Programming
A practical guide for reading error logs, finding the real file and line, separating warnings from failures, and asking for precise debugging help.
31 matching items
Message structure
First error line
TypeError: Cannot read properties of undefinedThe first clear error line is often the best starting point.
Ignore repeated follow-up logs at first and identify the first line that names the failure.
TypeError: Cannot read properties of undefinedFind the first real error line in this log.
Message structure
Error name
SyntaxError, TypeError, ReferenceErrorThe error name tells you the broad class of problem.
Use it to separate syntax, runtime, type, network, and environment problems.
SyntaxError, TypeError, ReferenceErrorClassify this error by error name before suggesting a fix.
Message structure
Error message
Module not found: Can't resolve '@/lib/error-message-guide'The message explains what the tool could not do.
Read the verb and object: could not resolve a module, could not read a property, or could not bind a port.
Module not found: Can't resolve '@/lib/error-message-guide'Rewrite this error message in plain Korean and point to the likely cause.
Message structure
Stack trace
at SiteHeader (src/components/SiteHeader.tsx:42:15)A list of function calls that led to the error.
Find the first line that points to your project file, then inspect that area.
The top stack line is not always your code. Some lines are framework internals.
at SiteHeader (src/components/SiteHeader.tsx:42:15)In this stack trace, identify the first line from my project.
Location clues
File path
src/components/LanguageSwitcher.tsxThe path tells you which file the tool thinks is involved.
Prefer paths under src, app, components, or lib before inspecting dependency folders.
src/components/LanguageSwitcher.tsxList the project files mentioned by this error.
Location clues
Line and column
src/app/page.tsx:27:12Line and column narrow the location to a specific character area.
Inspect the named line plus a few lines above it, because the real cause can start earlier.
src/app/page.tsx:27:12Open the file around the reported line and explain what looks wrong.
Location clues
Project file line
src/lib/git-github.ts:143:7A stack or build line that points to your source code instead of a dependency.
This is usually the line to inspect before reading long framework traces.
src/lib/git-github.ts:143:7Separate my project file lines from framework lines.
Location clues
node_modules line
node_modules/next/dist/server/app-render.jsA line inside an installed dependency or framework package.
Treat it as context unless the log also points to a package version or configuration problem.
Do not edit node_modules as a normal fix.
node_modules/next/dist/server/app-render.jsTell me whether this node_modules line is the cause or just context.
JavaScript runtime
SyntaxError
SyntaxError: Unexpected token '}'JavaScript or TypeScript could not parse the code.
Check brackets, quotes, commas, JSX tags, and incomplete expressions near the reported line.
SyntaxError: Unexpected token '}'Find the syntax problem around this line.
JavaScript runtime
TypeError
TypeError: Cannot read properties of undefinedA value did not have the shape or behavior the code expected at runtime.
Check whether the value can be undefined, null, a string instead of an object, or a missing function.
TypeError: Cannot read properties of undefinedTrace where this undefined value comes from.
JavaScript runtime
ReferenceError
ReferenceError: copy is not definedThe code tried to use a variable or function name that is not available in that scope.
Check imports, spelling, scope, and whether the variable is declared before use.
ReferenceError: copy is not definedFind why this name is not defined.
JavaScript runtime
Cannot read properties of undefined
Cannot read properties of undefined (reading 'navLabel')The code tried to access a property on a value that is undefined.
Check the object before the dot, such as copy, locale, params, or a map lookup result.
Cannot read properties of undefined (reading 'navLabel')Identify which value is undefined in this error.
JavaScript runtime
is not a function
setOpenMenu is not a functionThe code tried to call something with parentheses, but the value is not a function.
Check whether a prop, import, hook result, or object method has the expected name.
setOpenMenu is not a functionFind why this value is being called as a function.
Next.js and web
Module not found
Module not found: Can't resolve '@/lib/error-message-guide'The bundler could not find the file or package named in an import.
Check the file exists, the path spelling is exact, and the alias points to the expected folder.
Module not found: Can't resolve '@/lib/error-message-guide'Check the import path and the actual file path.
Next.js and web
Export not found
Export errorMessageGuideCopy doesn't exist in target moduleThe import name does not match any export from the target file.
Compare the imported name with the exact exported const, function, or type name.
Export errorMessageGuideCopy doesn't exist in target moduleFind the mismatched import or export name.
Next.js and web
Hydration failed
Hydration failed because the server rendered HTML didn't match the clientThe HTML generated on the server did not match what React rendered in the browser.
Check browser-only values, dates, random values, localStorage, and theme state used during initial render.
Hydration failed because the server rendered HTML didn't match the clientFind the likely server and client render mismatch.
Next.js and web
404 Not Found
GET /ko/reference/error-message-guide 404The requested route or file could not be found.
Check the route folder, generateStaticParams, link href, and exported static output path.
GET /ko/reference/error-message-guide 404Check why this route returns 404.
Next.js and web
500 Internal Server Error
GET /ko 500The server received the request but failed while processing it.
Look at the terminal log from the same request, not only the browser status code.
GET /ko 500Match this browser 500 with the terminal error that caused it.
TypeScript and lint
Property does not exist on type
Property 'navLabel' does not exist on type 'string'TypeScript says the value's declared type does not include that property.
Check whether the value has the correct type, whether it was indexed correctly, and whether the data shape changed.
Property 'navLabel' does not exist on type 'string'Explain why TypeScript thinks this property does not exist.
TypeScript and lint
Type is not assignable
Type 'string' is not assignable to type 'Locale'A value is being passed somewhere that expects a different type.
Compare the actual value type with the expected type named after 'to type'.
Type 'string' is not assignable to type 'Locale'Show the actual type and expected type in this error.
TypeScript and lint
ESLint error
React Hook useEffect has a missing dependencyA static analysis rule found code that may be unsafe, inconsistent, or against project style.
Read the rule name and the file line, then decide whether the code or dependency list should change.
React Hook useEffect has a missing dependencyExplain this lint error and fix it without changing behavior.
Next.js and web
Build failed
Failed to compile.The production build could not finish.
Scroll above the summary and find the first compile, type, lint, or route error.
Failed to compile.From this build log, identify the first blocking failure.
Terminal and system
EADDRINUSE
Error: listen EADDRINUSE: address already in use :::3001The server tried to use a port that another process is already using.
Stop the old server or run the new server on a different port.
Error: listen EADDRINUSE: address already in use :::3001Find what is using this port and suggest the safest next step.
Terminal and system
ENOENT
ENOENT: no such file or directory, open 'next.config.js'The process tried to open a file or folder that does not exist at that path.
Check the current working directory, filename spelling, extension, and generated files.
ENOENT: no such file or directory, open 'next.config.js'Check whether this file is missing or the command is running from the wrong folder.
Terminal and system
Permission denied
Operation not permittedThe process does not have permission to read, write, or execute the target.
Check sandbox rules, file permissions, protected folders, and whether approval is needed.
Do not fix permission errors by broadly changing permissions unless you understand the target.
Operation not permittedExplain why this permission error happened and what approval is needed.
Terminal and system
Command not found
zsh: command not found: nextThe shell cannot find an executable command with that name.
Check whether dependencies are installed, whether npm scripts should be used, and whether PATH is set.
zsh: command not found: nextTell me whether I should run an npm script instead of this command.
Debugging order
Warning
DeprecationWarning: module.register() is deprecatedA warning says something may need attention, but it may not stop the current command.
Check whether the command still completed successfully before treating the warning as a failure.
DeprecationWarning: module.register() is deprecatedSeparate blocking errors from non-blocking warnings in this output.
Debugging order
DeprecationWarning
DeprecationWarning: module.register() is deprecatedA feature still works now but is planned to be replaced or removed later.
If build passes, record it as a follow-up unless it points to your code or a required upgrade.
DeprecationWarning: module.register() is deprecatedTell me whether this DeprecationWarning blocks deployment.
Debugging order
Recent change
The error started after editing SiteHeader.tsxThe most recent related edit is often the highest-value clue.
Compare the error location with files modified in the current task.
The error started after editing SiteHeader.tsxCompare this error with the files changed most recently.
Debugging order
Reproduce step
Open /ko, click the language menu, then click the site menuThe smallest action sequence that makes the error appear again.
Clear reproduce steps make it possible to verify that the fix actually works.
Open /ko, click the language menu, then click the site menuTurn this description into exact reproduce steps.
Debugging order
Minimal log
Error line + file path + stack line from srcA short set of log lines that preserves the actual failure and location.
Keep the first error, the project file path, the command, and the final success or failure summary.
Error line + file path + stack line from srcCompress this long log into the minimal lines needed for debugging.