- Alpha
- On npm: @ts-r.dev/check
ts-r-check: type checker
A type checker with tsc's command line and output format. It reports only what it is sure of; many checks are still missing.
ts-r-check type-checks a TypeScript project and prints diagnostics in tsc's format, with tsc's error codes. Like tsc, it also writes the project's JavaScript, source maps and declaration files unless noEmit is set; pass --noEmit to only check, as with tsc --noEmit.
The checker is the least finished part of ts-r. Read Status before you rely on it.
How to read its results
The checker follows one rule: no false positives first. When it meets something it doesn't implement yet, it gives that expression a silent type that is never reported. So:
- An error from ts-r-check is very likely an error tsc reports too.
- A clean run from ts-r-check does not mean tsc is clean. It can miss errors.
Keep tsc in CI until ts-r-check covers the features your project uses.
What works today
- Loading the project from
tsconfig.json, includingextends, with tsc's config diagnostics such as TS6053. - Module resolution that matches tsc's: 158,174 resolutions across 114 configs of five public repos, 10 mismatches (one
requireof vitest, in tRPC). - Assignability, calls and overloads, generics and inference, unions and narrowing, type operators (
keyof, mapped, conditional and literal types), classes and declarations, and unused-code checks. Errors such as TS2322, TS2304, TS2307 and TS7006 print with tsc's wording. - tsc's output in both plain and pretty form.
Not yet
- Octal escapes in strings (TS1487) are reported only with
--no-history, not by a default run or with--noCheck. Regular expression errors don't carry tsc's "Did you mean" suggestions (TS1369). - Some inference: contextual types that flow through a reverse mapped type print wider than tsc's (
number | T["a"]where tsc printsnumber). - Some relation and narrowing results differ from tsc's, mostly in TS2322, TS2339 and TS2345: 385 of tsc's 12,750 conformance test variants (3.0%) don't match exactly.
The full list is in ts_r_checker's notes.
Command line
usage: ts-r-check [-p tsconfig.json] [options]--help lists every option. It accepts tsc 7's command line; the flags below are the common ones.
| Flag | What it does |
|---|---|
-p <path> | The project's tsconfig.json. Without it, the nearest tsconfig.json above the current directory (TS5058 or TS5057 when there is none). |
--noEmit | Only check: write no JavaScript, source maps or declarations. Without it (and without noEmit in the tsconfig), ts-r-check writes them as tsc does; syntax it cannot lower for the target yet is reported as TSR2003. |
--pretty false | Plain output: path(line,col): error TSxxxx: message, chain lines indented two spaces per level, no summary. |
--threads N | Checker threads. Default 4, clamped to 1 to 4. |
Pretty output is the default on a terminal. It adds code frames and a Found N errors in M files. line. Paths are relative to the current directory, and output is sorted by path, then position. Config diagnostics print first.
Exit codes follow tsc:
| Code | When |
|---|---|
| 0 | No errors |
| 1 | Errors, and --noEmit, noEmit or noEmitOnError is in effect |
| 2 | Errors otherwise (tsc's "errors, outputs generated") |
You can also run it through the launcher as ts-r check.
Lib files
ts-r-check needs TypeScript's lib.*.d.ts files. It looks in this order:
- The directory in
TS_R_LIB_DIR. - A
lib/directory next to the executable (or../lib). target/upstream/typescript/tsc/internal/bundled/libsin a source checkout, aftercargo xtask upstream sync --minimal.
See Getting started for the source setup.
Next to tsc
ts-r-check reads the same tsconfig.json and prints the same format, so editor problem matchers and CI annotations that parse tsc's output work unchanged. It also takes tsc's --build and --watch, and writes the same output files. Running both and comparing their output is a good way to try it; ts-r-build makes bundles.
For stricter checks than tsc's, see Sound mode.
Measured
Against tsc 7.0.2:
| Test | Result |
|---|---|
| tsc's conformance baselines, 12,750 compared test variants | 97.0% exact (all diagnostics and chains); 191 variants with a diagnostic tsc does not report, 10 of them in tests where tsc reports nothing |
| Six public projects: tRPC, date-fns, TypeORM, Playwright, RxJS, the React Compiler | 3 of 6 match exactly; Playwright 2 extra; RxJS 6 extra and 5 missing of 21,908; React Compiler 71 extra, 0 missing of 156 |
| Ecosystem check stage | 652 packages pass, 57 fail (92.0%) |
Most extra diagnostics in the ecosystem run are TS2344, TS2536, TS2417 and TS2716. Some of the 220 conformance extras are runner artefacts, such as tests for module kinds tsc 7 removed, which have no baseline.
Speed on four dedicated Linux cores (the review box's bench of pull request #42 on 2026-10-24, cargo xtask bench --repos, cold check, no history, median of 5; peak memory not measured):
| Project | ts-r-check | tsc 7.0.2 |
|---|---|---|
tRPC packages/server | 474.4 ms | 979.0 ms |
date-fns pkgs/core | 998.4 ms | 1,980.1 ms |
RxJS packages/rxjs | 1,202.1 ms | 6,513.9 ms |
With its saved history and nothing changed, ts-r-check took 140.9 ms on RxJS, 401.0 ms on tRPC and 1,020.4 ms on date-fns in that run.
An older run with two more projects and peak memory (Apple M-series shared laptop, 2026-10-16 at commit 9c3a371, median of 5, peak memory of one run):
| Project | ts-r-check | tsc 7.0.2 |
|---|---|---|
tRPC packages/server | 108 ms, 117 MB | 189 ms, 294 MB |
date-fns pkgs/core | 226 ms, 216 MB | 323 ms, 558 MB |
RxJS packages/rxjs | 462 ms, 458 MB | 1,280 ms, 1,168 MB |
TypeORM packages/typeorm | 548 ms, 377 MB | 567 ms, 826 MB |
| Playwright (root project) | 592 ms, 463 MB | 685 ms, 1,094 MB |
With its saved history and nothing changed, ts-r-check took 23 to 126 ms on those projects. These repos don't type-check cleanly under TypeScript 7. ts-r-check reports exactly tsc's diagnostics on tRPC, date-fns and TypeORM, 2 extra on Playwright, and 6 extra and 5 missing of about 21,900 on RxJS (2026-10-24, 087afc9). ts-r also skips work it doesn't implement yet, so these numbers will move as the checker grows.
Method and commits: check on Linux, check on the laptop, ecosystem and correctness.