Ways of setting Readonly to true or false through Javascript.

1st way:-

document.getElementById("txt_description").readonly = true;

2nd way:-

document.forms[0].txt_description.setAttribute('readonly','readonly');

3rd way:-

document.forms['form1'].txt_description.setAttribute('readonly','readonly');

4th way:-

document.forms['form1'].txt_description.readonly = true;

//txt_description is defined in aspx page as:

<asp:TextBox ID="txt_description" runat="server" TextMode="MultiLine"></asp:TextBox>