How to initialize toasts with JavaScript in Bootstrap 5?

Since the second parameter of the constructor is optional, it is unnecessary. However, if you would like to supply options, you should do so in the form of an object:

// Defaults according to the documentation
{
  animation: true,
  autohide: true,
  delay: 500
}

<!doctype html>
<html lang="en">

  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/5.0.0-alpha1/css/bootstrap.min.css">
  </head>

  <body>

    <div class="container mt-5">

      <!-- button to initialize toast -->
      <button type="button" class="btn btn-primary" id="toastbtn">Initialize toast</button>

      <!-- Toast -->
      <div class="toast">
        <div class="toast-header">
          <strong class="mr-auto">Bootstrap</strong>
          <small>11 mins ago</small>
          <button type="button" class="ml-2 mb-1 close" data-dismiss="toast">
            <span>&times;</span>
          </button>
        </div>
        <div class="toast-body">
          Hello, world! This is a toast message.
        </div>
      </div>

    </div>


    <!-- Popper.js first, then Bootstrap JS -->
    <script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/umd/popper.min.js"></script>
    <script src="https://stackpath.bootstrapcdn.com/bootstrap/5.0.0-alpha1/js/bootstrap.min.js"></script>
    <script>
      document.getElementById("toastbtn").onclick = function() {
        var toastElList = [].slice.call(document.querySelectorAll('.toast'))
        var toastList = toastElList.map(function(toastEl) {
        // Creates an array of toasts (it only initializes them)
          return new bootstrap.Toast(toastEl) // No need for options; use the default options
        });
       toastList.forEach(toast => toast.show()); // This show them
   
        console.log(toastList); // Testing to see if it works
      };

    </script>
  </body>

</html>

If you would to show only the part with a definited id this is the code

document.getElementById("toastbtn").onclick = function() {
    var myAlert =document.getElementById('toastNotice');//select id of toast
    var bsAlert = new bootstrap.Toast(myAlert);//inizialize it
    bsAlert.show();//show it
}

<!doctype html>
<html lang="en">

  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/5.0.0-alpha1/css/bootstrap.min.css">
  </head>

  <body>

    <div class="container mt-5">

      <!-- button to initialize toast -->
      <button type="button" class="btn btn-primary" id="toastbtn">Initialize toast</button>

      <!-- Toast -->
      <div id="toastNotice" class="toast">
        <div class="toast-header">
          <strong class="mr-auto">Bootstrap</strong>
          <small>11 mins ago</small>
          <button type="button" class="ml-2 mb-1 close" data-dismiss="toast">
            <span>&times;</span>
          </button>
        </div>
        <div class="toast-body">
          Hello, world! This is a toast message.
        </div>
      </div>

    </div>


    <!-- Popper.js first, then Bootstrap JS -->
    <script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/umd/popper.min.js"></script>
    <script src="https://stackpath.bootstrapcdn.com/bootstrap/5.0.0-alpha1/js/bootstrap.min.js"></script>
    <script>
      document.getElementById("toastbtn").onclick = function() {
        var myAlert =document.getElementById('toastNotice');//select id of toast
          var bsAlert = new bootstrap.Toast(myAlert);//inizialize it
          bsAlert.show();//show it
      };

    </script>
  </body>

</html>