JavaScript, TypeScript, Python, Go, Java & Docker Error Reference

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Every error below links to a full reference page with the exact message, root cause, and fix. The same errors are in the searchable table above — these sections group them by type for browsing.

JSON Parse & Serialization Errors

Errors thrown by JSON.parse(), response.json(), and JSON.stringify() when data isn't valid JSON or a value can't be serialized.

• Unexpected token < in JSON at position 0the server returned HTML, not JSON
• Unexpected end of JSON inputempty or truncated response body
• Unexpected token o in JSON at position 1parsing an object that is already parsed
• JSON SyntaxError: missing commaa comma between values is missing
• JSON SyntaxError: trailing commastray comma after the last value
• JSON SyntaxError: single quotes not allowedJSON requires double quotes
• JSON SyntaxError: comments not allowedJSON has no comment syntax
• JSON SyntaxError: NaN / Infinity not allowednon-finite numbers are invalid JSON
• TypeError: Converting circular structure to JSONthe object references itself
• JSON.parse returns a string, not an objectthe value was double-encoded
• SyntaxError in JSON.parsepatterns to catch parse errors safely
• JSON.stringify drops a propertyundefined, function, and symbol values
• TypeError: Do not know how to serialize a BigIntconvert BigInt before stringify
• SyntaxError: Unexpected non-whitespace character after JSON datamore than one JSON value
• SyntaxError: Bad control character in string literal in JSONa raw newline or tab in a string
• json: cannot unmarshal string into Go value of type intGo struct field type doesn't match the JSON

Python Errors

Errors from Python's json module, file reading, and the requests library — parsing, serialization, and text encoding.

• JSONDecodeError: Expecting value: line 1 column 1 (char 0)empty or non-JSON response
• JSONDecodeError: Expecting property name enclosed in double quotessingle quotes or a trailing comma
• JSONDecodeError: Extra data: line 1 column N (char N)more than one JSON value
• JSONDecodeError: Unterminated string starting ata missing closing quote
• TypeError: Object of type datetime is not JSON serializabledatetime, Decimal, set, or bytes
• UnicodeDecodeError: 'utf-8' codec can't decode byte 0xffwrong file encoding or a BOM
• TypeError: string indices must be integersindexed a JSON string as a dict
• TypeError: the JSON object must be str, bytes or bytearray, not dictre-parsed an already-parsed object
• KeyError in Pythondict key that doesn't exist
• ValueError: Trailing datamultiple JSON docs / NDJSON
• AttributeError: 'NoneType' object has no attribute 'X'a variable was None where an object was expected
• TypeError: 'NoneType' object is not subscriptableindexed with [ ] into a None value
• IndentationError: unexpected indentstray whitespace or mixed tabs/spaces
• error: externally-managed-environmentpip / PEP 668 — use a venv or pipx
• ssl.SSLCertVerificationError: certificate verify failedCA bundle, certifi, or a corporate proxy CA

Encoding & Base64 Errors

Errors decoding base64 and binary data in the browser and Node.js.

• InvalidCharacterError: Failed to execute 'atob' — the string to be decoded is not correctly encodedbase64url, a data URI, or whitespace

URI & URL Errors

Errors from percent-encoding and decoding URLs and query strings.

• URIError: URI malformeda lone % or bad percent-encoding

Node.js & Build Errors

Errors from the Node.js runtime, the npm CLI, and JavaScript build tools like webpack and Vite.

• Error: listen EADDRINUSE: address already in usethe port is already taken
• npm ERR! code ERESOLVE — unable to resolve dependency treea peer dependency conflict
• Error: Cannot find module (MODULE_NOT_FOUND)missing package or wrong path
• JsonWebTokenError: invalid signature & jwt malformedwrong secret or a bad token
• process.env.X is undefineddotenv not loaded or wrong prefix
• SyntaxError: Cannot use import statement outside a moduleCommonJS vs ES modules
• ReferenceError: require is not defined in ES module scopeESM vs CommonJS
• FATAL ERROR: JavaScript heap out of memoryraise the V8 heap limit
• Error: error:0308010C:digital envelope routines::unsupportedNode 17+ OpenSSL 3 vs old webpack
• UnhandledPromiseRejectionWarninga rejected promise with no catch (Node 15+ crashes)
• Error: read ECONNRESET / socket hang upthe peer closed the TCP connection
• npm ERR! code EACCES: permission deniedfix without sudo — version manager or prefix
• gyp ERR! build errornative addon needs a compiler + Python
• Module not found: Can't resolve 'fs'server-only code bundled for the browser
• 504 (Outdated Optimize Dep)Vite's dependency pre-bundle went stale

Network & Fetch Errors

Errors where the request fails at the network level — before or instead of a normal HTTP response.

• TypeError: Failed to fetchthe request failed before any response
• CORS policy: No 'Access-Control-Allow-Origin' header is presentthe server is missing CORS headers
• net::ERR_NAME_NOT_RESOLVEDDNS could not resolve the host
• TypeError: Failed to execute 'json' on 'Response': body stream already readthe body was read twice
• AbortError: The operation was abortedthe request was cancelled or timed out
• net::ERR_CONNECTION_REFUSEDnothing is listening on that port
• net::ERR_CERT_AUTHORITY_INVALIDthe certificate's issuer isn't trusted
• net::ERR_SSL_PROTOCOL_ERRORthe TLS handshake itself failed
• net::ERR_TOO_MANY_REDIRECTSa redirect loop with no terminating response
• net::ERR_CONNECTION_TIMED_OUTno response at all, not even a rejection
• net::ERR_INTERNET_DISCONNECTEDthe OS's own connectivity check disagrees with reality
• net::ERR_BLOCKED_BY_CLIENTan extension or ad blocker cancelled the request
• CORS: Response to preflight request doesn't pass access control checkthe OPTIONS preflight itself failed
• CORS: Request header field is not allowed by Access-Control-Allow-Headersa header isn't on the server's allow-list
• Mixed Content: was loaded over HTTPS, but requested an insecure resourcean HTTPS page requested a plain-HTTP resource

Database Errors

Errors from PostgreSQL — connections, locking, and constraint violations.

• FATAL: sorry, too many clients alreadymax_connections reached — pool connections
• ERROR: deadlock detectedtwo transactions waiting on each other
• duplicate key value violates unique constrainta unique/primary key already exists

Docker Errors

Errors from building and running Docker containers.

• Error response from daemon: port is already allocatedhost port already bound by something
• standard_init_linux.go: exec format errorCPU architecture mismatch (amd64 vs arm64)
• Cannot connect to the Docker daemon. Is the docker daemon running?the CLI can't reach dockerd at all
• Container exited with code 137 (OOMKilled)SIGKILL — usually the OOM killer
• Got permission denied while trying to connect to the Docker daemon socketadd your user to the docker group
• no space left on deviceprune images, containers & build cache
• Conflict. The container name "/X" is already in usea stopped container still owns the name
• OCI runtime create failed: executable file not found in $PATHthe command, not the image, is missing

Kafka Errors

Errors from Kafka producers and consumers — serialization, offsets, rebalancing, and cluster connectivity.

• SerializationException: Unknown magic byte!producer and consumer disagree on the serializer
• CommitFailedException: Offset commit cannot be completedpoll() took longer than max.poll.interval.ms
• TimeoutException: Expiring N record(s)a batch never reached a broker in time
• OffsetOutOfRangeException: Fetch offset is out of rangethe committed offset was deleted by retention
• Error while fetching metadata: LEADER_NOT_AVAILABLEno elected leader — often advertised.listeners

TypeScript Errors

Compiler errors from tsc — type mismatches, null safety, and missing declarations. Many come from untyped JSON; generate accurate types with JSON to TypeScript.

• TS2322: Type 'X' is not assignable to type 'Y'a value's type doesn't match the target
• TS2345: Argument of type 'X' is not assignable to parameter of type 'Y'the same mismatch on a function call
• TS2339: Property 'X' does not exist on type 'Y'property not declared on the type
• TS2532: Object is possibly 'undefined'strictNullChecks — narrow before access
• TS2349: This expression is not callablevalue isn't a function for all its types
• TS7016: Could not find a declaration file for module 'X'install @types or add a declaration
• TS7053: Element implicitly has an 'any' type (index signature)dynamic key needs keyof / Record
• TS2307: Cannot find module 'X' or its corresponding type declarationsnothing resolves at all — not even a fallback any
• TS2304: Cannot find name 'X'missing import, @types, or DOM lib
• TS7006: Parameter 'x' implicitly has an 'any' typenoImplicitAny — annotate or type the source
• TS2769: No overload matches this callargs match none of the overload signatures

JavaScript Runtime Errors

Errors thrown while JavaScript runs — missing values and runaway recursion.

• TypeError: Cannot read properties of null (reading 'X')property access on null/undefined
• RangeError: Maximum call stack size exceededinfinite or too-deep recursion
• TypeError: Cannot read properties of undefined (reading 'X')the undefined sibling of the null error
• Uncaught TypeError: X is not a functionwrong import, typo, or shadowed name
• Uncaught ReferenceError: X is not definedtypo, missing script, or scope issue
• TypeError: Assignment to constant variablereassigning a const — use let instead
• ReferenceError: Cannot access 'X' before initializationthe temporal dead zone
• TypeError: Cannot set properties of undefined (setting 'X')writing to a nested object that isn't built yet
• TypeError: Cannot convert undefined or null to objectObject.keys/values/entries on null
• TypeError: X.map is not a functionthe value isn't actually an array
• TypeError: Failed to construct 'URL': Invalid URLrelative path with no base, or bad input
• RangeError: Invalid array lengthnegative, fractional, or NaN array length
• ReferenceError: process is not definedNode's process global doesn't exist in the browser

Go, Rust & Java Errors

Runtime panics and compile errors from Go, Rust, and Java — nil/null dereferences, bounds, deadlocks, and the Rust borrow checker.

• panic: invalid memory address or nil pointer dereferenceGo — dereferenced a nil pointer/interface
• panic: index out of range [N] with length MGo — slice/array index past the end
• fatal error: all goroutines are asleep - deadlock!Go — every goroutine blocked on a channel
• error[E0382]: borrow of moved valueRust — used a value after it moved
• error[E0499]: cannot borrow as mutable more than onceRust — two &mut borrows overlap
• Exception in thread "main" java.lang.NullPointerExceptionJava — dereferenced a null reference
• undefined: XGo — typo, missing import, or unexported name
• imported and not used: "X"Go — a hard compile error, not a lint warning
• cannot find module providing package XGo — module resolution walk failed
• error[E0308]: mismatched typesRust — no implicit type coercion
• error[E0277]: the trait bound is not satisfiedRust — the type doesn't implement a required trait
• error[E0597]: does not live long enoughRust — a reference outlived its value
• Exception in thread "main" java.lang.ArrayIndexOutOfBoundsExceptionJava — index outside 0..length-1
• ClassNotFoundException / NoClassDefFoundErrorJava — two different classloading failures
• error: cannot find symbolJava — javac couldn't resolve an identifier
• panic: interface conversion: interface {} is X, not YGo — failed type assertion (comma-ok fix)
• called `Option::unwrap()` on a `None` valueRust — unwrap panicked; use the safe ladder
• java.util.ConcurrentModificationExceptionJava — modified a collection while iterating
• declared and not used: xGo — an unused local is a hard compile error
• error[E0599]: no method named 'X' foundRust — usually a trait that isn't in scope
• java.lang.OutOfMemoryError: Java heap spaceJava — leak vs undersized heap