Support TypeScript const enums (#13324)

packages/babel-plugin-transform-typescript/src/const-enum.ts

@@ -0,0 +1,89 @@
+import type * as t from "@babel/types";
+import type { NodePath } from "@babel/traverse";
+
+import { translateEnumValues } from "./enum";
+
+export default function transpileConstEnum(
+  path: NodePath<t.TSEnumDeclaration & { const: true }>,
+  t: typeof import("@babel/types"),
+) {
+  const { name } = path.node.id;
+
+  const parentIsExport = path.parentPath.isExportNamedDeclaration();
+  let isExported = parentIsExport;
+  if (!isExported && t.isProgram(path.parent)) {
+    isExported = path.parent.body.some(
+      stmt =>
+        t.isExportNamedDeclaration(stmt) &&
+        !stmt.source &&
+        stmt.specifiers.some(
+          spec => t.isExportSpecifier(spec) && spec.local.name === name,
+        ),
+    );
+  }
+
+  const entries = translateEnumValues(path, t);
+
+  if (isExported) {
+    const obj = t.objectExpression(
+      entries.map(([name, value]) =>
+        t.objectProperty(
+          t.isValidIdentifier(name)
+            ? t.identifier(name)
+            : t.stringLiteral(name),
+          value,
+        ),
+      ),
+    );
+
+    if (path.scope.hasOwnBinding(name)) {
+      (parentIsExport ? path.parentPath : path).replaceWith(
+        t.expressionStatement(
+          t.callExpression(
+            t.memberExpression(t.identifier("Object"), t.identifier("assign")),
+            [path.node.id, obj],
+          ),
+        ),
+      );
+    } else {
+      path.replaceWith(
+        t.variableDeclaration("var", [t.variableDeclarator(path.node.id, obj)]),
+      );
+      path.scope.registerDeclaration(path);
+    }
+
+    return;
+  }
+
+  const entriesMap = new Map(entries);
+
+  // TODO: After fixing https://github.com/babel/babel/pull/11065, we can
+  // use path.scope.getBinding(name).referencePaths rather than doing
+  // a full traversal.
+  path.scope.path.traverse({
+    Scope(path) {
+      if (path.scope.hasOwnBinding(name)) path.skip();
+    },
+    MemberExpression(path) {
+      if (!t.isIdentifier(path.node.object, { name })) return;
+
+      let key: string;
+      if (path.node.computed) {
+        if (t.isStringLiteral(path.node.property)) {
+          key = path.node.property.value;
+        } else {
+          return;
+        }
+      } else if (t.isIdentifier(path.node.property)) {
+        key = path.node.property.name;
+      } else {
+        return;
+      }
+      if (!entriesMap.has(key)) return;
+
+      path.replaceWith(t.cloneNode(entriesMap.get(key)));
+    },
+  });
+
+  path.remove();
+}

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

@@ -5,9 +5,6 @@ import type { NodePath } from "@babel/traverse";
 
 export default function transpileEnum(path, t) {
   const { node } = path;
-  if (node.const) {
-    throw path.buildCodeFrameError("'const' enums are not supported.");
-  }
 
   if (node.declare) {
     path.remove();
@@ -105,7 +102,10 @@ type PreviousEnumMembers = {
   [name: string]: number | string;
 };
 
-function translateEnumValues(path, t) {
+export function translateEnumValues(
+  path: NodePath<t.TSEnumDeclaration>,
+  t: typeof import("@babel/types"),
+): Array<[name: string, value: t.Expression]> {
   const seen: PreviousEnumMembers = Object.create(null);
   // Start at -1 so the first enum member is its increment, 0.
   let prev: number | typeof undefined = -1;

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

@@ -3,6 +3,7 @@ import syntaxTypeScript from "@babel/plugin-syntax-typescript";
 import { types as t, template } from "@babel/core";
 import { injectInitialization } from "@babel/helper-create-class-features-plugin";
 
+import transpileConstEnum from "./const-enum";
 import transpileEnum from "./enum";
 import transpileNamespace from "./namespace";
 import type { NodePath } from "@babel/traverse";
@@ -60,6 +61,7 @@ export default declare((api, opts) => {
     jsxPragma = "React.createElement",
     jsxPragmaFrag = "React.Fragment",
     onlyRemoveTypeImports = false,
+    optimizeConstEnums = false,
   } = opts;
 
   if (!process.env.BABEL_8_BREAKING) {
@@ -462,7 +464,11 @@ export default declare((api, opts) => {
       },
 
       TSEnumDeclaration(path) {
-        transpileEnum(path, t);
+        if (optimizeConstEnums && path.node.const) {
+          transpileConstEnum(path, t);
+        } else {
+          transpileEnum(path, t);
+        }
       },
 
       TSImportEqualsDeclaration(path: NodePath<t.TSImportEqualsDeclaration>) {

packages/babel-plugin-transform-typescript/test/fixtures/declarations/export-declare-enum/input.ts

@@ -0,0 +1,3 @@
+export declare enum A {}
+
+;

packages/babel-plugin-transform-typescript/test/fixtures/declarations/export-declare-enum/options.json

@@ -0,0 +1,3 @@
+{
+  "sourceType": "module"
+}

packages/babel-plugin-transform-typescript/test/fixtures/declarations/export-declare-enum/output.mjs

@@ -0,0 +1 @@
+;

packages/babel-plugin-transform-typescript/test/fixtures/enum/const/input.ts

@@ -1 +1,2 @@
-const enum E {}
+// With --isolatedModules, TSC ignores the "const" modifier when compiling enums
+const enum E {}

packages/babel-plugin-transform-typescript/test/fixtures/enum/const/output.js

@@ -0,0 +1,4 @@
+// With --isolatedModules, TSC ignores the "const" modifier when compiling enums
+var E;
+
+(function (E) {})(E || (E = {}));

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/custom-values-exported/input.ts

@@ -0,0 +1,11 @@
+export const enum A {
+  x = 3,
+  y = "f",
+  z = 4 << 2,
+  w = y
+}
+
+A.x;
+A.y;
+A.z;
+A.w;

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/custom-values-exported/output.mjs

@@ -0,0 +1,10 @@
+export var A = {
+  x: 3,
+  y: "f",
+  z: 16,
+  w: "f"
+};
+A.x;
+A.y;
+A.z;
+A.w;

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/custom-values/input.ts

@@ -0,0 +1,11 @@
+const enum A {
+  x = 3,
+  y = "f",
+  z = 4 << 2,
+  w = y
+}
+
+A.x;
+A.y;
+A.z;
+A.w;

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/custom-values/output.mjs

@@ -0,0 +1,4 @@
+3;
+"f";
+16;
+"f";

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/declare/input.ts

@@ -0,0 +1,3 @@
+declare const enum A { x }
+
+A.x;

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/declare/output.mjs

@@ -0,0 +1 @@
+0;

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/export-const-enum/input.ts

@@ -0,0 +1,3 @@
+export const enum A { y }
+
+let x = A.y;

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/export-const-enum/output.mjs

@@ -0,0 +1,4 @@
+export var A = {
+  y: 0
+};
+let x = A.y;

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/exported/input.ts

@@ -0,0 +1,5 @@
+const enum A { y }
+
+let x = A.y;
+
+export { A };

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/exported/output.mjs

@@ -0,0 +1,5 @@
+var A = {
+  y: 0
+};
+let x = A.y;
+export { A };

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/local-shadowed/input.ts

@@ -0,0 +1,8 @@
+const enum A { x }
+
+{
+  let A = {};
+  A.x;
+}
+
+A.x;

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/local-shadowed/output.mjs

@@ -0,0 +1,5 @@
+{
+  let A = {};
+  A.x;
+}
+0;

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/local/input.ts

@@ -0,0 +1,8 @@
+const enum A {
+  x, y
+}
+
+A.x;
+A["y"];
+A.z;
+A;

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/local/output.mjs

@@ -0,0 +1,4 @@
+0;
+1;
+A.z;
+A;

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/merged-exported/input.ts

@@ -0,0 +1,13 @@
+export const enum A {
+  x, y
+}
+
+export const enum A {
+  z
+}
+
+A.x;
+A["y"];
+A.z;
+A.w;
+A;

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/merged-exported/output.mjs

@@ -0,0 +1,12 @@
+export var A = {
+  x: 0,
+  y: 1
+};
+Object.assign(A, {
+  z: 0
+});
+A.x;
+A["y"];
+A.z;
+A.w;
+A;

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/merged/input.ts

@@ -0,0 +1,13 @@
+const enum A {
+  x, y
+}
+
+const enum A {
+  z
+}
+
+A.x;
+A["y"];
+A.z;
+A.w;
+A;

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/merged/output.mjs

@@ -0,0 +1,5 @@
+0;
+1;
+0;
+A.w;
+A;

packages/babel-plugin-transform-typescript/test/fixtures/optimize-const-enums/options.json

@@ -0,0 +1,4 @@
+{
+  "plugins": [["transform-typescript", { "optimizeConstEnums": true }]],
+  "sourceType": "module"
+}

packages/babel-preset-typescript/src/index.js

@@ -12,6 +12,7 @@ export default declare((api, opts) => {
     jsxPragma,
     jsxPragmaFrag,
     onlyRemoveTypeImports,
+    optimizeConstEnums,
   } = normalizeOptions(opts);
 
   const pluginOptions = process.env.BABEL_8_BREAKING
@@ -21,6 +22,7 @@ export default declare((api, opts) => {
         jsxPragma,
         jsxPragmaFrag,
         onlyRemoveTypeImports,
+        optimizeConstEnums,
       })
     : isTSX => ({
         allowDeclareFields: opts.allowDeclareFields,
@@ -29,6 +31,7 @@ export default declare((api, opts) => {
         jsxPragma,
         jsxPragmaFrag,
         onlyRemoveTypeImports,
+        optimizeConstEnums,
       });
 
   return {