Optional arbitrary data to send to the client, data this is a string will
be sent unmodified, otherwise JSON.parse() will be used to serialize the
value.
optional
id: number
An optional id which will be sent with the event and exposed in the
client EventSource.
optional
replacer: (string | number)[] | ((
this: any,
key: string,
value: any,
) => any)
The replacer is passed to JSON.stringify when converting the data
property to a JSON string.
optional
space: string | number
Space is passed to JSON.stringify when converting the data property
to a JSON string.