This PoC adds a newlib value,baseline, to TypeScript.
When users setcompilerOptions.lib: ["baseline"] in tsconfig.json, the compiler only exposes types for JavaScript built‑ins that have reached Baseline Widely Available ("high"). Any built‑ins that are not yet in Baseline (low/false) are left undeclared and thus surface as ordinary type errors when referenced.
Language syntax (operators/statements/grammar/classes) is out of scope for this PR.

Scope (In/Out)

In: ECMAScript (JS) built‑in APIs only.
Data source: Features underjavascript.builtins.* in web‑features whosestatus.baseline == "high" (Widely Available).
Out: Language syntax (operators/statements/grammar/classes), DOM/Web API/Worker/ScriptHost (host environment APIs). (TODO)

Spec

baseline.
- Concretely, we introducesrc/lib/baseline.d.ts, and reference only those existingsrc/lib/es20xx.*.d.ts fragments that correspond to Baseline Widely features via/// <reference lib="...">.
- Built‑ins that haven't reached Baseline remain undeclared, so referencing them produces normal type errors.

Data Source and Verification (jq)

Data:web-featuresdata.json
- Path:/Users/ru/Documents/eslint-plugin/eslint-plugin-baseline-js/node_modules/web-features/data.json
We extract feature IDs wherestatus.baseline == "high" and anycompat_features entry starts withjavascript.builtins.. We verified both the count and the list.

Set a variable:

DATA_PATH="/Users/ru/Documents/eslint-plugin/eslint-plugin-baseline-js/node_modules/web-features/data.json"

Count (unique feature IDs):

jq -r'.features | to_entries[] \  | select(.value.status?.baseline=="high") \  | select(any(.value.compat_features[]?; startswith("javascript.builtins."))) \  | .key'"$DATA_PATH"| sort -u| wc -l

List (unique feature IDs):

jq -r'.features | to_entries[] \  | select(.value.status?.baseline=="high") \  | select(any(.value.compat_features[]?; startswith("javascript.builtins."))) \  | .key'"$DATA_PATH"| sort -u

At the time of writing, we extract 65 JS built‑in features as Baseline Widely.

TODO

Scope and variants
- Should we offer abaseline.full variant similar toes2015.full that also includes host libs likedom? (Note: this would mean JS is Baseline‑gated, but DOM is not.)
Definingbaseline.d.ts
- Should we keep the allowlist references in the TS repo manually, or provide a mapper/tooling on the WebDX CG side? Either way, this repo needs the final mapping.
- Known mismatch example between web‑features granularity and TS lib fragment granularity:
  - src/lib/es2023.array.d.ts currently contains both:
    - findLast/findLastIndex (web‑features:array-findlast, Baseline = high), and
    - toReversed/toSorted/toSpliced/with (web‑features:array-by-copy, Baseline = low)
  - We should decide whether to split this into separate fragments (e.g.es2023.array.findlast vses2023.array.by-copy) sobaseline can safely include only the Baseline‑high set.
Sustainable data maintenance