TypeScript undefined coercing

Question

I was writing a Redux reducer when I found that this code:

export function users(state: { myself: IUser | undefined, users: IUser[] } = { myself: undefined, users: [] }, action: UserActions) {
  const mself = state.myself;
  if (action.type === EMyselfAction.CHANGE_MYSELF_CONFIG && state.myself !== undefined) {
    return {
      myself: myself(mself, action),
      users: state.users.map((u) => user(u, action, state.myself.id)),
    };
  }
  ...
}

Is outputting the error Object is possibly 'undefined':

How can be state.myself undefined if it is already checked that it is not?

Answer 1

I think that's because state.myself is in a different scope.

export function users(state: { myself: IUser | undefined, users: IUser[] } = { myself: undefined, users: [] }, action: UserActions) {
  const mself = state.myself;
  if (action.type === EMyselfAction.CHANGE_MYSELF_CONFIG && state.myself !== undefined) {
    state.myself // (property) myself: IUser

    const foo = () => {
       state.myself // but here is (property) myself: IUser | undefined
    }
    // ...
  }
  ...
}

I don't think you can guarantee otherwise, since you can use it somewhere else (not only in that branch where state.myself !== undefined). So you could do this:

export function users(state: { myself: IUser | undefined, users: IUser[] } = { myself: undefined, users: [] }, action: UserActions) {
  const mself = state.myself;
  if (action.type === EMyselfAction.CHANGE_MYSELF_CONFIG && state.myself !== undefined) {
    const id = state.myself.id;
    return {
      myself: myself(mself, action),
      users: state.users.map((u) => user(u, action, id)),
    };
  }
  ...
}