There are  basically three types of pop-up boxes in javascript :
- Alert –>> it just displays a message with an OK button.
        <!DOCTYPE html>
        <html>
       <body>
       <p>Click the button to display an alert box.</p>
      <button onclick="myFunction()">Click me</button>
     <script>
    function myFunction() {
           alert("Hello! Welcome to edureka!");
       }
   </script>
      </body>
     </html>
    2 .Confirm –>> it pops up a confirmation message window with OK and Cancel button.
              
<!DOCTYPE html>
<html>
<body>
<p>Click the button to display a confirm box.</p>
<button onclick="myFunction()">Click me</button>
<script>
function myFunction() {
  confirm("Press a button!");
}
</script>
</body>
</html>
3.Prompt –>> it pops up a dialog box asking user input followed confirmation buttons.
<!DOCTYPE html>
<html>
<body>
<p>Click the button to demonstrate the prompt box.</p>
<button onclick="myFunction()">Click me</button>
<p id="demo"></p>
<script>
function myFunction() {
  var person = prompt("Please enter your name", "Edureka");
  if (person != null) {
    document.getElementById("demo").innerHTML =
    "Hello " + person + "! How are you today?";
  }
}
</script>
</body>
</html>