The setFormValue() method of the ElementInternals interface sets the element's submission value and state, communicating these to the user agent.
setFormValue(value)
setFormValue(value, state)valueA File, a string, or a FormData as the value to be submitted to the server.
state OptionalA File, a string, or a FormData representing the input made by the user. This allows the application to re-display the information that the user submitted, in the form that they submitted it, if required.
Note: In general, state is used to pass information specified by a user, the value is suitable for submission to a server, post sanitization. For example, if a custom element asked a user to submit a date, the user might enter "3/15/2019". This would be the state. The server expects a date format of 2019-03-15, the date in this format would be passed as the value.
Undefined.
NotSupportedError DOMExceptionThrown if the element does not have its formAssociated property set to true.
In the following example, a checkbox custom element sets on as the value to send to the server, and checked as the state.
this.internals_.setFormValue("on", "checked");