storing-a-line-in-state-array.js

const handleDrawingEvent = line => ({
  line,
  type: 'LINE_DRAWN'
})

const defaultState = { lines: [] };

const reducer = (state = defaultState, action) => {
  switch (action.type) {
    case 'LINE_DRAWN': 
      return { ...state, lines: [...state.lines, action.line] };
    default:
      return state;
  }
}