Set ASP.Net TextBox height automatically as on type in JavaScript

Hi George616,

Refer below sample code.

HTML

<asp:TextBox ID="textBox1" runat="server" TextMode="MultiLine" Style="overflow: hidden; resize: none;" oninput="Resize(this)" />
<script type="text/javascript">
    function Resize(textbox) {
        textbox.style.height = "";
        textbox.style.height = Math.min(textbox.scrollHeight, 300) + "px";
    }
</script>