InterventionReportBody.message

The message read-only property of the InterventionReportBody interface returns a human-readable description of the intervention, including information such as how the intervention could be avoided. This typically matches the message a browser will display in its DevTools console when an intervention is imposed, if one is available.

let options = {
  types: ['intervention'],
  buffered: true
}

let observer = new ReportingObserver(function(reports, observer) {
  let firstReport = reports[0];
  console.log(firstReport.type); // intervention
  console.log(firstReport.body.message);
}, options);