feat: add PackageTypesResolver

Author: velut

src/package-types-resolver.ts

@@ -0,0 +1,49 @@
+import type { NormalizedPackageJson } from "read-pkg";
+import { exports } from "resolve.exports";
+
+export class PackageTypesResolver {
+	#pkgJson: Partial<NormalizedPackageJson>;
+	#subpath: string;
+
+	constructor(pkgJson: Partial<NormalizedPackageJson>, subpath: string) {
+		this.#pkgJson = pkgJson;
+		this.#subpath = subpath.trim();
+	}
+
+	resolveTypes(): string | undefined {
+		return this.resolveExportsMapTypes() ?? this.resolveTypesOrTypings();
+	}
+
+	resolveExportsMapTypes(): string | undefined {
+		try {
+			const entries =
+				exports(this.#pkgJson, this.#subpath, {
+					conditions: ["types", "import", "node"],
+					unsafe: true,
+				}) ?? [];
+			const entry = entries.at(0);
+			if (!entry || !this.isTypesFile(entry)) return undefined;
+			return entry;
+		} catch {
+			// The package may not have an `exports` map.
+			return undefined;
+		}
+	}
+
+	resolveTypesOrTypings(): string | undefined {
+		// Try to find the `package.json#/types` (or `typings`) file
+		// but accept it only to describe the types for the root subpath.
+		if (!this.isRootSubpath()) return undefined;
+		const entry = this.#pkgJson.types || this.#pkgJson.typings;
+		if (!entry || !this.isTypesFile(entry)) return undefined;
+		return entry;
+	}
+
+	isRootSubpath(): boolean {
+		return [".", this.#pkgJson.name].includes(this.#subpath);
+	}
+
+	isTypesFile(filename: string): boolean {
+		return [".d.ts", ".d.mts", ".d.cts"].some((ext) => filename.endsWith(ext));
+	}
+}