[ts] Insert export {} when necessary to imply ESM (#13314)

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

@@ -21,8 +21,12 @@ function isInType(path) {
   }
 }
 
-const PARSED_PARAMS = new WeakSet();
 const GLOBAL_TYPES = new WeakMap();
+// 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 unamibiguously that results are ESM.
+const NEEDS_EXPLICIT_ESM = new WeakMap();
+const PARSED_PARAMS = new WeakSet();
 
 function isGlobalType(path, name) {
   const program = path.find(path => path.isProgram()).node;
@@ -175,7 +179,8 @@ export default declare((api, opts) => {
       Identifier: visitPattern,
       RestElement: visitPattern,
 
-      Program(path, state) {
+      Program: {
+        enter(path, state) {
           const { file } = state;
           let fileJsxPragma = null;
           let fileJsxPragmaFrag = null;
@@ -211,6 +216,10 @@ export default declare((api, opts) => {
           // remove type imports
           for (let stmt of path.get("body")) {
             if (stmt.isImportDeclaration()) {
+              if (!NEEDS_EXPLICIT_ESM.has(state.file.ast.program)) {
+                NEEDS_EXPLICIT_ESM.set(state.file.ast.program, true);
+              }
+
               if (stmt.node.importKind === "type") {
                 stmt.remove();
                 continue;
@@ -218,10 +227,13 @@ export default declare((api, opts) => {
 
               // If onlyRemoveTypeImports is `true`, only remove type-only imports
               // and exports introduced in TypeScript 3.8.
-            if (!onlyRemoveTypeImports) {
+              if (onlyRemoveTypeImports) {
+                NEEDS_EXPLICIT_ESM.set(path.node, false);
+              } else {
                 // Note: this will allow both `import { } from "m"` and `import "m";`.
                 // In TypeScript, the former would be elided.
                 if (stmt.node.specifiers.length === 0) {
+                  NEEDS_EXPLICIT_ESM.set(path.node, false);
                   continue;
                 }
 
@@ -249,6 +261,7 @@ export default declare((api, opts) => {
                     importsToRemove.push(binding.path);
                   } else {
                     allElided = false;
+                    NEEDS_EXPLICIT_ESM.set(path.node, false);
                   }
                 }
 
@@ -285,8 +298,24 @@ export default declare((api, opts) => {
             }
           }
         },
+        exit(path) {
+          if (
+            path.node.sourceType === "module" &&
+            NEEDS_EXPLICIT_ESM.get(path.node)
+          ) {
+            // If there are no remaining value exports, this file can no longer
+            // be inferred to be ESM. Leave behind an empty export declaration
+            // so it can be.
+            path.pushContainer("body", t.exportNamedDeclaration());
+          }
+        },
+      },
+
+      ExportNamedDeclaration(path, state) {
+        if (!NEEDS_EXPLICIT_ESM.has(state.file.ast.program)) {
+          NEEDS_EXPLICIT_ESM.set(state.file.ast.program, true);
+        }
 
-      ExportNamedDeclaration(path) {
         if (path.node.exportKind === "type") {
           path.remove();
           return;
@@ -307,7 +336,10 @@ export default declare((api, opts) => {
           )
         ) {
           path.remove();
+          return;
         }
+
+        NEEDS_EXPLICIT_ESM.set(state.file.ast.program, false);
       },
 
       ExportSpecifier(path) {
@@ -317,14 +349,22 @@ export default declare((api, opts) => {
         }
       },
 
-      ExportDefaultDeclaration(path) {
+      ExportDefaultDeclaration(path, state) {
+        if (!NEEDS_EXPLICIT_ESM.has(state.file.ast.program)) {
+          NEEDS_EXPLICIT_ESM.set(state.file.ast.program, true);
+        }
+
         // remove whole declaration if it's exporting a TS type
         if (
           t.isIdentifier(path.node.declaration) &&
           isGlobalType(path, path.node.declaration.name)
         ) {
           path.remove();
+
+          return;
         }
+
+        NEEDS_EXPLICIT_ESM.set(state.file.ast.program, false);
       },
 
       TSDeclareFunction(path) {

packages/babel-plugin-transform-typescript/test/fixtures/declarations/erased/output.mjs

@@ -1 +1,3 @@
 ; // Otherwise-empty file
+
+export {};

packages/babel-plugin-transform-typescript/test/fixtures/declarations/nested-namespace/output.mjs

@@ -1 +1,3 @@
 ; // Otherwise-empty file
+
+export {};

packages/babel-plugin-transform-typescript/test/fixtures/exports/export-type-from/output.mjs

@@ -1 +1,2 @@
 ;
+export {};

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

@@ -1 +1,2 @@
 ;
+export {};

packages/babel-plugin-transform-typescript/test/fixtures/imports/elide-preact/output.mjs

@@ -1 +1,2 @@
 const x = 0;
+export {};

packages/babel-plugin-transform-typescript/test/fixtures/imports/elide-react/output.mjs

@@ -1 +1,2 @@
 const x = 0;
+export {};

packages/babel-plugin-transform-typescript/test/fixtures/imports/elide-typeof/output.mjs

@@ -1 +1,2 @@
 const x = 0;
+export {};

packages/babel-plugin-transform-typescript/test/fixtures/imports/elision-qualifiedname/output.mjs

@@ -1 +1,2 @@
 const x = 0;
+export {};

packages/babel-plugin-transform-typescript/test/fixtures/imports/elision-rename/output.mjs

@@ -1 +1,2 @@
 const x = 0;
+export {};

packages/babel-plugin-transform-typescript/test/fixtures/imports/enum-id/output.mjs

@@ -5,3 +5,4 @@ var Enum;
 })(Enum || (Enum = {}));
 
 ;
+export {};

packages/babel-plugin-transform-typescript/test/fixtures/imports/import-type-not-removed/output.mjs

@@ -1,2 +1,3 @@
 // TODO: This should not be removed
 ;
+export {};

packages/babel-plugin-transform-typescript/test/fixtures/imports/import-type/output.mjs

@@ -1 +1,2 @@
 ;
+export {};

packages/babel-plugin-transform-typescript/test/fixtures/imports/property-signature/output.mjs

@@ -1,3 +1,4 @@
 const obj = {
   A: 'foo'
 };
+export {};

packages/babel-plugin-transform-typescript/test/fixtures/namespace/canonical/output.mjs

@@ -37,3 +37,5 @@ for (let s of strings) {
     console.log(`"${s}" - ${validators[name].isAcceptable(s) ? "matches" : "does not match"} ${name}`);
   }
 }
+
+export {};