Error fixes

Fix TypeError: Cannot read properties of undefined by finding which value is missing, reading the stack trace and repairing the data flow.

Fix TypeError: x is not a function in JavaScript: wrong imports, shadowed names, lost this, a missing semicolon or .map on a non-array.

Fix ReferenceError: x is not defined: typos, block scope, the TDZ, window or require missing in Node, and scripts that load in the wrong order.

Fix 'blocked by CORS policy' fetch errors: which headers the server must send, preflights, credentials, dev proxies and why no-cors fails.

Fix SyntaxError: Unexpected token '<' in JSON from fetch: the server sent HTML. Check res.ok and content-type, then read the body as text.

Fix Uncaught (in promise) errors and Node crashes from unhandled rejections: missing await, async forEach, fire-and-forget and rethrown errors.

Fix RangeError: Maximum call stack size exceeded: missing base cases, self-calling setters, circular data, huge spreads and deep recursion.

Fix SyntaxError: Cannot use import statement outside a module in Node and browsers: type module, .mjs, require of ESM and __dirname in ESM.

Fix 'Failed to configure a DataSource: url attribute is not specified' in Spring Boot 3 by setting the URL, adding the driver or a profile.

Seeing the Whitelabel Error Page with status 404 in Spring Boot 3? Check the mapping, component scan, @RestController and context path.

Fix 'the dependencies of some of the beans in the application context form a cycle' in Spring Boot 3 by redesigning beans, not flags.

Fix 'required a bean of type that could not be found' and 'No qualifying bean' in Spring Boot 3: scanning, conditions, qualifiers, tests.

Fix 'Web server failed to start. Port 8080 was already in use' in Spring Boot: find the process on the port, stop it or change server.port.

Fix 'blocked by CORS policy' for a Spring Boot 3 API: global CORS config, Spring Security preflight handling, credentials and exposed headers.

Fix 'Could not write JSON: Infinite recursion (StackOverflowError)' for bidirectional JPA entities in Spring Boot 3 with DTOs or annotations.

Why is an @Autowired or @Value field null in Spring Boot? Objects built with new, static fields and constructor timing, and how to fix them.

Fix java.lang.NullPointerException by reading the helpful message, finding the null in the top stack frame and validating inputs early.

Fix java.util.ConcurrentModificationException when removing items in a loop: use removeIf or Iterator.remove, and see when concurrent lists help.

Fix java.lang.OutOfMemoryError: Java heap space by checking -Xmx, capturing a heap dump and finding what retains memory in Eclipse MAT.

ClassNotFoundException vs NoClassDefFoundError in Java: what each error means, why classes go missing at runtime and how to fix the classpath.

Fix UnsupportedClassVersionError (class file version 65.0 vs 61.0) by matching the runtime JDK to your build or compiling with javac --release.

Why Arrays.asList and List.of throw UnsupportedOperationException on add or remove, and how to choose a mutable copy or an immutable list.

Fix 'Too many re-renders' in React: a state setter runs during render, often onClick={setX(1)} or state synced in the component body.

Fix 'Objects are not valid as a React child' in React: rendering objects, Dates, Errors or props, plus async components on the client.

Fix 'Invalid hook call' in React: hooks in class components, callbacks or plain functions, duplicate React copies and mismatched react-dom.

Fix the React warning 'Cannot update a component while rendering a different component': a child sets parent state during render.

Fix 'Maximum update depth exceeded' in React: effects with no or unstable dependencies, setState in componentDidUpdate and ref callbacks.

Why React state isn't updated immediately after setState: render snapshots, batching, functional updaters, mutation and when to use useEffect.

Fix TS2532 'Object is possibly undefined' and TS18048 by finding where undefined enters the type, then narrowing or failing early.

Fix TS2322 'Type X is not assignable to type Y' by reading the elaboration chain from the bottom and correcting the value, not casting.

Fix TS2339 'Property does not exist on type' for unions, empty objects, DOM elements and window globals without casting to any.

Fix TS2307 'Cannot find module or its corresponding type declarations' and TS7016 by tracing module resolution, @types, paths and exports.

Fix TS2345 'Argument of type string is not assignable to parameter of type never' from empty arrays, function unions and exhaustive checks.

Fix TS7053 'Element implicitly has an any type because expression of type string can't be used to index type' with keyof, Record and guards.

Fix Angular NG0100 ExpressionChangedAfterItHasBeenCheckedError: why the dev-mode check fires, and data-flow fixes that beat setTimeout.

Fix Angular NullInjectorError: No provider for X! (NG0201): read the injector path, add provideHttpClient(), providedIn root or a token.

Fix Angular NG8002 Can't bind to 'ngModel' since it isn't a known property of 'input': import FormsModule, then fix formControl and inputs.

Fix Angular NG8001 'app-x' is not a known element: standalone imports, selector typos, NgModule exports, schemas and TestBed setup.

Fix 'blocked by CORS policy' errors when Angular ng serve calls a backend: proxy.conf.json, angular.json wiring and production CORS setup.

Fix Node.js 'listen EADDRINUSE: address already in use :::3000': find the process holding the port, close servers on restart, use PORT and port 0.

Fix Node.js 'Error: Cannot find module' and ERR_MODULE_NOT_FOUND: missing installs, wrong paths, case on Linux, ESM extensions and exports.

Fix npm 'code ERESOLVE unable to resolve dependency tree': read the peer conflict report, upgrade the right package, use overrides with care.

Fix Node.js 'Reached heap limit Allocation failed - JavaScript heap out of memory': tell leaks from big data, take heap snapshots, stream.

Fix Node.js 'Error: connect ECONNREFUSED 127.0.0.1:5432': check the service, localhost vs ::1 since Node 17, Docker networking and startup retries.

Fix 'npm error code EACCES permission denied' on global installs without sudo: change the npm prefix, use nvm or fnm, prefer npx, fix Docker.

Fix ModuleNotFoundError: No module named 'x': installs into the wrong interpreter, inactive venvs, pip vs import names and python -m.

Fix TypeError: 'NoneType' object is not subscriptable: missing returns, list.sort(), failed re.match, dict.get and Optional type checks.

Fix Python IndentationError: expected an indented block, unindent does not match any outer indentation level, and TabError from mixed tabs.

Fix UnboundLocalError: cannot access local variable (once 'referenced before assignment'): why assignment makes a name local, global, nonlocal.

Fix Python KeyError: 'x': d[key] vs .get, in checks, setdefault and defaultdict, JSON payload validation, pandas columns and odd key reprs.

Fix pip's error: externally-managed-environment (PEP 668) on Homebrew, Debian and Ubuntu: use a venv or pipx, not --break-system-packages.

Fix Git's fatal: refusing to merge unrelated histories: why it happens, when --allow-unrelated-histories is right, and safer alternatives.

Fix Git's error: failed to push some refs and ! [rejected] main -> main (fetch first): fetch, rebase or merge, and force-with-lease safely.

How to undo the last commit in Git: git reset --soft, --mixed or --hard, commit --amend, git revert for pushed commits, and reflog recovery.

Fix 'Your branch and origin/main have diverged' and fatal: Need to specify how to reconcile divergent branches: merge, rebase or ff-only.

Fix git@github.com: Permission denied (publickey) and Could not read from remote repository: test SSH, load keys, set IdentityFile, or use HTTPS.

Fix 'permission denied while trying to connect to the Docker daemon socket': docker group, re-login, rootless mode, contexts and a dead daemon.

Fix Docker 'port is already allocated' and 'address already in use': find the container or process on the port, then remap or bind to 127.0.0.1.

Fix Docker 'no space left on device' during build or pull: measure with docker system df, prune in a safe order, and cap logs and build cache.

Fix 'exec format error' in Docker: confirm the CPU architecture mismatch with uname and docker inspect, then rebuild with the right --platform or buildx target.

Why a Docker container exits immediately: check the exit code, read docker logs, open a shell and fix a command that only starts a background job.

Fix PostgreSQL 'column must appear in the GROUP BY clause or be used in an aggregate function' by choosing the grain, aggregates or DISTINCT ON.

Fix PostgreSQL 'duplicate key value violates unique constraint': resync sequences after imports, use ON CONFLICT and remove check-then-insert races.

Fix PostgreSQL 'relation does not exist': check the search_path, qualify the schema, confirm migrations ran here, and fix quoted, case-sensitive identifiers.

Fix PostgreSQL 'FATAL: sorry, too many clients already': count connections per app instance, find leaks and idle transactions, and pool safely.

Fix PostgreSQL 'operator does not exist: integer = text': read the operand types, cast the constant or parameter, and fix the column type at the source.

Fix Kubernetes CrashLoopBackOff: read the exit code, check kubectl logs --previous, fix command, config and probes, and understand backoff timing.

Fix Kubernetes ImagePullBackOff and ErrImagePull: read the pull error, then fix tag typos, imagePullSecrets, CPU architecture or Docker Hub rate limits.

Fix Kubernetes OOMKilled (exit code 137): read lastState.terminated.reason, tell node eviction from the container limit, and size memory limits from data.

Fix a Kubernetes Pod stuck in Pending: read the '0/3 nodes are available' event, then fix requests, affinity, taints, PVCs or quotas.

Fix CreateContainerConfigError: read the kubectl describe event, repair missing ConfigMap or Secret names and keys, and use optional refs sparingly.