Support lib: "baseline-widely-available"

[will-stone]

🔍 Search Terms

"Baseline Widely Available"

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

When creating websites, it is now an industry standard to support Baseline Widely Available JS APIs. Rather than only being able to select an ECMAScript year in the lib config, how about adding "baseline-widely-available" where it updates with each TS release, only allowing JS API and syntax that is marked as Widely Available?

📃 Motivating Example

To ensure that you only use API and syntax supported by all major browsers, you will be able to set:

{
  "compilerOptions": {
    "lib": ["baseline-widely-available"]
  }
}

This means that you can be assured that all the JS you write has been supported for at least 30 months.

💻 Use Cases

1. Writing modern websites and always knowing which APIs are available for use.
2. You could use ESLint rules to ban too modern JS API but that is an opt-out structure, where this would prevent mistakes at the lowest level, when writing the code.
3. Currently using lib: "ES2022" as this covers the time gap but it's not self-updating and could still include items that some browsers are yet to support in the 30 month version history.

[3ru]

+1 to having this in TypeScript!

To make the discussion more concrete and move it forward, I opened a draft PR (#62540) with a PoC baseline lib.

It’d be great to sync with the Baseline team on feasibility, mapping granularity, and the update process. cc @rviscomi @foolip @captainbrosset.

Happy to iterate in whatever direction the team prefers.

[3ru]

Hi @RyanCavanaugh, I’d love your perspective on the idea’s feasibility and design.

Do you think a “Baseline Widely Available” lib fits TS? I’m leaning toward an external generator (like the TypeScript-DOM-lib-generator) that emits opt‑in files in the tree: src/lib/baseline.generated.d.ts and src/lib/baseline.dom.generated.d.ts.
Rationale: files like es2023.array.d.ts mix features (e.g., findLast = widely, toReversed = limited), so file‑level <reference lib="..."> can’t include one and exclude the other—whereas a generator can.

Does this approach sound reasonable, or would you prefer another path (e.g., more native composition inside src/lib)?

[compeek]

This is maybe not quite the exact same idea as the original suggestion, but it would be nice to have a lib option like current-year.

TypeScript 6 now defaults target to the "current-year ES version", and target already implies a matching lib, which means that lib now effectively defaults to the current-year ES version as well. However, there seems to be no way to specify that value explicitly, which is unfortunate because it means there's no way to exclude dom (which is included by default) without hardcoding a year.

I suppose for consistency it might be nice to have an explicit current-year option for target as well.