Mock event callback

scan.spec.ts

import { scan } from '../scan';
import { BleManager, Device } from 'react-native-ble-plx';
import {
  BleErrorCodeMessage,
  BleError,
} from 'react-native-ble-plx/src/BleError';
let cb: Parameters<BleManager['startDeviceScan']>[2] = null;
jest.mock('react-native-ble-plx', () => ({
  BleManager: () => ({
    startDeviceScan: jest.fn(((_, __, receivedCb) => {
      cb = receivedCb;
    }) as BleManager['startDeviceScan']),
  }),
}));
it('should log the callback param', () => {
  scan();
  // send a scanned device
  cb(null, { id: 'DEVICE_A' } as Device);
  // send an error
  cb(new BleError('Something', BleErrorCodeMessage), null);

  // expect
  // ...
});

scan.ts

import { BleManager } from 'react-native-ble-plx';
export const scan = () => {
  const bleManager = new BleManager();
  bleManager.startDeviceScan(null, null, (error, scannedDevice) => {
    console.log('ERROR', error);
    console.log('SCANNED DEVICE', scannedDevice);
  });
};