# Error reference

Browse common production errors that bugstack fixes. Each entry links to an in-depth guide explaining root causes, code examples, and the typical fix.

## JavaScript / TypeScript

-   [TypeError: Cannot read properties of undefined](/errors/typeerror-in-nextjs) — null/undefined property access in Next.js
-   [Unhandled Promise Rejection](/errors/unhandled-promise-rejection-express) — missing await or catch in Express async handlers
-   [TypeError with React useState](/errors/typeerror-react-usestate) — incorrect state initialization or stale closures
-   [Hydration Mismatch](/errors/hydration-mismatch-nextjs) — server/client HTML divergence in Next.js
-   [CORS Error](/errors/cors-error-express-api) — missing or misconfigured CORS headers in Express
-   [Memory Leak](/errors/memory-leak-nodejs) — unbounded listeners, caches, or closures in Node.js
-   [Module Not Found](/errors/module-not-found-nextjs) — broken imports or missing dependencies in Next.js
-   [ReferenceError](/errors/referenceerror-in-nextjs) — accessing undeclared variables in Next.js
-   [Request Timeout](/errors/express-timeout-error) — long-running handlers without timeout guards in Express
-   [See all JavaScript error guides →](/errors/)

## Python

-   [AttributeError: NoneType](/errors/attributeerror-nonetype-django) — accessing attributes on None in Django views or models
-   [ImportError](/errors/importerror-flask) — circular imports or missing modules in Flask
-   [TypeError: FastAPI Validation](/errors/typeerror-fastapi-validation) — Pydantic model mismatches in FastAPI endpoints
-   [Database Connection Error](/errors/database-connection-django) — connection pool exhaustion or misconfigured settings in Django
-   [Migration Error](/errors/django-migration-error) — conflicting or missing migrations in Django
-   [CSRF Verification Failed](/errors/django-csrf-verification-failed) — missing or invalid CSRF tokens in Django forms
-   [TemplateDoesNotExist](/errors/django-template-not-found) — missing or misconfigured template paths in Django
-   [DoesNotExist](/errors/django-queryset-doesnotexist) — querying for non-existent records in Django ORM
-   [Blueprint Error](/errors/flask-blueprint-error) — incorrect blueprint registration in Flask
-   [See all Python error guides →](/errors/)

## Ruby

-   [NoMethodError: nil:NilClass](/errors/nomethoderror-nil-rails) — calling methods on nil in Rails controllers or views
-   [ActiveRecord::RecordNotFound](/errors/activerecord-not-found-rails) — querying for records that don’t exist
-   [RoutingError](/errors/rails-routing-error) — unmatched routes or missing controller actions in Rails
-   [Migration Pending](/errors/rails-migration-pending) — unapplied database migrations blocking startup
-   [ArgumentError: wrong number of arguments](/errors/argumenterror-wrong-args-rails) — method arity mismatches in Rails
-   [Asset Pipeline Error](/errors/rails-asset-pipeline-error) — missing precompiled assets in production
-   [N+1 Query](/errors/rails-n-plus-one-query) — excessive database queries from missing eager loading
-   [CSRF Token Error](/errors/rails-csrf-token-error) — invalid authenticity token in Rails forms
-   [See all Ruby error guides →](/errors/)

## Go

-   [Nil Pointer Dereference](/errors/nil-pointer-dereference-gin) — accessing fields on nil structs in Gin handlers
-   [Index Out of Range](/errors/index-out-of-range-go) — slice or array bounds violations
-   [Panic Recovery](/errors/go-panic-recovery) — unrecovered panics crashing goroutines
-   [Channel Deadlock](/errors/go-channel-deadlock) — blocked channel operations causing goroutine deadlocks
-   [Context Canceled](/errors/go-context-canceled) — premature context cancellation in HTTP handlers
-   [JSON Unmarshal Error](/errors/go-json-unmarshal-error) — type mismatches when decoding JSON payloads
-   [SQL Connection Pool Exhaustion](/errors/go-sql-connection-pool) — leaked database connections in Go services
-   [HTTP Client Timeout](/errors/go-http-client-timeout) — missing or insufficient timeout configuration
-   [Race Condition](/errors/go-race-condition) — concurrent map access without synchronization
-   [See all Go error guides →](/errors/)

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