Sure, here's an example of a simple text-based MMORPG game implemented as a single HTML file using JavaScript. Please note that this is just a basic example for illustrative purposes and the game mechanics can be expanded upon and improved further.
In this simple game, the player starts at level 1 with 100 health and 0 experience. The `displayStats()` function shows the player's stats, and the `attack()` function calculates a random amount of damage dealt to the player and updates their health and experience accordingly. If the player's health reaches 0 or below, the game ends with a "Game Over" message. Additionally, if the player's experience reaches 50 or above, they level up and gain extra health.
Please keep in mind that this is just a basic example, and a full-fledged MMORPG game would require more complex game mechanics, graphics, networking, etc.