asp.net执行JS函数后TextBox颜色值消失？

Question

当通过下面的JS函数设置属性时，TexBox的颜色属性消失（闪烁）。请指教，如何保留颜色属性?

（即单击按钮后，颜色（奇数为绿色，偶数为红色）闪烁，颜色变回空白）

function numbercheck() {
  var num = document.getElementById("txtnum").value;

  if (num == "") {
    alert("Please Enter number")
  } else {
    if (num % 2 == 0) {
      alert("You have entered even number");
      document.getElementById("txtnum").style.backgroundColor = "green";
    } else {
      alert("You have enetered Odd number");
      document.getElementById("txtnum").style.backgroundColor = "red";
    }
  }
}

<p> Please enter a Number :
  <asp:TextBox ID="txtnum" runat="server"></asp:TextBox>
</p>
<asp:Button Text="Check Number" ID="btnnum" runat="server" OnClientClick="return numbercheck()" />

Answer 1

    Please change your script function as
      function numbercheck() {
                debugger;
                var num = document.getElementById("<%=txtnum.ClientID%>").value;

                if (num == "") {
                    alert("Please Enter number")
                } else {
                    if (num % 2 == 0) {
                        alert("You have entered even number");
                        document.getElementById("<%=txtnum.ClientID%>").style.backgroundColor = "green";
                    } else {`enter code here`
                        alert("You have enetered Odd number");
                        document.getElementById("<%=txtnum.ClientID%>").style.backgroundColor = "red";
                    }
                }
            }

and 
<asp:Button Text="Check Number" ID="btnnum" runat="server" OnClientClick="return numbercheck()"  CausesValidation="false" UseSubmitBehavior="false" />

asp.net执行JS函数后TextBox颜色值消失？

Colour value of TextBox vanishes after JS function is executed in asp.net?

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