asp: textbox readonly

In asp file I have two asp:textbox

<asp:TextBox ID="textValue" runat="server" Width="100px"/>
<asp:TextBox ID="textValue2" runat="server" Width="100px" ReadOnly="true"/>

then I set the value via javascript getting

<asp:TextBox ID="textValue" runat="server" Width="100px" value="aaa"/>
<asp:TextBox ID="textValue2" runat="server" Width="100px" ReadOnly="true" value="bbb"/>

but when refresh the webpage finally get

<asp:TextBox ID="textValue" runat="server" Width="100px" value="aaa"/>
<asp:TextBox ID="textValue2" runat="server" Width="100px" ReadOnly="true"/>

Why the value bbb is "lost"? How can I avoid this?