Use JSON Validation as a Pre-Commit Quality Check

A dependency was removed by hand, leaving a trailing comma after the last entry in dependencies. It commits fine, but the next person's npm install fails. The validator (and a jq empty hook) catches it pre-commit.

Input (package.json):
{
  "dependencies": {
    "react": "^18.0.0",
  }
}

Validator output:
Invalid JSON
Expected double-quoted property name in JSON at position 46 (line 4 column 3)

Fix: remove the comma after the react entry.

A translator's value contained an apostrophe and the editor 'helpfully' wrapped the whole value in single quotes. The locale file commits, then the app crashes on load in that language.

Input (fr.json):
{ "greeting": 'Bonjour' }

Validator output:
Invalid JSON
Expected property name or '}' in JSON at position 14

Fix: use double quotes -> "greeting": "Bonjour"
(json-format-fixer can convert single->double quotes automatically)

A test fixture was duplicated by pasting a second object below the first. Each is valid, but JSON allows one top-level value, so the fixture loader throws.

Input (user.fixture.json):
{ "id": 1, "role": "admin" }
{ "id": 2, "role": "user" }

Validator output:
Invalid JSON
Unexpected non-whitespace character after JSON at position 28 (line 2 column 1)

Fix: wrap both in an array, or split into two fixture files.

Resolving a merge conflict in a config fixture left the same key twice. This parses — V8 keeps the last value — so the validator passes it. This is the one class of bug syntax validation cannot catch.

Input (settings.fixture.json):
{ "timeout": 30, "timeout": 60 }

Validator output:
Valid JSON

Parsed output:
{ "timeout": 60 }   <- the first 'timeout' silently vanished

Takeaway: review merge-conflict resolutions by eye; neither this
tool nor jq flags duplicate keys.

After fixing, green plus a readable parsed shape confirms the fixture is both valid and structurally what the test expects.

Validator output:
Valid JSON  (118 B)

Parsed output:
{
  "id": 1,
  "role": "admin",
  "active": true
}

Safe to git add — and the lint-staged jq hook will pass it too.

{"a":1,} fails with Expected double-quoted property name. The most common reason a package.json or fixture breaks after a hand-edit. The validator points at the comma; json-format-fixer strips trailing commas automatically. A jq empty pre-commit hook blocks this for the whole team.

{"a":1,"a":2} parses and V8 keeps the last value. This validator does not detect duplicate keys — and neither does jq or node, so a jq-based hook won't either. The classic way this slips into a commit is a merge conflict resolved by keeping both sides. Review merge resolutions by eye.

JSON requires double quotes. A value or key in single quotes (often pasted from JS) fails immediately. Convert with json-format-fixer, which has a single→double quote pass, or fix by hand before staging.

A // TODO or /* */ in a strict JSON file is rejected (Unexpected token /). If you need a note inside JSON, add a string field like "_note": "…" instead. Files that are genuinely JSONC (tsconfig) shouldn't go through a strict pre-commit check at all.

JSON permits one top-level value. Pasting a second fixture object below the first yields Unexpected non-whitespace character after JSON. Wrap the records in a [ ] array, or use NDJSON with a line-by-line loader (NDJSON is not single-document JSON).

A file saved by a Windows editor may begin with U+FEFF. The tool trims it before parsing, so it validates here. But some tools and diff tooling treat a BOM as a content change — configure your editor to save UTF-8 without BOM to keep diffs clean and avoid surprising stricter parsers.

An empty .json fails with Unexpected end of JSON input. Usually means a generator wrote nothing or a file was created but not filled in. An empty JSON object is {}, an empty array is [] — pick the right empty value rather than committing zero bytes.

A fixture that is a bare scalar is valid JSON and validates green. If your loader expects an object/array, the mismatch is a test expectation, not a syntax error — but it's worth noticing in the parsed output before committing.

A raw tab/newline pasted into a string (instead of \t/\n) fails with Bad control character in string literal. Common when pasting multi-line copy into a locale string. Escape it, or move long text out of the JSON entirely.

Free tier caps files at 2 MB. Most fixtures and locale bundles are well under this. A multi-megabyte fixture is usually a sign the test data should be generated or loaded from a database, not committed. Larger files validate on Pro (100 MB) / Developer (5 GB).