Create a redux module for infinitely loading editable lists

src/redux/infinite-list.js

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+/**
+ * @typedef ReduxModule
+ * A redux "module" for reusable functionality. The module exports
+ * a reducer function, a set of action creators and a selector
+ * that are all scoped to the given "key". This allows us to reuse
+ * this reducer multiple times in the same redux store.
+ *
+ * @property {string} key The key to use when registering this
+ *      modules reducer in the redux state tree.
+ * @property {function} selector Function called with the full
+ *      state tree to select only this modules slice of the state.
+ * @property {object} actions An object of action creator functions
+ *      to call to make changes to the data in this reducer.
+ * @property {function} reducer A redux reducer that takes an action
+ *      from the action creators and the current state and returns
+ *      the next state.
+ */
+
+/**
+ * @typedef {function} InfiniteListFetcher
+ * A function to call that returns more data for the InfiniteList
+ * loadMore action. It must resolve to {items: [], moreToLoad} or
+ * reject with the error {statusCode}.
+ * @returns {Promise<{items:[], moreToLoad:boolean}>}
+ */
+
+/**
+ * A redux module to create a list of items where more items can be loaded
+ * using an API. Additionally, there are actions for prepending items
+ * to the list, removing items and handling load errors.
+ *
+ * @param {string} key - used to scope action names and the selector
+ *      This key must be unique among other instances of this module.
+ * @returns {ReduxModule} the redux module
+ */
+const InfiniteList = key => {
+    
+    const initialState = {
+        items: [],
+        offset: 0,
+        error: null,
+        loading: true,
+        moreToLoad: false
+    };
+
+    const reducer = (state, action) => {
+        if (typeof state === 'undefined') {
+            state = initialState;
+        }
+
+        switch (action.type) {
+        case `${key}_LOADING`:
+            return {
+                ...state,
+                error: null,
+                loading: true
+            };
+        case `${key}_APPEND`:
+            return {
+                ...state,
+                items: state.items.concat(action.items),
+                loading: false,
+                error: null,
+                moreToLoad: action.moreToLoad
+            };
+        case `${key}_REPLACE`:
+            return {
+                ...state,
+                items: state.items.map((item, i) => {
+                    if (i === action.index) return action.item;
+                    return item;
+                })
+            };
+        case `${key}_REMOVE`:
+            return {
+                ...state,
+                items: state.items.filter((_, i) => i !== action.index)
+            };
+        case `${key}_PREPEND`:
+            return {
+                ...state,
+                items: [action.item].concat(state.items)
+            };
+        case `${key}_ERROR`:
+            return {
+                ...state,
+                error: action.error,
+                loading: false,
+                moreToLoad: false
+            };
+        default:
+            return state;
+        }
+    };
+
+    const actions = {
+        create: item => ({type: `${key}_PREPEND`, item}),
+        remove: index => ({type: `${key}_REMOVE`, index}),
+        replace: (index, item) => ({type: `${key}_REPLACE`, index, item}),
+        error: error => ({type: `${key}_ERROR`, error}),
+        loading: () => ({type: `${key}_LOADING`}),
+        append: (items, moreToLoad) => ({type: `${key}_APPEND`, items, moreToLoad}),
+
+        /**
+         * Load more action returns a thunk. It takes a function to call to get more items.
+         * It will call the LOADING action before calling the fetcher, and call
+         * APPEND with the results or call ERROR.
+         * @param {InfiniteListFetcher} fetcher - function that returns a promise
+         *  which must resolve to {items: [], moreToLoad}.
+         * @returns {function} a thunk that sequences the load and dispatches
+         */
+        loadMore: fetcher => (dispatch => {
+            dispatch(actions.loading());
+            return fetcher()
+                .then(({items, moreToLoad}) => dispatch(actions.append(items, moreToLoad)))
+                .catch(error => dispatch(actions.error(error)));
+        })
+    };
+
+    const selector = state => state[key];
+    
+    return {
+        key, actions, reducer, selector
+    };
+};
+
+export default InfiniteList;

test/unit/redux/infinite-list.test.js

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+/* global Promise */
+import InfiniteList from '../../../src/redux/infinite-list';
+
+const module = InfiniteList('test-key');
+let initialState;
+describe('Infinite List redux module', () => {
+    beforeEach(() => {
+        initialState = module.reducer(undefined, {}); // eslint-disable-line no-undefined
+    });
+
+    describe('reducer', () => {
+        test('module contains a reducer', () => {
+            expect(typeof module.reducer).toBe('function');
+        });
+        
+        test('initial state', () => {
+            expect(initialState).toMatchObject({
+                loading: true,
+                error: null,
+                items: [],
+                moreToLoad: false
+            });
+        });
+
+        describe('LOADING', () => {
+            let action;
+            beforeEach(() => {
+                action = module.actions.loading();
+                initialState.loading = false;
+                initialState.items = [1, 2, 3];
+                initialState.error = new Error();
+            });
+            test('sets the loading state', () => {
+                const newState = module.reducer(initialState, action);
+                expect(newState.loading).toBe(true);
+            });
+            test('maintains any existing data', () => {
+                const newState = module.reducer(initialState, action);
+                expect(newState.items).toBe(initialState.items);
+            });
+            test('clears any existing error', () => {
+                const newState = module.reducer(initialState, action);
+                expect(newState.error).toBe(null);
+            });
+        });
+
+        describe('APPEND', () => {
+            let action;
+            beforeEach(() => {
+                action = module.actions.append([4, 5, 6], true);
+            });
+            test('appends the new items', () => {
+                initialState.items = [1, 2, 3];
+                const newState = module.reducer(initialState, action);
+                expect(newState.items).toEqual([1, 2, 3, 4, 5, 6]);
+            });
+            test('sets the moreToLoad state', () => {
+                initialState.moreToLoad = false;
+                const newState = module.reducer(initialState, action);
+                expect(newState.moreToLoad).toEqual(true);
+            });
+            test('clears any existing error and loading state', () => {
+                initialState.error = new Error();
+                initialState.loading = true;
+                const newState = module.reducer(initialState, action);
+                expect(newState.error).toBe(null);
+                expect(newState.error).toBe(null);
+            });
+        });
+
+        describe('REPLACE', () => {
+            let action;
+            beforeEach(() => {
+                action = module.actions.replace(2, 55);
+            });
+            test('replaces the given index with the new item', () => {
+                initialState.items = [8, 9, 10, 11];
+                const newState = module.reducer(initialState, action);
+                expect(newState.items).toEqual([8, 9, 55, 11]);
+            });
+        });
+
+        describe('REMOVE', () => {
+            let action;
+            beforeEach(() => {
+                action = module.actions.remove(2);
+            });
+            test('removes the given index', () => {
+                initialState.items = [8, 9, 10, 11];
+                const newState = module.reducer(initialState, action);
+                expect(newState.items).toEqual([8, 9, 11]);
+            });
+        });
+
+        describe('CREATE', () => {
+            let action;
+            beforeEach(() => {
+                action = module.actions.create(7);
+            });
+            test('prepends the given item', () => {
+                initialState.items = [8, 9, 10, 11];
+                const newState = module.reducer(initialState, action);
+                expect(newState.items).toEqual([7, 8, 9, 10, 11]);
+            });
+        });
+
+        describe('ERROR', () => {
+            let action;
+            let error = new Error();
+            beforeEach(() => {
+                action = module.actions.error(error);
+            });
+            test('sets the error state', () => {
+                const newState = module.reducer(initialState, action);
+                expect(newState.error).toBe(error);
+            });
+            test('resets loading to false', () => {
+                initialState.loading = true;
+                const newState = module.reducer(initialState, action);
+                expect(newState.loading).toBe(false);
+            });
+            test('maintains any existing data', () => {
+                initialState.items = [1, 2, 3];
+                const newState = module.reducer(initialState, action);
+                expect(newState.items).toEqual([1, 2, 3]);
+            });
+        });
+    });
+
+    describe('action creators', () => {
+        test('module contains actions creators', () => {
+            // The actual action creators are tested above in the reducer tests
+            for (let key in module.actions) {
+                expect(typeof module.actions[key]).toBe('function');
+            }
+        });
+
+        describe('loadMore', () => {
+            test('returns a thunk function, rather than a standard action object', () => {
+                expect(typeof module.actions.loadMore()).toBe('function');
+            });
+            test('calls loading and the fetcher', () => {
+                let dispatch = jest.fn();
+                let fetcher = jest.fn(() => new Promise(() => { })); // that never resolves
+                module.actions.loadMore(fetcher)(dispatch);
+                expect(dispatch).toHaveBeenCalledWith(module.actions.loading());
+                expect(fetcher).toHaveBeenCalled();
+            });
+            test('calls append with resolved result from fetcher', async () => {
+                let dispatch = jest.fn();
+                let fetcher = jest.fn(() => Promise.resolve({items: ['a', 'b'], moreToLoad: false}));
+                await module.actions.loadMore(fetcher)(dispatch);
+                expect(dispatch.mock.calls[1][0]) // the second call to dispatch, after LOADING
+                    .toEqual(module.actions.append(['a', 'b'], false));
+            });
+            test('calls error with rejecting promise from fetcher', async () => {
+                let error = new Error();
+                let dispatch = jest.fn();
+                let fetcher = jest.fn(() => Promise.reject(error));
+                await module.actions.loadMore(fetcher)(dispatch);
+                expect(dispatch.mock.calls[1][0]) // the second call to dispatch, after LOADING
+                    .toEqual(module.actions.error(error));
+            });
+        });
+    });
+
+    describe('selector', () => {
+        test('will return the slice of state defined by the key', () => {
+            const state = {
+                [module.key]: module.reducer(undefined, {}) // eslint-disable-line no-undefined
+            };
+            expect(module.selector(state)).toBe(initialState);
+        });
+    });
+});