Callbacks
WebSerial comes with a single callback called onMessage. It's responsible for notifying your firmware whenever it has received a command/message from user via webserial terminal.
If the callback is not registered, then the command received from terminal is simply discarded.
Example Usage
...
#include <WebSerial.h>
void setup() {
  ...
  /* Attach Message Callback */
  WebSerial.onMessage([&](uint8_t *data, size_t len) {
    Serial.printf("Received %lu bytes from WebSerial: ", len);
    Serial.write(data, len);
    Serial.println();
  });
}
void loop() {
  ...
}