A pre-commit hook that checks whether .json files parse as
JSON5 (JSON with comments, trailing commas, and more). A drop-in replacement
for the standard check-json hook that also accepts JSON5, without giving up the speed of the C
parser on ordinary JSON.
- Accepts JSON5 syntax (comments, trailing commas) that the standard
check-jsonhook rejects. - Fast path: strict JSON is parsed with the C-speed stdlib
jsonparser, falling back to the pure-Pythonjson5parser only when a file actually uses JSON5 features (~500× faster on large strict-JSON files; see Performance). - Duplicate-key detection on both paths.
- Drop-in replacement for
check-json.
In .pre-commit-config.yaml, under repos:, add:
- repo: https://github.com/simonvanlierde/check-json5
rev: v1.2.0
hooks:
- id: check-json5Remove the original check-json hook if it is already included. A file passes when it parses as
JSON5 and has no duplicate keys.
JSON5 is similar to JSON but permits comments, trailing commas, and more. It resembles "JSONC" (JSON
with Comments) but, unlike JSONC, has an actual specification. Many config files in the wild are
really JSON5, so the standard check-json hook, which rejects comments outright, flags them as
invalid. This hook accepts them.
Strict-JSON files are parsed with the fast built-in json
library; only files that use JSON5 features fall back to the pure-Python
json5 library.
The pure-Python json5 parser is slow on large files. Since most .json files are strict JSON, the
hook tries the C-speed stdlib json parser first and only falls back to json5 when a file uses
comments or trailing commas. Duplicate-key detection works on both paths.
Parse time per file (strict JSON, taking the fast path):
| File size | Before (json5 only) |
After (fast path) | Speedup |
|---|---|---|---|
| ~1 KB config | 7 ms | <0.1 ms | ~600× |
| ~50 KB | 235 ms | 0.4 ms | ~645× |
| ~500 KB | 2.4 s | 4 ms | ~575× |
| ~2 MB | 10.4 s | 21 ms | ~500× |
Files that use JSON5 syntax still go through the json5 parser and are unchanged. For tiny config
files the absolute saving is negligible next to Python's own startup cost; the win matters on large
JSON files.
The project uses uv for dependency management, pytest for
tests, pre-commit for local checks, and GitHub Actions for CI with coverage reported to Codecov.
uv sync # install dependencies
uv run pytest # run the test suite
uv run pytest --cov # tests with coverage
pre-commit run -a # run the configured hooks locallyAdapted by Simon van Lierde from the original check-json5 by Ben Mares, itself based on the pre-commit-hooks repository by Anthony Sottile and contributors. This version switches dependency management from Poetry to uv and adds the fast-path parser and benchmark above.
MIT, consistent with the upstream projects it is based on (Ben Mares, pre-commit-hooks).