Compile export default <expr> to module.exportDefault(<expr>).

This allows us to simplify the module.export API significantly, since it
no longer sometimes takes a string as its first argument. Now `getters` is
always an object of methods that get the values of local variables, and
`constant` is an optional boolean to indicate that the getter functions
always return the same values, which helps with #134.

lib/entry.js

@@ -67,7 +67,7 @@ Entry.getOrCreate = function (exported) {
   return entry;
 };
 
-Ep.addGetters = function (getters) {
+Ep.addGetters = function (getters, constant) {
   var names = Object.keys(getters);
   var nameCount = names.length;
 
@@ -81,6 +81,7 @@ Ep.addGetters = function (getters) {
         // Should this throw if this.getters[name] exists?
         ! (name in this.getters)) {
       this.getters[name] = getter;
+      getter.constant = !! constant;
     }
   }
 };

lib/import-export-visitor.js

@@ -8,8 +8,6 @@ const MagicString = require("./magic-string.js");
 const Visitor = require("./visitor.js");
 
 const codeOfCR = "\r".charCodeAt(0);
-const exportDefaultPrefix = 'module.export("default",exports.default=(';
-const exportDefaultSuffix = "));";
 
 class ImportExportVisitor extends Visitor {
   finalizeHoisting() {
@@ -214,15 +212,28 @@ class ImportExportVisitor extends Visitor {
       }, "declaration");
 
     } else {
-      // Otherwise, since the exported value is an expression, it's
-      // important that we wrap it with parentheses, in case it's something
-      // like a comma-separated sequence expression.
-      overwrite(this, decl.start, dd.start, exportDefaultPrefix);
+      // Otherwise, since the exported value is an expression, we use the
+      // special module.exportDefault(value) form.
 
       path.call(this.visitWithoutReset, "declaration");
       assert.strictEqual(decl.declaration, dd);
 
-      overwrite(this, dd.end, decl.end, exportDefaultSuffix, true);
+      let prefix = "module.exportDefault(";
+      let suffix = ");";
+
+      if (dd.type === "SequenceExpression") {
+        // If the exported expression is a comma-separated sequence
+        // expression, this.code.slice(dd.start, dd.end) may not include
+        // the vital parentheses, so we should wrap the expression with
+        // parentheses to make absolutely sure it is treated as a single
+        // argument to the module.exportDefault method, rather than as
+        // multiple arguments.
+        prefix += "(";
+        suffix = ")" + suffix;
+      }
+
+      overwrite(this, decl.start, dd.start, prefix);
+      overwrite(this, dd.end, decl.end, suffix, true);
 
       if (this.modifyAST) {
         // A Function or Class declaration has become an expression on the
@@ -234,7 +245,22 @@ class ImportExportVisitor extends Visitor {
           dd.type = "ClassExpression";
         }
 
-        path.replace(buildExportDefaultStatement(this, dd));
+        // Almost every JS parser parses this expression the same way, but
+        // we should still give custom parsers a chance to parse it.
+        let ast = this.parse("module.exportDefault(ARG);");
+        if (ast.type === "File") ast = ast.program;
+        assert.strictEqual(ast.type, "Program");
+
+        const callExprStmt = ast.body[0];
+        assert.strictEqual(callExprStmt.type, "ExpressionStatement");
+
+        const callExpr = callExprStmt.expression;
+        assert.strictEqual(callExpr.type, "CallExpression");
+
+        // Replace the ARG identifier with the exported expression.
+        callExpr.arguments[1] = dd;
+
+        path.replace(callExprStmt);
       }
 
       this.madeChanges = true;
@@ -349,29 +375,6 @@ function addToSpecifierMap(map, __ported, local) {
   return map;
 }
 
-function buildExportDefaultStatement(visitor, declaration) {
-  let ast = visitor.parse(exportDefaultPrefix + "VALUE" + exportDefaultSuffix);
-
-  if (ast.type === "File") {
-    ast = ast.program;
-  }
-
-  assert.strictEqual(ast.type, "Program");
-
-  const stmt = ast.body[0];
-  assert.strictEqual(stmt.type, "ExpressionStatement");
-  assert.strictEqual(stmt.expression.type, "CallExpression");
-
-  const arg1 = stmt.expression.arguments[1];
-  assert.strictEqual(arg1.right.type, "Identifier");
-  assert.strictEqual(arg1.right.name, "VALUE");
-
-  // Replace the VALUE identifier with the desired declaration.
-  arg1.right = declaration;
-
-  return stmt;
-}
-
 // Returns a map from {im,ex}ported identifiers to lists of local variable
 // names bound to those identifiers.
 function computeSpecifierMap(specifiers) {

lib/runtime.js

@@ -10,8 +10,9 @@ exports.enable = function (mod) {
   if (typeof mod.export !== "function" ||
       typeof mod.importSync !== "function") {
     mod.export = moduleExport;
-    mod.watch = moduleWatch;
+    mod.exportDefault = moduleExportDefault;
     mod.runSetters = runSetters;
+    mod.watch = moduleWatch;
 
     // To be deprecated:
     mod.runModuleSetters = runSetters;
@@ -40,16 +41,12 @@ function importSync(id, setters, key) {
 }
 
 // Register getter functions for local variables in the scope of an export
-// statement. The keys of the getters object are exported names, and the
-// values are functions that return local values.
-function moduleExport(getters) {
+// statement. Pass true as the second argument to indicate that the getter
+// functions always return the same values.
+function moduleExport(getters, constant) {
   utils.setESModule(this.exports);
   var entry = Entry.getOrCreate(this.exports);
-
-  if (utils.isObject(getters)) {
-    entry.addGetters(getters);
-  }
-
+  entry.addGetters(getters, constant);
   if (this.loaded) {
     // If the module has already been evaluated, then we need to trigger
     // another round of entry.runSetters calls, which begins by calling
@@ -58,6 +55,15 @@ function moduleExport(getters) {
   }
 }
 
+// Register a getter function that always returns the given value.
+function moduleExportDefault(value) {
+  return this.export({
+    default: function () {
+      return value;
+    }
+  }, true);
+}
+
 // Platform-specific code should find a way to call this method whenever
 // the module system is about to return module.exports from require. This
 // might happen more than once per module, in case of dependency cycles,

test/babel-plugin-tests.js

@@ -34,7 +34,7 @@ describe("babel-plugin-transform-es2015-modules-reify", () => {
     const ast = parse(code);
     delete ast.tokens;
     const result = transformFromAst(ast, code, options);
-    assert.ok(/\bmodule\.(?:watch|importSync|export)\b/.test(result.code));
+    assert.ok(/\bmodule\.(?:export(?:Default)?|import(?:Sync)?|watch)\b/.test(result.code));
     return result;
   }
 

test/compiler-tests.js

@@ -90,7 +90,7 @@ describe("compiler", () => {
     isVarDecl(ast.body[firstIndex + 2], ["bar"]);
     isCallExprStmt(ast.body[firstIndex + 3], "module", "watch");
     isCallExprStmt(ast.body[firstIndex + 4], "console", "log");
-    isCallExprStmt(ast.body[firstIndex + 5], "module", "export");
+    isCallExprStmt(ast.body[firstIndex + 5], "module", "exportDefault");
   });
 
   it("should respect options.enforceStrictMode", () => {
@@ -255,7 +255,7 @@ describe("compiler", () => {
 
     assert.strictEqual(anonAST.body.length - anonFirstIndex, 1);
     assert.strictEqual(
-      anonAST.body[anonFirstIndex].expression.arguments[1].right.type,
+      anonAST.body[anonFirstIndex].expression.arguments[1].type,
       "ClassExpression"
     );
 

test/export-tests.js

@@ -85,16 +85,12 @@ describe("export declarations", () => {
     assert.strictEqual(val, "value-1");
 
     exportAgain();
-    assert.strictEqual(def, "default-2");
-    assert.strictEqual(val, "value-2");
-
-    exportYetAgain();
-    assert.strictEqual(def, "default-3");
+    assert.strictEqual(def, "default-1");
     assert.strictEqual(val, "value-2");
 
     setTimeout(() => {
       oneLastExport();
-      assert.strictEqual(def, "default-3");
+      assert.strictEqual(def, "default-1");
       assert.strictEqual(val, "value-3");
       done();
     }, 0);

test/export/later.js

@@ -4,15 +4,18 @@ export default "default-1";
 export let val = "value-1";
 
 export function exportAgain() {
-  module.export("default", exports.default = "default-2");
+  // Neither of these re-export styles should work, because the original
+  // export default still takes precedence over anything else.
+  module.exportDefault(exports.default = "default-2");
+
+  // This style also does not work, because the getter function for the
+  // variable val is all that matters.
+  exports.val = "ignored";
+
   val = +(val.split("-")[1]);
   val = "value-" + ++val;
 }
 
-export function exportYetAgain() {
-  module.export("default", exports.default = "default-3");
-}
-
 export function oneLastExport() {
   strictEqual(
     val = "value-3",

test/output/anon-class/expected.js

@@ -1,5 +1,5 @@
-"use strict";module.export("default",exports.default=(class {
+"use strict";module.exportDefault(class {
   constructor(value) {
     this.value = value;
   }
-}));
+});

test/output/default-expression/expected.js

@@ -2,4 +2,4 @@
 
 // This default expression will evaluate to 0 if the parentheses are
 // mistakenly stripped away.
-module.export("default",exports.default=(count++, count));
+module.exportDefault((count++, count));

test/output/export-all/expected.js

@@ -1,2 +1,2 @@
 "use strict";module.watch(require("./abc"),{"*":function(v,k){exports[k]=v}},0);
-module.export("default",exports.default=("default"));
+module.exportDefault("default");

test/setter/grandchild.js

@@ -1,5 +1,5 @@
 export let c = 0;
 export function increment() {
   ++c;
-  module.export();
+  module.runSetters();
 };