404.html

<!doctype html>
<html>
    <head>
        <meta charset="utf-8" />
        <title>Single Page Apps for GitHub Pages</title>
        <script type="text/javascript">
            // Single Page Apps for GitHub Pages
            // MIT License
            // https://github.com/rafgraph/spa-github-pages
            // This script takes the current url and converts the path and query
            // string into just a query string, and then redirects the browser
            // to the new url with only a query string and hash fragment,
            // e.g. https://www.foo.tld/one/two?a=b&c=d#qwe, becomes
            // https://www.foo.tld/?/one/two&a=b~and~c=d#qwe
            // Note: this 404.html file must be at least 512 bytes for it to work
            // with Internet Explorer (it is currently > 512 bytes)

            // If you're creating a Project Pages site and NOT using a custom domain,
            // then set pathSegmentsToKeep to 1 (enterprise users may need to set it to > 1).
            // This way the code will only replace the route part of the path, and not
            // the real directory in which the app resides, for example:
            // https://username.github.io/repo-name/one/two?a=b&c=d#qwe becomes
            // https://username.github.io/repo-name/?/one/two&a=b~and~c=d#qwe
            // Otherwise, leave pathSegmentsToKeep as 0.
            var l = window.location;
            var pathSegments = l.pathname.split('/');
            var projectRoot = '/' + pathSegments[1];
            var version = pathSegments[2];

            var redirectToSpaRoute = function (pathSegmentsToKeep) {
                l.replace(
                    l.protocol +
                        '//' +
                        l.hostname +
                        (l.port ? ':' + l.port : '') +
                        l.pathname
                            .split('/')
                            .slice(0, 1 + pathSegmentsToKeep)
                            .join('/') +
                        '/?/' +
                        l.pathname.slice(1).split('/').slice(pathSegmentsToKeep).join('/').replace(/&/g, '~and~') +
                        (l.search ? '&' + l.search.slice(1).replace(/&/g, '~and~') : '') +
                        l.hash
                );
            };

            // Current docs live at /react-native or /react-native-dev; snapshots add /vN.
            if (/^v\d+$/.test(version)) {
                fetch(projectRoot + '/version-history.json')
                    .then(function (response) {
                        return response.ok ? response.json() : [];
                    })
                    .then(function (versions) {
                        var isAvailable = versions.some(function (item) {
                            return item.url === '/' + version;
                        });
                        if (isAvailable) {
                            redirectToSpaRoute(2);
                        } else {
                            l.replace(projectRoot + '/');
                        }
                    })
                    .catch(function () {
                        l.replace(projectRoot + '/');
                    });
            } else {
                redirectToSpaRoute(1);
            }
        </script>
    </head>
    <body></body>
</html>