index.js

const isSongPage =
    !!window.location.href.match(
        /http(?:s)?:\/\/genius\.com\/[a-z0-9-]*-lyrics/gi,
    ) && !!document.querySelector("[class^=AlbumTracklist]");

const getNextUrl = (songs, prev) => {
    const currentIndex = songs.indexOf(window.location.pathname);
    const nextTrack = songs[prev ? currentIndex - 1 : currentIndex + 1];

    if (nextTrack) {
        window.location.pathname = nextTrack;
    } else {
        alert("No track comes before or after this");
    }
};

const keys = new Set();

document.addEventListener("keydown", (event) => {
    if (isSongPage) {
        keys.add(event.key);
        const songs = [];
        const tracklist = document.querySelectorAll(
            "[class^=AlbumTracklist__TrackName]",
        );

        for (const track of tracklist) {
            songs.push(
                track.querySelector("[class^=StyledLink]")
                    ? track.querySelector("[class^=StyledLink]").pathname
                    : window.location.pathname,
            );
        }

        switch (event.key) {
            case "ArrowRight":
            case "ArrowLeft":
                if (keys.size === 1) {
                    getNextUrl(songs, event.key === "ArrowLeft");
                }
                break;
            default:
                return;
        }
    }
});

document.addEventListener("keyup", (event) => {
    keys.delete(event.key);
});