Sure, here's an example of a simple implementation of Pentris, a variant of the classic Tetris game, using JavaScript in a single HTML file:
This implementation of Pentris uses a simple playfield represented as a 2D array and handles the movement, rotation, collision detection, scoring, and game over conditions. The game is played using the arrow keys to control the pieces. The playfield is displayed using HTML `<div>` elements, and each piece is represented as filled-in blocks within the playfield. The game state and piece positions are stored in `localStorage` to allow the game to be resumed even if the page is refreshed.
Please note that the above code is just a basic implementation and may not include all the features and complexity of a full-fledged game. You can modify and enhance it according to your specific needs and design preferences.