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A guide to common error messages and their solutions.

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Build Errors

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Module not found

Error: Cannot find module '@/components/Button'
Cause: The imported file doesn’t exist or path is wrong. Fix: 
The import '@/components/Button' can't be found.
Can you check if the file exists and fix the import path?

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Unexpected token

SyntaxError: Unexpected token '<'
Cause: JavaScript syntax error, often in JSX. Fix: 
Getting a syntax error "Unexpected token".
Can you find and fix the syntax issue?

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Type error

Type 'string' is not assignable to type 'number'
Cause: TypeScript type mismatch. Fix: 
Getting this type error: [full error message]
Can you fix the type issue?

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Export not found

Attempted import error: 'Button' is not exported from './components'
Cause: Component exists but isn’t exported. Fix: 
The 'Button' component isn't exported properly.
Can you add the export to the component file?

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Runtime Errors

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Cannot read properties of undefined

TypeError: Cannot read properties of undefined (reading 'map')
Cause: Trying to use a method on undefined/null value. Fix: 
Getting "Cannot read properties of undefined" when mapping over data.
Can you add a check for undefined and handle the empty state?

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X is not a function

TypeError: response.json is not a function
Cause: Variable isn’t what you expect. Fix: 
Getting "[x] is not a function" error.
Can you check the variable type and fix the function call?

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Network error

TypeError: Failed to fetch
Cause: API request failed (network, CORS, or server error). Fix: 
API fetch is failing with a network error.
Can you check the API endpoint and error handling?

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React/Next.js Errors

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Hydration mismatch

Hydration failed because the initial UI does not match what was rendered on the server
Cause: Server and client render different content. Fix: 
Getting a hydration mismatch error.
Can you fix the component to render the same on server and client?
Common causes:
  • Using Date.now() or Math.random() directly
  • Accessing window or localStorage without checks
  • Conditional rendering based on browser-only values

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Invalid hook call

Invalid hook call. Hooks can only be called inside of the body of a function component
Cause: Hook used incorrectly. Fix: 
Getting "Invalid hook call" error.
Can you check that hooks are only used in function components and not inside conditions or loops?

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Too many re-renders

Error: Too many re-renders. React limits the number of renders
Cause: Component is in infinite render loop. Fix: 
Getting "Too many re-renders" error.
Can you find and fix the infinite loop in the component?
Usually caused by:
  • Calling setState directly in render
  • Missing dependency array in useEffect
  • State update triggering another state update

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Database Errors

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RLS policy violation

new row violates row-level security policy for table "posts"
Cause: RLS blocking the operation. Fix: 
Getting RLS policy violation when [inserting/updating/deleting].
Can you check and fix the RLS policies for this table?

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Foreign key violation

insert or update on table "posts" violates foreign key constraint
Cause: Referenced record doesn’t exist. Fix: 
Getting foreign key violation error.
The referenced [table] record doesn't exist.
Can you fix the query or create the referenced record first?

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Duplicate key

duplicate key value violates unique constraint
Cause: Trying to insert duplicate value in unique column. Fix: 
Getting duplicate key error.
Can you add a check for existing records or use upsert?

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Authentication Errors

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Session not found

AuthSessionMissingError: Auth session missing
Cause: User not logged in or session expired. Fix: 
Getting "session missing" error.
Can you add authentication check and redirect to login if not authenticated?

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Invalid login credentials

AuthApiError: Invalid login credentials
Cause: Wrong email or password. Fix: This is expected for wrong credentials. Ensure your error handling shows a user-friendly message.

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Environment Variable Errors

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Missing environment variable

Error: Missing environment variable: NEXT_PUBLIC_SUPABASE_URL
Cause: Required variable not set. Fix:
  1. Go to Settings > Environment
  2. Add the missing variable
  3. Redeploy if in production

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Quick Reference Table

Error ContainsLikely CauseQuick Fix
Cannot find moduleWrong importCheck file path
undefinedMissing dataAdd null check
not a functionWrong variable typeCheck variable
HydrationServer/client mismatchFix SSR
Invalid hookHook misuseCheck hook rules
Too many re-rendersInfinite loopFix useEffect/setState
RLS policySecurity policyCheck Supabase RLS
Failed to fetchNetwork/API issueCheck endpoint
NEXT_PUBLIC_Env var missingAdd to settings

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Getting More Help

If an error isn’t listed here:
  1. Copy the full error message
  2. Paste it to Kleap AI with context about what you were doing
  3. Check browser console for additional details

Troubleshooting Guide

Step-by-step solutions to common problems