redux-devtools/packages/redux-devtools-app/src/middlewares/exportState.ts

import { stringifyJSON } from '../utils/stringifyJSON';
import { UPDATE_STATE, LIFTED_ACTION, EXPORT } from '../constants/actionTypes';
import { getActiveInstance } from '../reducers/instances';
import { Dispatch, MiddlewareAPI } from 'redux';
import { ExportRequest, StoreAction } from '../actions';
import { StoreState } from '../reducers';

let toExport: string | number | undefined;

function download(state: string) {
  const blob = new Blob([state], { type: 'octet/stream' });
  const href = window.URL.createObjectURL(blob);
  const a = document.createElement('a');
  a.style.display = 'none';
  a.download = 'state.json';
  a.href = href;
  document.body.appendChild(a);
  a.click();
  setTimeout(() => {
    document.body.removeChild(a);
    window.URL.revokeObjectURL(href);
  }, 0);
}

export const exportStateMiddleware =
  (store: MiddlewareAPI<Dispatch<StoreAction>, StoreState>) =>
  (next: Dispatch<StoreAction>) =>
  (action: StoreAction) => {
    const result = next(action);

    if (
      toExport &&
      action.type === UPDATE_STATE &&
      action.request!.type === 'EXPORT'
    ) {
      const request = action.request!;
      const id = request.instanceId || request.id;
      if (id === toExport) {
        toExport = undefined;
        download(
          JSON.stringify(
            {
              payload: request.payload,
              preloadedState: request.committedState,
            },
            null,
            '\t',
          ),
        );
      }
    } else if (action.type === EXPORT) {
      const instances = store.getState().instances;
      const instanceId = getActiveInstance(instances);
      const options = instances.options[instanceId];
      if (options.features.export === true) {
        download(
          stringifyJSON(instances.states[instanceId], options.serialize),
        );
      } else {
        toExport = instanceId;
        next({ type: LIFTED_ACTION, message: 'EXPORT', toExport: true });
      }
    }
    return result;
  };