React Router: Optional Parameters

React Router makes it easy to use parameters, but these are mandatory by default. While this is enough for most cases, there are some situations where optional parameters would be beneficial.

Creating a Route With an Optional Parameter

As with regular parameters, declaring an optional parameter is just a matter of the path property of a Route; any parameter that ends with a question mark will be treated as optional:

class App extends Component {
  render() {
    return (
      <BrowserRouter>
        <div>
          {/* Optional parameters are defined by placing a question mark at the end of a parameter. */}
          {/* In this case, `line` is an optional parameter. */}
          {/* Optional parameters will be passed to the component, but will be undefined if not present in the URL. */}
          <Route path="/Lyrics/:song/:line?" component={LyricSheet}/>
        </div>
      </BrowserRouter>
    );
  }
}

The route will be rendered if it matches the path, with or without the optional parameter. So ‘/Lyrics/Spoonman’ and ‘/Lyrics/Spoonman/3’ would both be accepted.

Using Optional Parameters

Optional parameters are passed alongside mandatory ones as props. But if they’re not in the URL, they’ll be undefined:

export default function LyricSheet({ match }) {
  const {line, song} = match.params;
  // Since `line` is an optional parameter, it'll either be undefined or a string.
  const lineNumber = line ? parseInt(line, 10) : -1;
  // Get the lyrics to the song.
  const lyrics = getLyrics(song)
    // Map the lyrics to components.  If `index` is `lineNumber`, set `highlight` to `true`.
    .map((lyric, index) => (<LyricLine highlight={index === lineNumber} key={index} lyric={lyric} />));
  return (
    <div>{lyrics}</div>
  );
}

The component will be rendered to show the lyrics of song. If the optional parameter line is defined, that line will be highlighted. You’ve probably seen something like this if you’ve flipped through a file on GitHub.

If you’d like to learn more about React, take a look at our How To Code in React.js series, or check out our React topic page for exercises and programming projects.