What are selectors in redux?

Question

I am trying to follow this code in redux-saga

export const getUser = (state, login) => state.entities.users[login] export const getRepo = (state, fullName) => state.entities.repos[fullName] 

Which is then used in the saga like this:

import { getUser } from '../reducers/selectors'  // load user unless it is cached function* loadUser(login, requiredFields) {   const user = yield select(getUser, login)   if (!user || requiredFields.some(key => !user.hasOwnProperty(key))) {     yield call(fetchUser, login)   } } 

This getUser reducer (is it even a reducer) looks very different from what I would normally expect a reducer to look like.

Can anyone explain what a selector is and how getUser is a reducer and how it fits in with redux-saga?

Answer 1

getUser is not a reducer, it is indeed a selector, that is, a function that knows how to extract a specific piece of data from the store.

Selectors provide an additional layer such that if you altered your store structure and all of a sudden your users were no longer at state.entities.users but instead at state.users.objects.entities (or whatever) then you only need to update the getUser selector and not every place in your app where you were making a reference to the old location.

That makes them particularly handy when it comes to refactoring your Redux store.