为什么此Ajax代码会延迟？

I'm trying to make searching address form.

This is the structure and methods.

search_address.html :

<form id="search_address_form" onSubmit="sendAddress(); return false;">
    <input id="input_address" class="input_address" name="input_address" type="text" placeholder="example: blahblahblah"/>
</form>
<div id="addressList_layer" class="addressList_layer">

<script>
    input_address = document.getElementById('input_address');
    input_address.addEventListener('keydown',function(){    
       sendAddress();    
    });

    function sendAddress(){
        xhrDocOpen('./php/search_address.php?searchingFor='+inputAddress_data.value,'addressList_layer','get');
        return false;
    }
</script>

search_address.php :

<?php
include_once('./config.php');
extract($_GET);
echo ($searchingFor);
?>

This code works fine.

But it runs with little bit delayed...

I mean, for example,

input_address.value : "1"
addressList_layer.value : ""

input_address.value : "123"
addressList_layer.value : "12"

input_address.value : "123456"
addressList_layer.value : "12345"

input_address.value : "123abc"
addressList_layer.value : "123ab"

Like this, addressList_layer show always one step delayed than input_address.

I don't know why...

Is there anyone who can guess the reason of this situation and give me the solution?

ps. xhrDocOpen() is...

var xhr = new XMLHttpRequest();
function xhrDocOpen(doc,placeID,method){
    xhr.onreadystatechange=function(){
        if(xhr.readyState==4 && xhr.status==200){
            document.getElementById(placeID).innerHTML=xhr.responseText;
        }
    xhr.open(method,doc,true);
    xhr.send('');
}

This is usual issue if you trying to use "keydown" event.

Just try to change it to "keyup" and it should work.