- 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, 0 mismatches.
- 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: 242 of tsc's 12,750 conformance test variants (1.9%) 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 | 98.1% exact (all diagnostics and chains); 110 variants with a diagnostic tsc does not report, 4 of them in tests where tsc reports nothing |
| Six public projects: tRPC, date-fns, TypeORM, Playwright, RxJS, the React Compiler | All 6 match exactly (React Compiler: all 156; RxJS: all 21,908) |
| Ecosystem check stage | 709 packages pass, 0 fail (100%) |
Some of the 220 conformance extras are runner artefacts, such as tests for module kinds tsc 7 removed, which have no baseline.
Speed on a shared GitHub-hosted ubuntu-24.04 runner (4 vCPU, 4 threads; the bench of pull request #133 on 2026-11-10, at 63756ad, merged as b748322; 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 | 269.8 ms | 589.9 ms |
date-fns pkgs/core | 598.7 ms | 1,154.3 ms |
RxJS packages/rxjs | 3,458.3 ms | 3,645.7 ms |
On RxJS the two are close (about 1.05×, within this runner's spread inside one run), and on the same run's Linux arm64 runner tsc 7 was faster there (3,392.1 ms against 4,031.2 ms). With its saved history and nothing changed, ts-r-check took 31.7 ms on tRPC, 81.8 ms on date-fns and 62.7 ms on RxJS 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, TypeORM, Playwright and RxJS (about 21,900 diagnostics; 2026-11-06, 3042fe0). 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.