<input type="button" value="button-text" onclick="handler"> button.onclick
The onclick property of a Button object refers to an event handler function that is invoked when the user clicks on the button. See HTMLElement.onclick for complete details. Note, however, that Button.onclick has been supported since JavaScript 1.0, unlike the generalized HTMLElement.onclick handler.
HTMLElement.onclick; Chapter 19; EventListener, EventTarget, and MouseEvent in the DOM reference section