Always Check Error Before Using Data from Supabase Queries

Always Check Error Before Using Data from Supabase Queries | BeforeMerge Rules | BeforeMerge

Impact: HIGH (prevents silent failures and null reference crashes in production)

Every Supabase query returns { data, error }. When error is non-null, data is always null. Destructuring only { data } and using it without checking error leads to TypeError: Cannot read properties of null at runtime. These crashes are especially hard to diagnose because the actual cause (RLS violation, network timeout, invalid column) is silently discarded.

This is the Supabase equivalent of unchecked return values — the query failed, but your code proceeds as if it succeeded.

Incorrect (ignoring error entirely):

// ❌ Destructures data without checking error
export default async function DashboardPage() {
  const supabase = await createClient()
 
  const { data: projects } = await supabase
    .from('projects')
    .select('id, name, created_at')
    .order('created_at', { ascending: false })
 
  // ❌ If query fails (RLS, network, typo in table name), projects is null
  // This crashes with: TypeError: Cannot read properties of null (reading 'map')
  return (
    <ul>
      {projects.map((project) => (
        <li key={project.id}>{project.name}</li>
      ))}
    </ul>
  )
}

Incorrect (checking error but continuing to use data):

// ❌ Logs error but still uses data (which is null)
export async function GET() {
  const supabase = await createClient()
 
  const { data, error } = await supabase
    .from('projects')
    .select('*')
 
  if (error) {
    console.warn('Query had an error:', error.message)
    // ❌ Falls through — data is null below
  }
 
  // ❌ data is null when error is non-null
  return Response.json({ projects: data }) // Returns { projects: null }
}

Incorrect (optional chaining hides the real problem):

// ❌ Optional chaining silences the error but shows blank UI
const { data: profile } = await supabase
  .from('profiles')
  .select('display_name, avatar_url')
  .eq('id', userId)
  .single()
 
// User sees "Unknown" with default avatar — no indication of failure
return (
  <div>
    <h1>{profile?.display_name ?? 'Unknown'}</h1>
    <img src={profile?.avatar_url ?? '/default.png'} />
  </div>
)

Correct (check error and return early in Server Components):

// ✅ Check error before using data
export default async function DashboardPage() {
  const supabase = await createClient()
 
  const { data: projects, error } = await supabase
    .from('projects')
    .select('id, name, created_at')
    .order('created_at', { ascending: false })
 
  if (error) {
    // ✅ Log for debugging, throw for error boundary
    console.error('Projects query failed:', { code: error.code, message: error.message })
    throw new Error('Failed to load projects')
  }
 
  // ✅ TypeScript narrows: projects is non-null here
  return (
    <ul>
      {projects.map((project) => (
        <li key={project.id}>{project.name}</li>
      ))}
    </ul>
  )
}

Correct (check error in Route Handlers):

// ✅ Return appropriate HTTP status on error
export async function GET() {
  const supabase = await createClient()
 
  const { data: projects, error } = await supabase
    .from('projects')
    .select('id, name, created_at')
 
  if (error) {
    console.error('Projects query failed:', { code: error.code, message: error.message })
    return Response.json(
      { error: 'Failed to load projects' },
      { status: 500 }
    )
  }
 
  return Response.json({ projects })
}

Correct (check error in Client Components):

'use client'
 
export function ProjectList() {
  const [projects, setProjects] = useState<Project[]>([])
  const [error, setError] = useState<string | null>(null)
 
  useEffect(() => {
    async function load() {
      const supabase = createClient()
 
      const { data, error } = await supabase
        .from('projects')
        .select('id, name, created_at')
 
      if (error) {
        console.error('Projects query failed:', error.message)
        setError('Could not load projects. Please try again.')
        return // ✅ Early return — don't touch data
      }
 
      setProjects(data) // ✅ Safe — data is non-null
    }
 
    load()
  }, [])
 
  if (error) return <div role="alert">{error}</div>
 
  return (
    <ul>
      {projects.map((project) => (
        <li key={project.id}>{project.name}</li>
      ))}
    </ul>
  )
}

Helper to enforce error checking:

// ✅ Utility that throws on error, returning only data
async function queryOrThrow<T>(
  query: PromiseLike<{ data: T | null; error: { message: string; code: string } | null }>
): Promise<T> {
  const { data, error } = await query
 
  if (error) {
    console.error('Supabase query failed:', { code: error.code, message: error.message })
    throw new Error(`Query failed: ${error.code}`)
  }
 
  return data as T
}
 
// Usage — cannot forget to check error
const projects = await queryOrThrow(
  supabase.from('projects').select('id, name').order('created_at', { ascending: false })
)

Detection hints:

# Find destructured data without error check
grep -rn "const { data }" src/ --include="*.ts" --include="*.tsx"
grep -rn "const { data:" src/ --include="*.ts" --include="*.tsx"
# Find .from() calls to audit error handling
grep -rn "\.from(" src/ --include="*.ts" --include="*.tsx"

Reference: Supabase Fetch Data · CWE-252: Unchecked Return Value

Always Check Error Before Using Data from Supabase Queries

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Always Check Error Before Using Data from Supabase Queries