How to Find the Duplicate Value in Array using JavaScript

In this blog you will learn how to find the duplicate value in an 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.