Flow | Maybe type defining

flow-maybe-type.js

/**
 * `Maybe` type allows us to type annotate a potentially `null` or `undefined` value. 
 * They have the type `T|void|null` for some type `T`, meaning it is either type `T` or it is `undefined` or `null`.
 */
var message: ?string = null; //here I’m saying that message is either a `string`, or it’s `null` or `undefined`

/**
 * You can also use maybe to indicate that an object property will be either of some type `T` or `undefined`.
 *
 * By putting the `?` next to the property name for `middleInitial`, you can indicate that this field is optional.
 */
 type Person = {
  firstName: string,
  middleInitial?: string,
  lastName: string,
};