signup1.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Signup</title>

    <style>
        *{
            margin: 0px;
            padding: 0px;
        }
        body{
            background-color:#252525;
        }

        form{
            width: 40%;
            background-color:#252525;
            margin:  10%  auto;
            padding: 5%;
        }

        input,button{
            width: 50%;
            display: block;
            margin-top: 10px;
            margin: auto;
            padding: 10px;
        }

        #signup{
            width: 55%;
            margin-top: 20px;
            background-color: red;
            color: white;
            border: none;
            
           
        }
        h1{
          text-align: center;
            color: white;
            font-size: 40px;
            margin-top: 10px;
           
        }
        p{
            text-align: center;
            color: white;
            margin-top: 30px;
            font-size: 20px;

        }
        #login{
            color:gray;
        }
        input{
            margin-bottom: 10px;
            border-radius: 7px;
            
        }
        

    </style>
</head>
<body>
    <h1>Register</h2>
    <p>Please fill in the fields below:</p>
    <div>
        <form>
            
            <input id="firstname" placeholder="First Name" type="text" >
            <input type="text"  id="lastname" placeholder="Last Name">
            <input type="email" id="email" placeholder="Email">
            <input type="password" id="password" placeholder="Password">
            <button id="signup">CREATE ACCOUNT</button>
            <p id="login">Already Have An Account? <a href="login.html">Login</a></p> 
           
        </form>
    </div>
   
</body>
</html>
<script>
    let form=document.querySelector("form");
    let userData= JSON.parse(localStorage.getItem("userData"))|| [] ; 
    form.addEventListener("submit",function(event){
        event.preventDefault() ;
        let data={
            fname:form.firstname.value,
            lname:form.lastname.value,
            email:form.email.value,
            password:form.password.value 
          
        }
        if(checkEmail(data.email)===true){
            userData.push(data)
            localStorage.setItem("userData",JSON.stringify(userData));
            window.location.href="login.html"
        }else{
            alert("Account Already Exists")
            window.location.href="login.html"
        }
       
       
    }) 
    function checkEmail(email){
        let filtered=userData.filter(function(elm){
            return email===elm.email;
        })
        if(filtered.length>0){
            return false
        }else{
            return true
        }
    }

</script>