Interface representing an FCM legacy API data message payload. Data messages let developers send up to 4KB of custom key-value pairs. The keys and values must both be strings. Keys can be any custom string, except for the following reserved strings:

* "from" * Anything starting with "google.".

See [Build send requests](/docs/cloud-messaging/send-message) for code samples and detailed documentation.

Signature:

export interface DataMessagePayload