customers.actions.js

import someAPI from '../services/someAPI';

export const GET_CUSTOMER = 'GET_CUSTOMER';

export const getCustomer = customerId => ({
  type: GET_CUSTOMERS,
  payload: someAPI.get(`customers/${customerId}`)
});

customers.reducer.js

import { GET_CUSTOMER } from '../actions/customers.actions';

const initialState = {
  loading: false,
  error: false,
  detail: {}
};

export default (state = initialState, action) => {
  switch (action.type) {
    case `${GET_CUSTOMER}_FULFILLED`:
      state.detail: action.payload.data
      return state
    // outros cases ...
    default:
      return state;
  }
};