[alexander-akait]

Part of the performance series (#376–#382).

Problem

In watch mode the TypeScript check keeps its builder program warm. But every new process starts from nothing: a one-shot build, a CI run, a restarted dev server. In the default readonly mode the check never wrote any state, and no mode ever read a .tsbuildinfo back, so each run type-checked the whole program again.

The survey for this series found rollup's TypeScript plugins and tsc --incremental avoiding exactly this, with readBuilderProgram and emitBuildInfo.

Change

Applies when the program writes nothing (readonly, the default) and cache isn't false:

• Where the state goes: a .tsbuildinfo under the check's cacheLocation. The new default is node_modules/.cache/diagnostics-webpack-plugin/typescript/, with one file per config file and set of compiler options. A cacheLocation ending in .tsbuildinfo is used as the file itself.
• Building: with incremental and that tsBuildInfoFile, using the EmitAndSemantic builder (only it has emitBuildInfo). The state is written after each check.
• Reading back: a compiler's first program reads the state with ts.readBuilderProgram. If it's unreadable or from another TypeScript version, the check starts from nothing.
• File versions: a hash of each file's content instead of mtime:size, so a state written by another process still matches after a checkout or a CI cache restore. The stat-based reuse of parsed files in watch mode is unchanged.
• Unchanged: the modes that write keep using the config file's own incremental, and so does build.

Measured

301 files, TypeScript check (no skipLibCheck), two runs each:

	wall	CPU	peak RSS
one-shot, main (every run)	4.7–4.9 s	12.2–12.6 s	560–575 MB
one-shot, this PR, first run	4.7 s	12.1 s	580 MB
one-shot, this PR, later runs	1.96–2.04 s	5.9–6.2 s	456–466 MB
watch, this PR, restart with state: first build / rebuild 1	1.95 s / 150 ms
watch, main: first build / rebuild 1	4.0–4.2 s / 520–610 ms

webpack alone takes about 1.6 s on this project, so on a later run the check adds about 0.4 s. Watch rebuilds after the first are unchanged.

Tests

• New test/typescript/state.test.js:
  • a second run starts from the state the first one wrote;
  • an edit between runs that breaks a file which didn't itself change is reported;
  • cache: false writes nothing and reads nothing.
  • Mutation-checked: giving every file the same version makes the second test fail.
• Test setup: the TypeScript test config now gives each compiler its own temporary cacheLocation, as the ESLint tests already do. The recorder mock records the EmitAndSemantic builder too.
• Full suite: 317/317, global module path cleared, as on CI. eslint, tsc, prettier, cspell and lint:schema are clean.
• README (TypeScript, cacheLocation) and a minor changeset are updated, and types regenerated.

---

Generated by Claude Code

[linux-foundation-easycla]

• ✅ Unknown (2fcf862, ef4efed)
• ❌ - login: @claude / name: Claude. The commit (2fcf862, ef4efed) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please visit our EasyCLA portal and chat with our support bot.

One or more co-authors of this pull request were not found. You must specify co-authors in commit message trailer via:

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📥 Commits

Reviewing files that changed from the base of the PR and between f62bdb5 and ef4efed.

📒 Files selected for processing: 8

• .changeset/ts-keep-state-between-runs.md
• README.md
• src/checks/typescript-program.js
• src/checks/typescript.js
• test/typescript/mock/typescript-recorder/index.js
• test/typescript/state.test.js
• test/typescript/utils/conf.js
• types/checks/typescript.d.ts

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[alexander-akait]

EasyCLA fails here for the same reason as on #366–#375: the commit's Co-Authored-By: Claude trailer doesn't match a GitHub user with a signed CLA. The code isn't involved. Clearing it means a maintainer override or removing the trailer, so I'm leaving it for the maintainers to decide.

---

Generated by Claude Code

[alexander-akait]

One CI job failed: Test - windows-latest - Node v26.x, Webpack latest, ESLint latest. The failing test is test/incremental.test.js › "should lint again what a change reaches among the files it globs" (3 files linted, 2 expected). It is not this PR's:

• The test drives the ESLint check through a mocked ESLint with a watch rebuild.
• This PR only touches the TypeScript check (typescript.js, typescript-program.js), and the test passed on every other OS/Node/ESLint combination of the same commit.

The likely cause is a late watcher event on Windows: a rebuild picks up the fixture's utimes reset on watch-other.js. No fix exists yet. I'm re-running the failed job once. If it fails again, I'll make that test ignore watcher events for files it did not edit, the same way #379's watch test does.

Separately, EasyCLA fails because the commits carry a Co-Authored-By: Claude trailer that maps to no GitHub account. That is for the maintainers to decide on; I haven't rewritten history.

---

Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.64%. Comparing base (f62bdb5) to head (ef4efed).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #383    +/-   ##
========================================
  Coverage   99.64%   99.64%            
========================================
  Files          19       19            
  Lines        5570     5689   +119     
========================================
+ Hits         5550     5669   +119     
  Misses         20       20            

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