Q:

React Router v4 Nested match params not accessible at root level

Test Case
https://codesandbox.io/s/rr00y9w2wm
Steps to reproduce

Click on Topics
Click on Rendering with React

OR

Go to https://rr00y9w2wm.codesandbox.io/topics/rendering

Expected Behavior

match.params.topicId should be identical from both the parent Topics component should be the same as match.params.topicId when accessed within the Topic component

Actual Behavior

match.params.topicId when accessed within the Topic component is undefined
match.params.topicId when accessed within the Topics component is rendering

I understand from this closed issue that this is not necessarily a bug.
This requirement is super common among users who want to create a run in the mill web application where a component Topics at a parent level needs to access the match.params.paramId where paramId is a URL param that matches a nested (child) component Topic:
const Topic = ({ match }) => (
  <div>
    <h2>Topic ID param from Topic Components</h2>
    <h3>{match.params.topicId}</h3>
  </div>
);

const Topics = ({ match }) => (
  <div>
    <h2>Topics</h2>
    <h3>{match.params.topicId || "undefined"}</h3>
    <Route path={`${match.url}/:topicId`} component={Topic} />
    ...
  </div>
);

In a generic sense, Topics could be a Drawer or Navigation Menu component and Topic could be any child component, like it is in the application I'm developing. The child component has it's own :topicId param which has it's own (let's say) <Route path="sections/:sectionId" component={Section} />  Route/Component.
Even more painful, the Navigation Menu needn't have a one-to-one relationship with the component tree. Sometimes the items at the root level of the menu (say Topics, Sections etc.) might correspond to a nested structure (Sections is only rendered under a Topic, /topics/:topicId/sections/:sectionId though it has its own normalized list that is available to the user under the title Sections in the Navigation Bar).
Therefore, when Sections is clicked, it should be highlighted, and not both Sections and Topics.
With the sectionId or sections path unavailable to the Navigation Bar component which is at the Root level of the application, it becomes necessary to write hacks like this for such a commonplace use case. 
I am not an expert at all at React Router, so if anyone can venture a proper elegant solution to this use case, I would consider this to be a fruitful endeavor. And by elegant, I mean

Uses match and not history.location.pathname
Does not involve hacky approaches like manually parsing the window.location.xxx 
Doesn't use this.props.location.pathname
Does not use third party libraries like path-to-regexp
Does not use query params

Other hacks/partial solutions/related questions:

React Router v4 - How to get current route?
React Router v4 global no match to nested route childs

TIA!

A:

React-router doesn't give you the match params of any of the matched children Route , rather it gives you the params based on the current match. So if you have your Routes setup like
<Route path='/topic' component={Topics} />

and in Topics component you have a Route like
<Route path=`${match.url}/:topicId` component={Topic} />

Now if your url is /topic/topic1 which matched the inner Route but for the Topics component, the matched Route is still, /topic and hence has no params in it, which makes sense.
If you want to fetch params of the children Route matched in the topics component, you would need to make use of matchPath utility provided by React-router and test against the child route whose params you want to obtain
import { matchPath } from 'react-router'

render(){
    const {users, flags, location } = this.props;
    const match = matchPath(location.pathname, {
       path: '/topic/:topicId',
       exact: true,
       strict: false
    })
    if(match) {
        console.log(match.params.topicId);
    }
    return (
        <div>
            <Route exact path="/topic/:topicId" component={Topic} />
        </div>
    )
}

EDIT:
One method to get all the params at any level is to make use of context and update the params as and when they match in the context Provider.
You would need to create a wrapper around Route for it to work correctly, A typical example would look like
RouteWrapper.jsx
import React from "react";
import _ from "lodash";
import { matchPath } from "react-router-dom";
import { ParamContext } from "./ParamsContext";
import { withRouter, Route } from "react-router-dom";

class CustomRoute extends React.Component {
  getMatchParams = props => {
    const { location, path, exact, strict } = props || this.props;
    const match = matchPath(location.pathname, {
      path,
      exact,
      strict
    });
    if (match) {
      console.log(match.params);
      return match.params;
    }
    return {};
  };
  componentDidMount() {
    const { updateParams } = this.props;
    updateParams(this.getMatchParams());
  }
  componentDidUpdate(prevProps) {
    const { updateParams, match } = this.props;
    const currentParams = this.getMatchParams();
    const prevParams = this.getMatchParams(prevProps);
    if (!_.isEqual(currentParams, prevParams)) {
      updateParams(match.params);
    }
  }

  componentWillUnmount() {
    const { updateParams } = this.props;
    const matchParams = this.getMatchParams();
    Object.keys(matchParams).forEach(k => (matchParams[k] = undefined));
    updateParams(matchParams);
  }
  render() {
    return <Route {...this.props} />;
  }
}

const RouteWithRouter = withRouter(CustomRoute);

export default props => (
  <ParamContext.Consumer>
    {({ updateParams }) => {
      return <RouteWithRouter updateParams={updateParams} {...props} />;
    }}
  </ParamContext.Consumer>
);

ParamsProvider.jsx
import React from "react";
import { ParamContext } from "./ParamsContext";
export default class ParamsProvider extends React.Component {
  state = {
    allParams: {}
  };
  updateParams = params => {
    console.log({ params: JSON.stringify(params) });
    this.setState(prevProps => ({
      allParams: {
        ...prevProps.allParams,
        ...params
      }
    }));
  };
  render() {
    return (
      <ParamContext.Provider
        value={{
          allParams: this.state.allParams,
          updateParams: this.updateParams
        }}
      >
        {this.props.children}
      </ParamContext.Provider>
    );
  }
}

Index.js
ReactDOM.render(
  <BrowserRouter>
    <ParamsProvider>
      <App />
    </ParamsProvider>
  </BrowserRouter>,
  document.getElementById("root")
);

Working DEMO

A:

Try utilizing query parameters ? to allow the parent and child to access the current selected topic. Unfortunately, you will need to use the module qs because react-router-dom doesn't automatically parse queries (react-router v3 does).
Working example: https://codesandbox.io/s/my1ljx40r9
URL is structured like a concatenated string: 
topic?topic=props-v-state 
Then you would add to the query with &: 
/topics/topic?topic=optimization&category=pure-components&subcategory=shouldComponentUpdate
✔ Uses match for Route URL handling
✔ Doesn't use this.props.location.pathname (uses this.props.location.search)
✔ Uses qs to parse location.search
✔ Does not involve hacky approaches
Topics.js
import React from "react";
import { Link, Route } from "react-router-dom";
import qs from "qs";
import Topic from "./Topic";

export default ({ match, location }) => {
  const { topic } = qs.parse(location.search, {
    ignoreQueryPrefix: true
  });

  return (
    <div>
      <h2>Topics</h2>
      <ul>
        <li>
          <Link to={`${match.url}/topic?topic=rendering`}>
            Rendering with React
          </Link>
        </li>
        <li>
          <Link to={`${match.url}/topic?topic=components`}>Components</Link>
        </li>
        <li>
          <Link to={`${match.url}/topic?topic=props-v-state`}>
            Props v. State
          </Link>
        </li>
      </ul>
      <h2>
        Topic ID param from Topic<strong>s</strong> Components
      </h2>
      <h3>{topic && topic}</h3>
      <Route
        path={`${match.url}/:topicId`}
        render={props => <Topic {...props} topic={topic} />}
      />
      <Route
        exact
        path={match.url}
        render={() => <h3>Please select a topic.</h3>}
      />
    </div>
  );
};

Another approach would be to create a HOC that stores params to state and children update the parent's state when its params have changed.
URL is structured like a folder tree: /topics/rendering/optimization/pure-components/shouldComponentUpdate
Working example: https://codesandbox.io/s/9joknpm9jy
✔ Uses match for Route URL handling
✔ Doesn't use this.props.location.pathname
✔ Uses lodash for object to object comparison
✔ Does not involve hacky approaches
Topics.js
import map from "lodash/map";
import React, { Fragment, Component } from "react";
import NestedRoutes from "./NestedRoutes";
import Links from "./Links";
import createPath from "./createPath";

export default class Topics extends Component {
  state = {
    params: "",
    paths: []
  };

  componentDidMount = () => {
    const urlPaths = [
      this.props.match.url,
      ":topicId",
      ":subcategory",
      ":item",
      ":lifecycles"
    ];
    this.setState({ paths: createPath(urlPaths) });
  };

  handleUrlChange = params => this.setState({ params });

  showParams = params =>
    !params
      ? null
      : map(params, name => <Fragment key={name}>{name} </Fragment>);

  render = () => (
    <div>
      <h2>Topics</h2>
      <Links match={this.props.match} />
      <h2>
        Topic ID param from Topic<strong>s</strong> Components
      </h2>
      <h3>{this.state.params && this.showParams(this.state.params)}</h3>
      <NestedRoutes
        handleUrlChange={this.handleUrlChange}
        match={this.props.match}
        paths={this.state.paths}
        showParams={this.showParams}
      />
    </div>
  );
}

NestedRoutes.js
import map from "lodash/map";
import React, { Fragment } from "react";
import { Route } from "react-router-dom";
import Topic from "./Topic";

export default ({ handleUrlChange, match, paths, showParams }) => (
  <Fragment>
    {map(paths, path => (
      <Route
        exact
        key={path}
        path={path}
        render={props => (
          <Topic
            {...props}
            handleUrlChange={handleUrlChange}
            showParams={showParams}
          />
        )}
      />
    ))}
    <Route
      exact
      path={match.url}
      render={() => <h3>Please select a topic.</h3>}
    />
  </Fragment>
);

