<FORM onSubmit="handler-statements" a definition of the handler ...attributes... other attributes here > form.onsubmit a reference to the handler form.onsubmit(); an explicit invocation of the handler
A typical use of the onsubmit() event handler is to check that the user's input is valid before sending it over the network to a server. If the data is not valid, the onsubmit() handler may cancel form submission by returning the value false. If the handler returns any other value, or returns no value at all then the form will be submitted normally.
Whenever you use the onsubmit() event handler to cancel the submission of a form, you should be sure to notify the user why the submission was canceled, so that they can correct the problem. Window.alert() is a useful way to provide this information to the user.