fix(typescript): erase type exports (babel#9944)

* fix(typescript): erase type exports

* use Set instead of array for tracking and checking TS type declarations

* add a test for an interface exported before its declaration

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

@@ -16,8 +16,22 @@ function isInType(path) {
   }
 }
 
+function isTSExportableDeclaration(node) {
+  // all kinds of type exports that transpile to nothing
+  // exception is enums, since they transpile to JS values
+  return (
+    t.isTSInterfaceDeclaration(node) ||
+    t.isTSTypeAliasDeclaration(node) ||
+    t.isTSModuleDeclaration(node) ||
+    (t.isVariableDeclaration(node) && node.declare) ||
+    (t.isClassDeclaration(node) && node.declare) ||
+    t.isTSDeclareFunction(node)
+  );
+}
+
 interface State {
   programPath: any;
+  exportableTSNames: Set<string>;
 }
 
 const PARSED_PARAMS = new WeakSet();
@@ -40,6 +54,7 @@ export default declare((api, { jsxPragma = "React" }) => {
 
       Program(path, state: State) {
         state.programPath = path;
+        state.exportableTSNames = new Set();
 
         const { file } = state;
 
@@ -52,6 +67,32 @@ export default declare((api, { jsxPragma = "React" }) => {
           }
         }
 
+        // find exportable top level type declarations
+        for (const stmt of path.get("body")) {
+          if (isTSExportableDeclaration(stmt.node)) {
+            if (stmt.node.id && stmt.node.id.name) {
+              state.exportableTSNames.add(stmt.node.id.name);
+            } else if (
+              stmt.node.declarations &&
+              stmt.node.declarations.length > 0
+            ) {
+              for (const declaration of stmt.node.declarations) {
+                if (declaration.id && declaration.id.name) {
+                  state.exportableTSNames.add(declaration.id.name);
+                }
+              }
+            }
+          } else if (
+            t.isExportNamedDeclaration(stmt.node) &&
+            stmt.node.specifiers.length === 0 &&
+            isTSExportableDeclaration(stmt.node.declaration) &&
+            stmt.node.declaration.id &&
+            stmt.node.declaration.id.name
+          ) {
+            state.exportableTSNames.add(stmt.node.declaration.id.name);
+          }
+        }
+
         // remove type imports
         for (const stmt of path.get("body")) {
           if (t.isImportDeclaration(stmt)) {
@@ -94,6 +135,26 @@ export default declare((api, { jsxPragma = "React" }) => {
         }
       },
 
+      ExportNamedDeclaration(path, { exportableTSNames }) {
+        // remove export declaration if it's exporting only types
+        if (
+          path.node.specifiers.length > 0 &&
+          !path.node.specifiers.find(
+            exportSpecifier =>
+              !exportableTSNames.has(exportSpecifier.local.name),
+          )
+        ) {
+          path.remove();
+        }
+      },
+
+      ExportSpecifier(path, { exportableTSNames }) {
+        // remove type exports
+        if (exportableTSNames.has(path.node.local.name)) {
+          path.remove();
+        }
+      },
+
       TSDeclareFunction(path) {
         path.remove();
       },

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

@@ -8,3 +8,40 @@ declare namespace N {}
 export interface I {}
 export type T = number;
 export class C2 {}
+
+export { x, f, E, C }; // only E
+export { M, N, I as I1, T as T1 }; // everything removed
+export {
+  x as x2,
+  f as f2,
+  C as CC2,
+  E as E2,
+  M as M2,
+  N as N2,
+  I as I2,
+  T as T2,
+  C2 as C3
+}; // only E and C2
+
+interface II2 {}
+type AA = {};
+enum BB {
+  K
+}
+enum BB {
+  L = "LL"
+}
+export { II2, AA, BB };  // only BB
+export { II2 as II3, AA as AA2 };  // everything removed
+export { BB as BB1 }; // as-is
+
+interface II3 {}
+type AA2 = {};
+enum BB2 {}
+function foo() {}
+export { II3 as default, AA2 as A, BB2 as BB3, foo }; // only BB2 and foo
+
+// export an interface before declaration
+export { Bar } // everything removed
+export { Bar as Bar2, C2 as C4 } // only C4
+interface Bar {}

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

@@ -1 +1,32 @@
 export class C2 {}
+export { E }; // only E
+
+// everything removed
+export { E as E2, C2 as C3 }; // only E and C2
+
+var BB;
+
+(function (BB) {
+  BB[BB["K"] = 0] = "K";
+})(BB || (BB = {}));
+
+(function (BB) {
+  BB["L"] = "LL";
+})(BB || (BB = {}));
+
+export { BB }; // only BB
+
+// everything removed
+export { BB as BB1 }; // as-is
+
+var BB2;
+
+(function (BB2) {})(BB2 || (BB2 = {}));
+
+function foo() {}
+
+export { BB2 as BB3, foo }; // only BB2 and foo
+// export an interface before declaration
+
+// everything removed
+export { C2 as C4 }; // only C4