How to Find the Duplicate Value in Array using JavaScript

While working on an ASP.Net project, we often will get a scenario to check the duplicate input value like on the textbox value. We can do it like this.

<%@ Page Title="" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true"
    CodeFile="Default3.aspx.cs" Inherits="Default3" %>

<asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" runat="Server">
    <script type="text/javascript">

    function testValues() {
        var no1 = document.getElementById('<%= TextBox1.ClientID %>').value;
        var no2 = document.getElementById('<%= TextBox2.ClientID %>').value;
        var no3 = document.getElementById('<%= TextBox3.ClientID %>').value;
        var no4 = document.getElementById('<%= TextBox4.ClientID %>').value;
        var arrInput = [no1,no2,no3,no4];
        var sorted_arr = arrInput.sort();
        var results = [];
        for (var i = 0; i < arrInput.length - 1; i++) {
            if (sorted_arr[i + 1] == sorted_arr[i]) {
                results.push(sorted_arr[i]);
            }
        }

        alert("duplicate Value:  " + results);
    }

    </script>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="Server">
    <asp:TextBox ID="TextBox1" runat="server" />
    <br />
    <asp:TextBox ID="TextBox2" runat="server" />
    <br />
    <asp:TextBox ID="TextBox3" runat="server" />
    <br />
    <asp:TextBox ID="TextBox4" runat="server" />
    <br />
    <br />
    <asp:Button ID="Button1" Text="Submit" OnClientClick="testValues();" runat="server" />
</asp:Content>

Summary

I have seen so many developers write custom validation logic using C# in a code behind file. But that is not a good practice since it will degrade the performance of the application. We should write maximum code on the client side using JavaScript.

I hope it will be helpful while implementing the custom validation in a web based project on the client side.