An example of a selector.

todoSelector.js

import { createSelector } from 'reselect';

// sub-selectors
const allTodoSelector = state => state.todo.todos;  // an array of todos
const filterSelector = (_, props) => props.params.filter;  // a url parameter

// a selector
const todoSelector = createSelector(
  [allTodoSelector, filterSelector],
  (todos, filter = 'all') => {
    switch (filter) {
      case 'all':
        return todos;

      case 'active':
        return todos.filter(todo => !todo.done);

      case 'done':
        return todos.filter(todo => todo.done);

      default:
        return todos;
    }
  },
);

export default todoSelector;