example.js

return new Promise((resolve, reject) => {
  // Get current state (required, otherwise requestLocationAuthorization
  // callback is triggered before change)
  cordova.plugins.diagnostic.getLocationAuthorizationStatus(state => {
    let res = validateState(state);

    if (res.code == 0) {
      return resolve();
    } else if (res.code == 1) {
      return reject(res);
    } else if (res.code == 2) {
      triggerPermissionPrompt();
    }

  }, err);

  function triggerPermissionPrompt() {
    cordova.plugins.diagnostic.requestLocationAuthorization(state => {
      let res = validateState(state);

      // At this point, if it's anything other than
      // granted, user must change manually
      if (res.code == 0) {
        return resolve();
      } else {
        return reject(res);
      }

    }, err, cordova.plugins.diagnostic.locationAuthorizationMode.ALWAYS);
  }

  function err(msg) {
    console.log(`Error requesting location: ${msg}`);
    return reject({ code: 3 });
  }

  // Check value of custom constants
  // 0: permission granted, resolve
  // 1: permission must be manually changed, reject
  // 2: not requested yet
  function validateState(state) {
    switch(state) {
      case cordova.plugins.diagnostic.permissionStatus.GRANTED:
        return { code: 0 }

      case cordova.plugins.diagnostic.permissionStatus.DENIED:
      case cordova.plugins.diagnostic.permissionStatus.GRANTED_WHEN_IN_USE:
        return { code: 1 }

      case cordova.plugins.diagnostic.permissionStatus.NOT_REQUESTED:
        return { code: 2 }
    } 
  }
})