Build a Custom React Router From Scratch

What is Route?
What is Link?
What is Router?
Dependency List
Project Structure
RouterContext Component
Route Component
Link Component
404 Component
Route.ts
Index.tsx

Live Example

Have you ever wondered how react-router works under the hood? If that's the case the best way to explore is to build a custom react-router from scratch.
To do that we need to know which APIs utilized by react-router. As we all know, react-router actually use History API, which is the native API of the browser.

The DOM Window object provides access to the browser's session history through the history object. It exposes useful methods and properties that let you navigate back and forth through the user's history, and manipulate the contents of the history stack.
MDN Web Docs

History object enables us to manipulate the routing on the current window. It has lot's of methods such as window.history.forward(),
window.history.back() which moves forward and backwards respectively, like click on a back and forward button on your favourite browser.
Since, we now have more knowledge about History API we can start talking about react-routers fundamental components like Link, Route, Router.

Route

Route is actually a basic component which accepts path and children as parameters to navigate to that URL and render the given children elements.

Link

Link also accepts path, onClick and children to render. If given path is valid pushes the user to the desired location.
The main way to allow users to navigate around your application.

Router

Router is the parent of all other components in the component tree. In essence, everything related to routing goes into Router.
In our case, Router gonna be our Provider which utilizes ContextAPI to pass down all the shared things across our router.

So, we get the basic idea. Let's start building our project.

npx create-react-app my-app --template typescript

This gives us a solid template to work on with Typescript support. Let's install our dependencies and check out our project structure.

Dependencies

yarn add history querystringify @types/querystringify @types/history

Project Structure

/src
  /router
    Link.tsx
    Route.tsx
    RouterContext.tsx
    Utils.ts
  404.tx
  index.tsx
  routes.tsx

Utils

import { Location, State } from 'history';
import qs from 'querystringify';

type Props = {
  location: Location<State>;
};

export function locationToRoute({ location }: Props) {
  return {
    path: location.pathname,
    hash: location.hash,
    query: qs.parse(location.search),
  };
}

We first define our locationToRoute() which accept location, by the way, Location contains
information about the URL path and things like path, hash, query. And, to capture the query parameters we use qs.parse().

RouterContext Component

import React, { useContext, useLayoutEffect, useState } from 'react';
import { createBrowserHistory, Location, State } from 'history';
import { locationToRoute } from './Utils';
import { NotFound } from '../404';
import { RoutesType } from '../routes';

const history = createBrowserHistory();
export const RouterContext = React.createContext({
  route: locationToRoute(history),
});

type Props = {
  routeList: RoutesType;
  children: React.ReactNode;
};

const RouterProvider = ({ routeList, children }: Props) => {
  const [routes] = useState(Object.keys(routeList).map((key) => routeList[key].path));
  const [route, setRoute] = useState(locationToRoute(history));

  const handleRouteChange = (location: { location: Location<State> }) => {
    const route = locationToRoute(location);
    setRoute(route);
  };
  const is404 = routes.indexOf(route.path) === -1;

  useLayoutEffect(() => {
    let unlisten = history.listen(handleRouteChange);
    return () => {
      unlisten();
    };
  }, []);

  return (
    <RouterContext.Provider value={{ route }}>
      {is404 ? <NotFound /> : children}
    </RouterContext.Provider>
  );
};

const useRouter = () => useContext(RouterContext);

export { useRouter, RouterProvider, history };

We first, define createBrowserHistory() to store the location in URLs. Then, create a React.createContext() to pass our values down.
Then, inside our RouterProvider we define a state called routes. routes let us check which routes are proper or not, and if they are not our is404() catches it.
And, we got handleRouteChange() function which is very crucial to set which route to navigate.

We now have only two parts left. One of them is useLayoutEffect() to be able to listen changes in the route we use useLayoutEffect() it also
fires synchronously right after all DOM manipulations end. In it's the cleanup function we just call unlisten(). And, finally, we return our
RouterContext.Provider with the value of the current route and define our useRouter() hook.

Route Component

import React from 'react';
import { RouterContext } from './RouterContext';

type Props = {
  children: React.ReactNode;
  path?: string;
};

export function Route({ path, children }: Props) {
  // Extract route from RouterContext
  const { route } = React.useContext(RouterContext);

  // Return null if the supplied path doesn't match the current route path
  if (route.path !== path) {
    return null;
  }

  return <>{children}</>;
}

Link Component

import React from 'react';
import { useRouter, history } from './RouterContext';

type Props = {
  to: string;
  children: React.ReactNode;
  onClick?: (e: React.MouseEvent<Element, MouseEvent>) => void;
};

export function Link({ to, children, onClick, ...props }: Props) {
  const { route } = useRouter();

  const handleClick = (e: React.MouseEvent) => {
    e.preventDefault();

    if (route.path === to) {
      // If it's not a valid path function will not trigger.
      return;
    }
    if (onClick) {
      onClick(e);
    }
    history.push(to);
  };

  return (
    <>
      <a {...props} onClick={handleClick}>
        {children}
      </a>
    </>
  );
}

404 Component

import React from 'react';
import { Link } from './router/Link';
import { routes } from './routes';

export function NotFound() {
  return (
    <div>
      <p>404 - Not Found</p>
      <Link to={routes.home.path}>Back to home</Link>
    </div>
  );
}

Route.ts

export type RoutesType = { [route: string]: { path: string } };

export const routes: RoutesType = {
  home: {
    path: '/',
  },
  about: {
    path: '/about',
  },
};

Let's bring everything together

Index.tsx

import React from 'react';
import ReactDOM from 'react-dom';
import { RouterProvider } from './router/RouterContext';
import { Link } from './router/Link';
import { Route } from './router/Route';
import { routes } from './routes';

function App() {
  return (
    <RouterProvider routeList={routes}>
      <Route path={routes.home.path}>
        <p>Homepage</p>
        <Link to={routes.about.path}>Go to about</Link>
      </Route>

      <Route path={routes.about.path}>
        <p>About</p>
        <Link to={routes.home.path}>Go to home</Link>
      </Route>
    </RouterProvider>
  );
}

const rootElement = document.getElementById('root');
ReactDOM.render(<App />, rootElement);

As if we use react-router, we first define our provider and supply it's routes which we defined earlier. Now, we can freely
use RouteComponent and LinkComponent in our entire app.

Reason behind building our custom react router is actually praticing the basics of historyAPI and see what's really going under the hood.
Before diving into using libraries every developer should learn the ideas behind those libraries, and try building a basic, functioning example of it.

Thanks for reading 🥳🥳🥳.