Mastering React Test-Driven Development

Building the reducer

Before we begin building the reducer, let's think about what we're going to build.

The reducer will be a higher-order function which, when called with an existing reducer, returns a new reducer that returns the state we're expecting. In our production code, we'll replace this store code:

combineReducers({  script: scriptReducer})

With this "decorated" reducer, which takes exactly the same reducer and wraps it in the withUndoRedo reducer that we'll build in this section:

combineReducers({  script: withUndoRedo(scriptReducer)})

To test this, we'll need to use a spy to act in place of the script reducer, which we'll call the decoratedReducerSpy.