Here's an example of a text-based Undertale game implemented as a single HTML file using JavaScript, enclosed in script tags:
In this example, the game presents the player with two options: attack or run. The player and enemy each have 100 health points. The game uses random number generation for attacks and simulates the battle until either the player or the enemy's health drops to 0. The game loop keeps running until the battle ends.
Please note that this is a simplified and text-based implementation for demonstration purposes only. The actual Undertale game offers complex mechanics, visuals, and interactions.