#debugging

Unexpected token '<' is not valid JSON — 5 Reasons Your API Returned HTML Instead

fetch throws SyntaxError: Unexpected token '<' when the response body is HTML, not JSON. Covers the 5 root causes: SPA history fallback returning index.html, auth 302 redirects, gateway 502 error pages, wrong baseURL, and missing res.ok checks — with curl diagnostics.

Permission denied (publickey): 6 Reasons Git Push Fails Over SSH (and the Fix for Each)

git clone or push rejected with Permission denied (publickey) fatal: Could not read from remote repository? Covers missing keygen, key not loaded in the agent, public key not added to GitHub/GitLab, multi-account key routing with ~/.ssh/config, deploy key limits, and wrong remote URLs — with ssh -v diagnostics.

npm ERR! ERESOLVE: peer dependency conflicts — the complete fix guide

npm install fails with ERESOLVE peer dependency conflicts? Understand what peer dependencies are, why npm 7+ enforces them, and 4 resolution strategies (force, legacy-peer-deps, overrides, manual) with trade-offs.

error:0308010C digital envelope routines::unsupported — Fixing Node 17+ Breakage in Older Projects

Node 17+ crashes webpack 4 builds with error:0308010C because OpenSSL 3.0 dropped MD4 from the default provider. Compare --openssl-legacy-provider, upgrading to webpack 5, and pinning an EOL Node — plus Node 22 / Next.js notes.

ECONNREFUSED: Connection Refused — 5 Causes Explained (Including Docker)

Node, Java, or curl reporting connect ECONNREFUSED 127.0.0.1:3306? It means nothing is listening on that port. Covers service not running, wrong port, 127.0.0.1-only binding, Docker container networking, and firewall REJECT rules — with ss/lsof diagnostic commands.

Docker permission denied on docker.sock: the Right Fix (and Its Security Cost)

docker commands fail with permission denied while trying to connect to the Docker daemon socket? The cause: your user isn't in the docker group. One-step fix with usermod -aG, the newgrp trick to avoid re-login, why chmod 777 is a trap, and why docker group membership equals passwordless root — plus the rootless alternative.

docker pull access denied: 5 Reasons Image Pulls Get Rejected (and the Fix)

docker pull fails with pull access denied for xxx, repository does not exist or may require docker login? Covers misspelled image names, missing tags, private registry login, Docker Hub rate limits, and registry mirrors for mainland China networks — with a one-minute triage flow.

Docker port is already allocated: Finding and Freeing the Taken Port

docker run fails with port is already allocated or bind: address already in use? The host-side port you mapped is taken — by another container's docker-proxy or a host process. How to tell the two apart, the compose project-name collision scenario, and why 127.0.0.1 and a specific IP binding can coexist.

Docker no space left on device: the Complete Cleanup Ladder, Safe to Aggressive

Docker build or run failing with no space left on device? Dangling images, stopped containers, build cache, and unbounded container logs — see exactly who eats the disk with docker system df. The prune ladder from container prune to system prune -a, the --volumes danger zone, and the log-rotation config that stops it coming back.

docker manifest unknown: 4 Reasons the Tag Can't Be Found

docker pull reports manifest for xxx:tag not found or manifest unknown? Covers tag typos, the latest-tag trap, renamed official images, and architecture mismatches (arm64/amd64 manifests) — with docker manifest inspect and the Hub Tags API for verification.

Executable File Not Found: 4 Reasons Docker Containers Exit Instantly (incl. exec format error)

Container fails with OCI runtime exec failed: executable file not found or standard_init_linux.go errors? Covers Windows CRLF breaking shebangs, ENTRYPOINT shell vs exec form, PATH and relative paths, and architecture mismatches — with docker inspect and --entrypoint debugging.

Cannot connect to the Docker daemon — 5 Reasons and How to Diagnose Each

docker ps fails with Cannot connect to the Docker daemon at unix:///var/run/docker.sock? Covers the Linux service not running, Docker Desktop not started, WSL2 integration reset, daemon.json syntax errors killing the daemon, wrong docker context, and missing rootless DOCKER_HOST — with journalctl diagnostics.

Container Name Is Already in Use: Resolving the Conflict Error

docker run fails with Conflict. The container name /xxx/ is already in use? An old container with the same name still exists — stopped containers keep their names too. Covers finding the squatter with docker ps -a, rm -f to free it, compose down for project leftovers, --rm for one-shot runs, and restart-policy resurrections.

Cannot find module: 6 Reasons Node.js Can't Find Your Module (and How to Fix Each)

Node.js ERR_MODULE_NOT_FOUND or Cannot find module errors decoded: require vs import path rules, node_modules corruption, case sensitivity, ESM/CommonJS mixing, monorepo workspaces — with fix commands for each.

Port Already in Use? The Complete EADDRINUSE Fix Guide (macOS/Linux/Windows)

Getting EADDRINUSE: address already in use or Port 3000 is already in use on startup? One set of commands to find and free the occupying process on every platform, plus prevention and the tricky cases (ControlCenter, TIME_WAIT, Docker).

npm ERR! network: How to Fix npm Install Timeouts with a Registry Mirror

Complete fix guide for npm install network failures (ETIMEDOUT, ECONNRESET, ERR_SOCKET_CONNECTION_TIMEOUT): switching to a fast registry mirror, cleaning caches, purging stale proxy configs, and the package-lock resolved-URL trap.

How to Fix 'Hydration failed / Text content does not match' in Next.js

The complete guide to Next.js/React SSR hydration errors: why server and client renders diverge, the three usual suspects (timestamps, random values, localStorage), correct suppressHydrationWarning usage, and a ClientOnly pattern for browser-only widgets.

How to Fix 'Cannot read properties of undefined (reading map)'

A complete debugging guide for the most common React runtime error: why undefined.map throws, handling async data correctly, optional chaining and default guards, empty-state rendering, and using our JSON tools to inspect the real payload.

JSON Troubleshooting Handbook: From Error Message to Fix (Symptom Index)

JSON errors that make no sense? This handbook organizes fixes by symptom — syntax errors, Unicode escaping, encoding mojibake, and Schema validation failures, with a diagnostic tree and a general debugging workflow.

Regex Matches Nothing / Greedy .* Swallows Results — How to Fix

Online regex test matches nothing, or greedy .* eats too much, or groups capture nothing? Explains greedy vs lazy, escaping special chars, and anchors — with a step-by-step debug using our regex tester.