Validate JSON Configuration Files Before Deployment

appsettings.json was hand-edited to add a log path on a Windows box. The single backslashes in C:\Logs\app are read as escape sequences, and \L isn't a valid JSON escape — the service won't start.

Input (appsettings.json):
{ "LogPath": "C:\Logs\app" }

Validator output:
Invalid JSON
Bad escaped character in JSON at position 14

Fix: double the backslashes -> "C:\\Logs\\app"  (or use / )

A CI step ran a plain JSON check over every *.json including tsconfig.json. The comment makes a strict parser reject it. This validator does the same — telling you the step is using the wrong parser, not that tsconfig is broken.

Input (tsconfig.json):
{
  // strict everywhere
  "compilerOptions": { "strict": true },
}

Validator output:
Invalid JSON
Unexpected token / … is not valid JSON

Takeaway: tsconfig is valid JSONC. Exclude it from strict-JSON steps,
or strip comments with json-format-fixer if a step truly needs JSON.

A missing comma between two top-level resources isn't always a hard error — but here it makes the parser fail. When a similar mistake does parse, the pretty output reveals the wrong nesting before deploy.

Input (template.json):
{
  "Resources": {
    "Bucket": { "Type": "AWS::S3::Bucket" }
    "Queue":  { "Type": "AWS::SQS::Queue" }
  }
}

Validator output:
Invalid JSON
Expected ',' or '}' after property value in JSON at position 71 (line 4 column 5)

Fix: add a comma after the Bucket block.

Generated *.tfvars.json got a trailing comma after the last variable from a script that appended entries. terraform rejects it; so does this validator, at the comma's position.

Input (prod.tfvars.json):
{
  "region": "us-east-1",
  "replicas": 3,
}

Validator output:
Invalid JSON
Expected double-quoted property name in JSON at position 47 (line 4 column 1)

Fix: remove the comma after "replicas": 3

After fixing, a green verdict plus a readable parsed structure is your go-signal. The parsed output confirms the nesting matches intent.

Validator output:
Valid JSON  (842 B)

Parsed output:
{
  "region": "us-east-1",
  "replicas": 3,
  "tls": true
}

Safe to deploy — and the same file passes the jq gate in CI.

These are JSON-with-Comments: they legitimately contain // comments and trailing commas. Strict JSON.parse rejects them, so this validator does too. That is correct behaviour — it tells you a strict-JSON step shouldn't own these files. Keep them with their JSONC-aware tool, or strip comments with json-format-fixer if a strict step truly needs them.

"C:\Users\app" fails because \U and \a aren't valid JSON escapes (Bad escaped character). Config files written on Windows hit this constantly. Either double every backslash (C:\\Users\\app) or use forward slashes, which work on Windows for most paths.

{"port":80,} is the most common hand-edit error. Strict JSON forbids it (Expected double-quoted property name). The validator gives the comma's position; json-format-fixer removes trailing commas automatically and re-parses.

{"port":80,"port":443} parses, and V8 keeps the last value (443). The validator does not warn about this — and neither does jq or a node loader. If two config sections set the same key, the file ships valid with the later value winning; review duplicates by hand.

JSON requires double quotes for strings and keys. {'port':80} fails immediately. This often comes from copying a JS object literal into a config file. Convert with json-format-fixer or fix by hand.

A config saved by a Windows editor may start with a U+FEFF BOM. The tool trims it before parsing (U+FEFF is whitespace in ECMAScript), so the file validates. Be aware some loaders (certain Java/Go config readers) don't strip the BOM — one that passes here may still trip a stricter loader.

An empty config file fails with Unexpected end of JSON input. Usually means a templating step rendered nothing (an undefined variable, a failed generator). Check that the file actually got written before the deploy step read it.

Some tools take a config that is a top-level array (e.g. a list of rules). [ {...}, {...} ] is valid JSON and validates green. If your loader expects an object at the root, that's a loader expectation, not a JSON error.

Adding a single // note line to an appsettings.json to leave yourself a reminder turns valid JSON into invalid JSON — strict parsers reject the /. If you need inline notes, add a string field like "_comment": "…" instead of a real comment.

Free tier caps files at 2 MB — far above any normal config. A config that large usually means data was embedded that belongs elsewhere. Validate big bundled config on Pro (100 MB) or Developer (5 GB).