Събитие на бутоните

document.getElementById("myBtn").addEventListener("click", function() {
  alert("Hello World!");
});

<button onclick="myFunction()">Click me</button>

<p id="demo"></p>

<script>
function myFunction() {
  document.getElementById("demo").innerHTML = "Hello World";
}
</script>

// EXAMPLE: 
document.querySelector(`.check`).addEventListener(`click`, 
   function () {
      console.log(document.querySelector(`.guess`).value;
   }
);

// SYNTAX:
//  <element-select>.addEventListener(`click`, 
//     function () {
//       <what-you-want-to-execute-when-clicking-on-element>
//     }
//  );

import java.awt.*;// import functions 
import java.awt.event.*;

public class buttonevent extends Frame implements ActionListener {
    int num;
    Button b1;
    TextField t1;

    public buttonevent() {//constructor
        setLayout(new FlowLayout());

        b1 = new Button("click me");//button named "click me" is created.
        t1 = new TextField(20);

        add(b1);//that button is added to the frame by add function.
        add(t1);

        b1.addActionListener(this);

    }

    public void actionPerformed(ActionEvent ae) {//event function
        String cmd = ae.getActionCommand();
        if (num == 0) {
            System.out.println(cmd);
        }
        if (cmd.equals("click me")) {
            num++;
        }
        if ((num % 2) != 0) {
            t1.setText("Hello Buddy!");
        }
        if ((num % 2) == 0) {
            t1.setText(" ");
            System.out.println(num);
        }
    }

    public static void main(String args[]) {//main function
        buttonevent ob = new buttonevent();
        ob.setSize(500, 500);
        ob.setVisible(true);
        ob.setTitle("button Event Handling");
    }
}