Sure, here's an example of a simple first-person shooter game implemented as a single HTML file using JavaScript:
This code sets up a simple first-person shooter game where the player can control a red circle using the arrow keys or WASD keys on a smartphone. The player can move around the canvas, and their movements are updated continuously using the `update` function. The player is represented by a red circle drawn on the canvas using the `draw` method of the player object.