JavaScript's interaction with HTML is handled through events that occur when the user or the browser manipulates a page.
When the page loads, it is called an event. When the user clicks a button, that click too is an event. Other examples include events like pressing any key, closing a window, resizing a window, etc. Events are a part of the Document Object Model (DOM) Level 3 and every HTML element contains a set of events which can trigger JavaScript Code.
Let’s see an example of onclick event in JavaScript, which is the most frequently used event type which occurs when a user clicks the left button of his mouse.
Example
You can put your validation, warning etc., against this event type.
<html> <head> <script> <!-- function sayHello() { alert("Hello World") } //--> </script> </head> <body> <p>Click the following button and see result</p> <form> <input type = "button" onclick = "sayHello()" value = "Say Hello" /> </form> </body> </html>