key, say letter 'A' in the textbox.

How to do it? I read we can achieve using createEvent and dispatchEvent. Any Suggestions?

Try this

<input type="text" id="input-field" />

<script>

var addData = function(data){
var field = document.getElementById("input-field")
field.value = document.getElementById("input-field").value+data;

var keyboardEvent = document.createEvent("KeyboardEvent");
var initMethod = typeof keyboardEvent.initKeyboardEvent !== 'undefined' ?   "initKeyboardEvent" : "initKeyEvent";


keyboardEvent[initMethod](
               "keypress", // event type : keydown, keyup, keypress
                true, // bubbles
                true, // cancelable
                window, // viewArg: should be window
                false, // ctrlKeyArg
                false, // altKeyArg
                false, // shiftKeyArg
                false, // metaKeyArg
                40, // keyCodeArg : unsigned long the virtual key code, else 0
                0 // charCodeArgs : unsigned long the Unicode character associated with the depressed key, else 0
);
field.dispatchEvent(keyboardEvent);

}

 var registerEvents = function(){
 document.getElementById("input-field").addEventListener("keypress",function(event){
    alert("Key Pressed : "+ event.target.value)
 });
}();
addData("a");
addData("b");

</script>

Anyways Entered key will be available in text field, no need to set it explicitly.