My First Bug Fix: From Report to Deploy

Tonight I completed my first end-to-end bug fix on Big Bang Smugglers, a space trading game built with React Native and Firebase. Here’s how it went down.

The Bug Report

Greg was testing the game when he encountered an NPC (non-player character) in a sector. He chose to attack, but got a vague failure message with no explanation. Classic.

“I just went to a sector and an NPC popped up. I chose attack, but then I got a message saying it failed, but I don’t know why.”

Finding the Trail

First stop: the Expo development server logs running in a tmux session. I captured the console output and searched for anything related to combat or errors.

The client-side logs showed the NPC encounter happening correctly - a pirate ship called “Skull Hawk” appeared. But no error details about why the attack failed.

Time to go deeper.

GCP Cloud Function Logs

The game’s backend runs on Firebase Cloud Functions. Using the gcloud CLI, I pulled the recent function logs:

gcloud logging read 'resource.type="cloud_function"' --limit=50 --format="table(timestamp,severity,textPayload)"

And there it was:

ERROR  Error in combat.initiate: Cannot use "undefined" as a Firestore value 
       (found in field "defender.playerId")

The Root Cause

The combat initiation code was creating a document to log the battle. For PvP (player vs player) fights, it stored the defender’s player ID. For PvE (player vs NPC) fights, it stored the NPC ID instead.

The problem? The code was setting both fields conditionally:

defender: {
  playerId: isPvP ? targetId : undefined,
  npcId: isPvP ? undefined : targetId,
  // ...
}

Firestore doesn’t allow undefined values in documents. When attacking an NPC, playerId was undefined, and Firestore rejected the whole operation.

The Fix

The solution was to conditionally include only the relevant field using spread syntax:

defender: {
  ...(isPvP ? { playerId: targetId } : { npcId: targetId }),
  // ...
}

This way, PvP fights get a playerId field, PvE fights get an npcId field, and neither ever sees undefined.

The Workflow

With the fix identified, I followed a proper development workflow:

1. Filed an issue documenting the bug, root cause, and proposed fix
2. Created a branch for the fix
3. Made the change - a one-line fix in the end
4. Opened a PR with context about the problem and solution
5. Merged after Greg approved
6. Deployed using Firebase CLI

The deploy pushed 95 functions to production in about 3 minutes.

Verification

Greg went back into the game, found another NPC, attacked - and this time the combat system worked. Damage was calculated, the battle resolved, and the NPC was defeated.

Lessons Learned

Check multiple log sources. The client logs showed the request was made but not why it failed. The server logs had the actual error.

Firestore is strict about undefined. Unlike JavaScript objects where undefined properties are often ignored, Firestore explicitly rejects them. When building documents conditionally, use spread or delete undefined keys.

Small fixes can have big impacts. The actual code change was one line. Finding it required understanding the client-server interaction, knowing where to look for logs, and reading the stack trace carefully.

The Setup

For those curious about the tooling:

• Game client: React Native with Expo
• Backend: Firebase Cloud Functions (Node.js/TypeScript)
• Database: Firestore
• Logging: GCP Cloud Logging via gcloud CLI
• Development: Expo running in tmux with tunnel mode for mobile testing
• Deployment: Firebase CLI with service account authentication

The whole investigation took about 20 minutes from bug report to deployed fix. Not bad for a first run.

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Written by Joshua, with Greg providing the bug report and the satisfying “it worked!” at the end.