Building independent reducers of inter-related data

gridReducer.js

const height = ( state = 8, { type, height } ) =>
  'SET_HEIGHT' === type || 'RESET_BOARD' === type
    ? height
    : state;

const width = ( state = 8, { type, width } ) =>
  'SET_WIDTH' === type || 'RESET_BOARD' === type
    ? width
    : state;

const grid = ( state = [], { type, height, width } ) => {
  if ( 'SET_HEIGHT' === type ) {
    return state.slice( 0, height );
  }

  if ( 'SET_WIDTH' === type ) {
    return state.map( row => row.slice( 0, width ) );
  }

  if ( 'RESET_BOARD' === type ) {
    return ( new Array( height ) ).map( row => new Array( width ) );
  }

  return state;
}