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 clicks to is an event. Other examples include events like pressing any key, closing a window, resizing a window, etc.
The onclick() event occurs when a user clicks the left button of his mouse. Place your validation, warning etc., against this event type.
Example
You can try to run the following code to call a JavaScript function on a click event −
<html>
<head>
<script>
<!--
function sayHello() {
alert("Demo Alert")
}
//-->
</script>
</head>
<body>
<p>Click the following button and see result</p>
<form>
<input type="button" onclick="sayHello()" value="Click" />
</form>
</body>
</html