reducers

Redux reducers and selectors - /src/reducers

This directory contains all of the reducers that are used to store state within this app. Reducers all accept some part of the store's state, and an action that is passed through a switch. This action is Flow typed as a union of all action objects, but Flow can understand the switch statements and will correctly infer which action is being used inside of the reducers. State is assumed to be immutable so that strict equality checks can tell when new bits of state are different.

How to write a reducer

const isThisTrueOrFalse: Reducer<boolean> = (state = false, action) => {
  switch (action.type) {
    case 'CHANGE_TO_TRUE':
      return true;
    case 'CHANGE_TO_FALSE':
      return false;
    default:
      return state;
  }
};

This simplistic reducer shows how to write a reducer for this project. Use a function that is typed as the Reducer<State> type. The generic type takes a single type parameter, the type definition for the state. This type then enforces that the function being set follows that type signature, so no types are needed on the arguments or return type. The action argument is correctly typed to be the Action type, and the state and return value should be correctly set to the State value.

Selectors

See src/selectors) for how this state is accessed.

Flow typing

The state is fully Flow typed and the definitions live in the src/types/state.js file.