how to enable or disable button while entering the textbox

what????

no

javascript:-

window.onload = function () {

var txt = document.getElementById("<%= txtSearch.ClientID %>");
var btn = document.getElementById("<%= BtnClearFilter.ClientID %>");
if (txt.value === "") {
btn.style.visibility = "hidden";
// when the window is loaded, hide the button if the textbox is empty
}
txt.onkeyup = function () {
if (txt.value !== "") {
btn.style.visibility = "visible";
} else {
btn.style.visibility = "hidden";
}
};
var clearFilterBtn = document.getElementById("<%= BtnClearFilter.ClientID %>");
clearFilterBtn.onclick = function () {
btn.style.visibility = "hidden";
}
};

Button:
<asp:Button ID="btnSearch" runat="server" Text="" CssClass="ButtonSearch6" EnableTheming="false" />

<asp:Button ID="BtnClearFilter" runat="server" OnClick="BtnClearFilter_Click" EnableTheming="false" Text="" CssClass="ClearFilter" />

here using this above script and then i have two button.one for search and another for clearbutton.

if i run my page it works fine ,but after i searching any one record then it will not fire,it always disable.