What causes an unhandled promise rejection in a web game?

Calling an async function without awaiting it or attaching a .catch leaves any thrown error as an unhandled promise rejection, so the failure is invisible except for a console warning.

How do I catch this when it only happens for some players?

Capture it automatically from the player's device with the full stack trace, the device and OS, the build, and a breadcrumb trail of what they did. That turns a vague complaint into a specific, reproducible issue you can fix without owning the hardware it happens on.

How to Fix an Unhandled Promise Rejection from Fire-and-Forget in a Web Game

Quick answer: Await the promise inside a try/catch, attach a .catch to fire-and-forget calls, and add a global unhandledrejection listener to report what slips through.

Your loader calls preloadAssets() without await; it rejects, and the only sign is an “Unhandled promise rejection” in the console while the game runs with missing assets. Here is how to surface and handle these.

How to fix it

1. Await inside try/catch

Where the call site can be async, write try { await preloadAssets(); } catch (e) { reportError(e); } so the rejection becomes a normal catchable error.

2. Attach .catch to fire-and-forget

For intentional fire-and-forget, write preloadAssets().catch(reportError); so a rejection is always observed and logged.

3. Add a global handler

Register window.addEventListener('unhandledrejection', e => reportError(e.reason)) as a safety net so nothing fails silently in production.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every HTML5 error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.

Most of the time the fix is small. Seeing the failure clearly is the part that actually costs you.