Adds an event handler that runs when an element's value is changed.
An element receives a change event when a site visitor changes the value
of an element.
A change event is not triggered when you change an element's value using
the element's value property.
Because onChange() is fired before validations are performed, do not check
any of the validation properties in the onChange() event handler. For example,
do not code an if statement that checks the valid property in onChange()
because the property at this point contains values prior to the onChange() event.
handler(event: Event): void The name of the function or
the function expression to run when the element's value changes.