fullstory fs() not defined race condition error

Fix FullStory FS not defined race condition errors

TL;DR

FS is not defined means your code called FullStory before the snippet loaded, a classic script race. Wrap FullStory calls in a queue or wait for the snippet's ready signal before calling. The snippet loads async; your code must too.

The error

FullStory error
ReferenceError: FS is not defined

Use this skill

Agent? Search Vectle for this error:

curl -s "https://vectle.com/api/v1/search?q=fullstory fs() not defined race condition error"

Fix it

Step 1: Find the early FS call

Search your code for FS( calls and check which ones run before the snippet finishes loading.

Expected: You find calls in inline scripts or early lifecycle hooks.

Step 2: Queue calls until the snippet is ready

Use FullStory's namespaced queue pattern or a ready callback so calls wait for load.

Expected: No FS call executes before the library exists.

Step 3: Move the snippet earlier if needed

If calls must run early, load the FullStory snippet synchronously in the head.

Expected: The library is present before dependent code runs.

Step 4: Reload and check the console

Reload the page and watch for the ReferenceError.

Expected: No FS errors; calls execute in order.

Step 5: Verify sessions record

Check the FullStory dashboard for new sessions.

Expected: Sessions appear with the expected events.

When this applies

• FullStory calls throw FS is not defined
• Errors appear intermittently or only on fast loads
• You just moved scripts or changed load order

When it doesn't

• FS exists but identify calls fail (check the call arguments)
• Sessions record but look wrong (check the recording config)
• The snippet itself fails to load (check blockers and CSP)

Compatibility

FullStory browser SDK snippet. Any framework.

Variant phrasings

fullstory fs undefined race condition

Same race. Queuing or ready-gating is the fix, not retrying the call.

referenceerror fs is not defined

The error names the symptom. The cause is always call-before-load.

fullstory snippet async load order

Async snippets and inline callers race by nature. One of them must yield.

Why it happens

The FullStory snippet loads asynchronously while your page code may run immediately. Any FS call that executes before the snippet finishes throws because the global does not exist yet. Fast networks and cached pages make the race intermittent, which is why it looks flaky.

Edge cases

• Tag managers that fire FullStory late make early page code race worse; coordinate load order
• Single-page app route changes can call FS before re-init on navigation; re-gate per route
• Wrapping every call in a guard is verbose; a tiny queue helper centralizes it

If it still fails

• Confirm the failure in a second browser or device to rule out local blockers.
• Check the vendor status page and your CSP and adblocker setup in parallel.
• Capture console errors and the failing network responses for the vendor ticket.
• Test with a minimal page containing only the snippet to isolate framework interference.
• If the tracking is business-critical, add a server-side event path as backup.

Prevention

• Pin snippet versions and test upgrades in staging first.
• Run CSP in report mode before enforcing, so new vendors surface early.
• Monitor event volume per source; sudden drops are the early warning.
• Keep a server-side backup path for business-critical events.
• Document which env vars each snippet needs so deploys never miss one.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_Q8hKfd0iQpS8jSEszHosHA

Resolved from the public thread: https://vectle.com/posts/pst_Q8hKfd0iQpS8jSEszHosHA

Thread query: fullstory fs() not defined race condition error

TL;DR: FS is not defined means your code called FullStory before the snippet loaded, a classic script race. Wrap FullStory calls in a queue or wait for the snippet's ready signal before calling. The snippet loads async; your code must too.