JSFiddle - React, Tailwind, and code Playground

HTML

<input id="fullname" type="text" />
<input id="fullname" type="text" />
<input id="street" type="text" />
<input id="postcode" type="text" />
<input id="phone" type="text" />
<input id="email" type="text" />
<button id="btn">Submit the form</button>
<ul id="error">
</ul>

JavaScript

document.getElementById("btn").onclick = function () {   
    var errorMsg = new Array();

    if(!document.getElementById("fullname").value){
        errorMsg.push("Please Enter Full Name");
    }

    if(!document.getElementById("street").value){
        errorMsg.push("Please Enter Street Name");
    }
    
    if(!document.getElementById("postcode").value){
        errorMsg.push("Please Enter Postal Code");
    }
    
    if(!document.getElementById("phone").value){
        errorMsg.push("Please Enter Phone Number");
    }
    
    if(!document.getElementById("email").value){
        errorMsg.push("Please Enter Email Id");
    }

    var messageHtml = "";
    
    errorMsg.forEach(function (message) {
        messageHtml += "<li>" + message + "</li>";
    });
    
    document.getElementById("error").innerHTML = messageHtml;
 };