[api-extractor] Please support typescript 3.8

[jrnail23]

Is this a feature or a bug?

• Feature
• Bug

Please describe the actual behavior.

What is the expected behavior?

[octogonz]

Are there any new features in 3.8 that could potentially break API Extractor?

[jrnail23]

Yes, it seems that import type { MyType } from "some-module" breaks it (or at least that's what I think broke my setup).
I kept getting warnings falsely complaining that my types were missing a release/scope annotation (i.e., @public).

[octogonz]

Yeah, if that gets emitted in the .d.ts file, it might be a new import pattern. API Extractor detects and walks each kind of import.

Some 3.8 features that might break something:

• ECMAScript private fields class Person { #name: string }
• export * as utilities from "./utilities.js"; - in the past this had to be an import/export pair instead of one statement

From a quick glance at the 3.9 features, I didn't see anything that looked like it would impact API Extractor.

[octogonz]

We can start work on a PR to add support for these issues. None of them seems particularly difficult.

But I should also point out:

• Using any of these features in your API might make your library unusable by consumers who haven't upgraded to the latest TypeScript compiler.
• A number of folks may be stuck on TypeScript 3.6 for a while, because TypeScript 3.7 broke compatibility with all previous compiler releases even for code that doesn't use any 3.7 language features (!)

[MattiasBuelens]

Using any of these features in your API might make your library unusable by consumers who haven't upgraded to the latest TypeScript compiler.

That's where a tool like downlevel-dts can come in, to transform these new features into older syntax for older TypeScript versions. 😉 So you would first use API Extractor to create a .d.ts file for TypeScript 3.8, and then you would run downlevel-dts to create a variant for older TypeScript versions. 🙂

[octogonz]

TypeScript 3.9 introduces some new syntax also: #1914

[octogonz]

Some progress on this:

• Here's a PR that upgrades the compiler engine: [api-extractor] Upgrade compiler engine to TypeScript 3.9 #1983
• Here's a PR that adds support for import type: [api-extractor] Add support for "import type" imports #1985

[octogonz]

• Here's a PR that adds support for ECMAScript private fields: [api-extractor] Add support for ECMAScript private fields #1987

With these changes, I think we can resolve this issue. There are still some remaining unsupported language features like #1914, but they have reasonable workarounds.

[octogonz]

This fix was released with API Extractor 7.9.0