Strip type-only TS namespaces

packages/babel-plugin-transform-typescript/src/global-types.ts

@@ -0,0 +1,24 @@
+import type { NodePath, Scope } from "@babel/traverse";
+
+export const GLOBAL_TYPES = new WeakMap<Scope, Set<string>>();
+
+export function isGlobalType({ scope }: NodePath, name: string) {
+  if (scope.hasBinding(name)) return false;
+  if (GLOBAL_TYPES.get(scope).has(name)) return true;
+
+  console.warn(
+    `The exported identifier "${name}" is not declared in Babel's scope tracker\n` +
+      `as a JavaScript value binding, and "@babel/plugin-transform-typescript"\n` +
+      `never encountered it as a TypeScript type declaration.\n` +
+      `It will be treated as a JavaScript value.\n\n` +
+      `This problem is likely caused by another plugin injecting\n` +
+      `"${name}" without registering it in the scope tracker. If you are the author\n` +
+      ` of that plugin, please use "scope.registerDeclaration(declarationPath)".`,
+  );
+
+  return false;
+}
+
+export function registerGlobalType(programScope: Scope, name: string) {
+  GLOBAL_TYPES.get(programScope).add(name);
+}

packages/babel-plugin-transform-typescript/src/index.ts

@@ -2,12 +2,17 @@ import { declare } from "@babel/helper-plugin-utils";
 import syntaxTypeScript from "@babel/plugin-syntax-typescript";
 import type { PluginPass, types as t } from "@babel/core";
 import { injectInitialization } from "@babel/helper-create-class-features-plugin";
-import type { Binding, NodePath, Scope } from "@babel/traverse";
+import type { Binding, NodePath } from "@babel/traverse";
 import type { Options as SyntaxOptions } from "@babel/plugin-syntax-typescript";
 
 import transpileConstEnum from "./const-enum.ts";
 import type { NodePathConstEnum } from "./const-enum.ts";
 import transpileEnum from "./enum.ts";
+import {
+  GLOBAL_TYPES,
+  isGlobalType,
+  registerGlobalType,
+} from "./global-types.ts";
 import transpileNamespace from "./namespace.ts";
 
 function isInType(path: NodePath) {
@@ -36,34 +41,12 @@ function isInType(path: NodePath) {
   }
 }
 
-const GLOBAL_TYPES = new WeakMap<Scope, Set<string>>();
 // Track programs which contain imports/exports of values, so that we can include
 // empty exports for programs that do not, but were parsed as modules. This allows
 // tools to infer unambiguously that results are ESM.
 const NEEDS_EXPLICIT_ESM = new WeakMap();
 const PARSED_PARAMS = new WeakSet();
 
-function isGlobalType({ scope }: NodePath, name: string) {
-  if (scope.hasBinding(name)) return false;
-  if (GLOBAL_TYPES.get(scope).has(name)) return true;
-
-  console.warn(
-    `The exported identifier "${name}" is not declared in Babel's scope tracker\n` +
-      `as a JavaScript value binding, and "@babel/plugin-transform-typescript"\n` +
-      `never encountered it as a TypeScript type declaration.\n` +
-      `It will be treated as a JavaScript value.\n\n` +
-      `This problem is likely caused by another plugin injecting\n` +
-      `"${name}" without registering it in the scope tracker. If you are the author\n` +
-      ` of that plugin, please use "scope.registerDeclaration(declarationPath)".`,
-  );
-
-  return false;
-}
-
-function registerGlobalType(programScope: Scope, name: string) {
-  GLOBAL_TYPES.get(programScope).add(name);
-}
-
 // A hack to avoid removing the impl Binding when we remove the declare NodePath
 function safeRemove(path: NodePath) {
   const ids = path.getBindingIdentifiers();

packages/babel-plugin-transform-typescript/src/namespace.ts

@@ -1,6 +1,8 @@
 import { template, types as t } from "@babel/core";
 import type { NodePath } from "@babel/traverse";
 
+import { registerGlobalType } from "./global-types.ts";
+
 export default function transpileNamespace(
   path: NodePath<t.TSModuleDeclaration>,
   allowNamespaces: boolean,
@@ -24,6 +26,11 @@ export default function transpileNamespace(
   const name = path.node.id.name;
   const value = handleNested(path, t.cloneNode(path.node, true));
   if (value === null) {
+    // This means that `path` is a type-only namespace.
+    // We call `registerGlobalType` here to allow it to be stripped.
+    const program = path.findParent(p => p.isProgram());
+    registerGlobalType(program.scope, name);
+
     path.remove();
   } else if (path.scope.hasOwnBinding(name)) {
     path.replaceWith(value);

packages/babel-plugin-transform-typescript/test/fixtures/namespace/export-type-only/input.ts

@@ -0,0 +1,11 @@
+// #15940
+namespace Platform {
+  export type Name = "web";
+
+  export namespace Data {
+    export type Type = string;
+    export declare namespace Empty {}
+  }
+}
+
+export { Platform };

packages/babel-plugin-transform-typescript/test/fixtures/namespace/export-type-only/output.mjs

@@ -0,0 +1 @@
+export {};