code: function setZoom(state, action) {
    let zoom = Math.min(DEFAULT_ZOOM.MAX, action.zoom);
    zoom = Math.max(DEFAULT_ZOOM.MIN, zoom);
    return Object.assign({}, state, {zoom});
  }
docs: Set the zoom level of the map.
  @param {Object} state Current state.
  @param {Object} action Action to handle.
  
  @returns {Object} The new state.