The first problem is preventing the page from loading and to do that, we pass the event object and use the preventDefault() method to stop the reload. Once that is done we grab the values of the input elements and use innerHTML to display it inside the

element with the help of the fancy template literals.

Here's the button

<button onClick="handleSubmit(event)">Submit</button>

Here's the JavaScript code

function handleSubmit (e) {
  e.preventDefault()
  let name = document.getElementById("fname").value
  let age = document.getElementById("age").value
  let email = document.getElementById("email").value
  document.getElementById("demo").innerHTML = `Name is : ${name} Age is: ${age} 
  and Email is : ${email}`;
}

Here's the code pen for this

Code Pen

I would not recommend implementing my answer as it is very unpolished and might cause problems in the long run, but I hope it helps.