How to use the mongodb-core.ReadPreference.primary function in mongodb-core

// Apply write concern to command
  command = writeConcern(command, self, options);

  // Use node md5 generator
  var md5 = crypto.createHash('md5');
  // Generate keys used for authentication
  md5.update(username + ":mongo:" + password);
  var userPassword = md5.digest('hex');

  // No password
  if(typeof password == 'string') {
    command.pwd = userPassword;
  }

  // Force write using primary
  commandOptions.readPreference = CoreReadPreference.primary;

  // Execute the command
  self.command(command, commandOptions, function(err, result) {
    if(err && err.ok == 0 && err.code == undefined) return handleCallback(callback, {code: -5000}, null);
    if(err) return handleCallback(callback, err, null);
    handleCallback(callback, !result.ok ? toError(result) : null
      , result.ok ? [{user: username, pwd: ''}] : null);
  })
}

var executeCommand = function(self, command, options, callback) {
  // Did the user destroy the topology
  if(self.serverConfig && self.serverConfig.isDestroyed()) return callback(new MongoError('topology was destroyed'));
  // Get the db name we are executing against
  var dbName = options.dbName || options.authdb || self.s.databaseName;

  // If we have a readPreference set
  if(options.readPreference == null && self.s.readPreference) {
    options.readPreference = self.s.readPreference;
  }

  // Convert the readPreference if its not a write
  if(options.readPreference) {
    options.readPreference = convertReadPreference(options.readPreference);
  } else {
    options.readPreference = CoreReadPreference.primary;
  }

  // Debug information
  if(self.s.logger.isDebug()) self.s.logger.debug(f('executing command %s against %s with options [%s]'
    , JSON.stringify(command), f('%s.$cmd', dbName), JSON.stringify(debugOptions(debugFields, options))));

  // Execute command
  self.s.topology.command(f('%s.$cmd', dbName), command, options, function(err, result) {
    if(err) return handleCallback(callback, err);
    if(options.full) return handleCallback(callback, null, result);
    handleCallback(callback, null, result.result);
  });
}

Db.prototype.listCollections = function(filter, options) {
  filter = filter || {};
  options = options || {};

  // Shallow clone the object
  options = shallowClone(options);
  // Set the promise library
  options.promiseLibrary = this.s.promiseLibrary;

  // Ensure valid readPreference
  if (options.readPreference) {
    options.readPreference = convertReadPreference(options.readPreference);
  } else {
    options.readPreference = this.s.readPreference || ReadPreference.primary;
  }

  // We have a list collections command
  if (this.serverConfig.capabilities().hasListCollectionsCommand) {
    // Cursor options
    var cursor = options.batchSize ? { batchSize: options.batchSize } : {};
    // Build the command
    var command = { listCollections: true, filter: filter, cursor: cursor };
    // Set the AggregationCursor constructor
    options.cursorFactory = CommandCursor;
    // Create the cursor
    cursor = this.s.topology.cursor(f('%s.$cmd', this.s.databaseName), command, options);
    // Do we have a readPreference, apply it
    if (options.readPreference) {
      cursor.setReadPreference(options.readPreference);
    }

var executeCommand = function(self, command, options, callback) {
  // Did the user destroy the topology
  if(self.serverConfig && self.serverConfig.isDestroyed()) return callback(new MongoError('topology was destroyed'));
  // Get the db name we are executing against
  var dbName = options.dbName || options.authdb || self.s.databaseName;

  // If we have a readPreference set
  if(options.readPreference == null && self.s.readPreference) {
    options.readPreference = self.s.readPreference;
  }

  // Convert the readPreference if its not a write
  if(options.readPreference) {
    options.readPreference = convertReadPreference(options.readPreference);
  } else {
    options.readPreference = CoreReadPreference.primary;
  }

  // Debug information
  if(self.s.logger.isDebug()) self.s.logger.debug(f('executing command %s against %s with options [%s]'
    , JSON.stringify(command), f('%s.$cmd', dbName), JSON.stringify(debugOptions(debugFields, options))));

  // Execute command
  self.s.topology.command(f('%s.$cmd', dbName), command, options, function(err, result) {
    if(err) return handleCallback(callback, err);
    if(options.full) return handleCallback(callback, null, result);
    handleCallback(callback, null, result.result);
  });
}

// Get additional values
  const maxTimeMS = typeof options.maxTimeMS === 'number' ? options.maxTimeMS : null;

  // Add maxTimeMS to options if set
  if (maxTimeMS != null) commandOptions.maxTimeMS = maxTimeMS;

  // Build the command to execute
  let command = {
    dropUser: username
  };

  // Apply write concern to command
  command = applyWriteConcern(command, { db }, options);

  // Force write using primary
  commandOptions.readPreference = ReadPreference.primary;

  // Execute the command
  executeCommand(db, command, commandOptions, (err, result) => {
    if (err && !err.ok && err.code === undefined) return handleCallback(callback, { code: -5000 });
    if (err) return handleCallback(callback, err, null);
    handleCallback(callback, null, result.ok ? true : false);
  });
}

var executeCommand = function(self, command, options, callback) {
  // Did the user destroy the topology
  if(self.serverConfig && self.serverConfig.isDestroyed()) return callback(new MongoError('topology was destroyed'));
  // Get the db name we are executing against
  var dbName = options.dbName || options.authdb || self.s.databaseName;

  // If we have a readPreference set
  if(options.readPreference == null && self.s.readPreference) {
    options.readPreference = self.s.readPreference;
  }

  // Convert the readPreference if its not a write
  if(options.readPreference) {
    options.readPreference = convertReadPreference(options.readPreference);
  } else {
    options.readPreference = CoreReadPreference.primary;
  }

  // Debug information
  if(self.s.logger.isDebug()) self.s.logger.debug(f('executing command %s against %s with options [%s]'
    , JSON.stringify(command), f('%s.$cmd', dbName), JSON.stringify(debugOptions(debugFields, options))));

  // Execute command
  self.s.topology.command(f('%s.$cmd', dbName), command, options, function(err, result) {
    if(err) return handleCallback(callback, err);
    if(options.full) return handleCallback(callback, null, result);
    handleCallback(callback, null, result.result);
  });
}

// Apply write concern to command
  command = writeConcern(command, self, options);

  // Use node md5 generator
  var md5 = crypto.createHash('md5');
  // Generate keys used for authentication
  md5.update(username + ":mongo:" + password);
  var userPassword = md5.digest('hex');

  // No password
  if(typeof password == 'string') {
    command.pwd = userPassword;
  }

  // Force write using primary
  commandOptions.readPreference = CoreReadPreference.primary;

  // Execute the command
  self.command(command, commandOptions, function(err, result) {
    if(err && err.ok == 0 && err.code == undefined) return handleCallback(callback, {code: -5000}, null);
    if(err) return handleCallback(callback, err, null);
    handleCallback(callback, !result.ok ? toError(result) : null
      , result.ok ? [{user: username, pwd: ''}] : null);
  })
}

// Apply write concern to command
  command = writeConcern(command, self, options);

  // Use node md5 generator
  var md5 = crypto.createHash('md5');
  // Generate keys used for authentication
  md5.update(username + ":mongo:" + password);
  var userPassword = md5.digest('hex');

  // No password
  if(typeof password == 'string') {
    command.pwd = userPassword;
  }

  // Force write using primary
  commandOptions.readPreference = CoreReadPreference.primary;

  // Execute the command
  self.command(command, commandOptions, function(err, result) {
    if(err && err.ok == 0 && err.code == undefined) return handleCallback(callback, {code: -5000}, null);
    if(err) return handleCallback(callback, err, null);
    handleCallback(callback, !result.ok ? toError(result) : null
      , result.ok ? [{user: username, pwd: ''}] : null);
  })
}

var executeCommand = function(self, command, options, callback) {
  // Did the user destroy the topology
  if(self.serverConfig && self.serverConfig.isDestroyed()) return callback(new MongoError('topology was destroyed'));
  // Get the db name we are executing against
  var dbName = options.dbName || options.authdb || self.s.databaseName;

  // If we have a readPreference set
  if(options.readPreference == null && self.s.readPreference) {
    options.readPreference = self.s.readPreference;
  }

  // Convert the readPreference if its not a write
  if(options.readPreference) {
    options.readPreference = convertReadPreference(options.readPreference);
  } else {
    options.readPreference = CoreReadPreference.primary;
  }

  // Debug information
  if(self.s.logger.isDebug()) self.s.logger.debug(f('executing command %s against %s with options [%s]'
    , JSON.stringify(command), f('%s.$cmd', dbName), JSON.stringify(debugOptions(debugFields, options))));

  // Execute command
  self.s.topology.command(f('%s.$cmd', dbName), command, options, function(err, result) {
    if(err) return handleCallback(callback, err);
    if(options.full) return handleCallback(callback, null, result);
    handleCallback(callback, null, result.result);
  });
}