Disallow duplicate named exports

src/parser/statement.js

@@ -844,7 +844,7 @@ pp.parseExport = function (node) {
     }
     node.declaration = expr;
     if (needsSemi) this.semicolon();
-    this.checkExport(node);
+    this.checkExport(node, true, true);
     return this.finishNode(node, "ExportDefaultDeclaration");
   } else if (this.state.type.keyword || this.shouldParseExportDeclaration()) {
     node.specifiers = [];
@@ -855,7 +855,7 @@ pp.parseExport = function (node) {
     node.specifiers = this.parseExportSpecifiers();
     this.parseExportFrom(node);
   }
-  this.checkExport(node);
+  this.checkExport(node, true);
   return this.finishNode(node, "ExportNamedDeclaration");
 };
 
@@ -903,7 +903,31 @@ pp.shouldParseExportDeclaration = function () {
   return this.isContextual("async");
 };
 
-pp.checkExport = function (node) {
+pp.checkExport = function (node, checkNames, isDefault) {
+  if (checkNames) {
+    // Check for duplicate exports
+    if (isDefault) {
+      // Default exports
+      this.checkDuplicateExports(node, "default", isDefault);
+    } else if (node.specifiers && node.specifiers.length) {
+      // Named exports
+      for (let specifier of node.specifiers) {
+        const name = specifier.exported.name;
+        if (name === "default") isDefault = true;
+        this.checkDuplicateExports(specifier, name, isDefault);
+      }
+    } else if (node.declaration) {
+      // Exported declarations
+      if (node.declaration.type === "FunctionDeclaration" || node.declaration.type === "ClassDeclaration") {
+        this.checkDuplicateExports(node, node.declaration.id.name, isDefault);
+      } else if (node.declaration.type === "VariableDeclaration") {
+        for (let declaration of node.declaration.declarations) {
+          this.checkDuplicateExports(declaration, declaration.id.name, isDefault);
+        }
+      }
+    }
+  }
+
   if (this.state.decorators.length) {
     let isClass = node.declaration && (node.declaration.type === "ClassDeclaration" || node.declaration.type === "ClassExpression");
     if (!node.declaration || !isClass) {
@@ -913,6 +937,20 @@ pp.checkExport = function (node) {
   }
 };
 
+pp.checkDuplicateExports = function(node, name, isDefault) {
+  if (this.state.exportedIdentifiers[name]) {
+    this.raiseDuplicateExportError(node, name, isDefault);
+  }
+  this.state.exportedIdentifiers[name] = true;
+};
+
+pp.raiseDuplicateExportError = function(node, name, isDefault) {
+  this.raise(node.start, isDefault ?
+    "Only one default export allowed per module." :
+    `\`${name}\` has already been exported. Exported identifiers must be unique.`
+  );
+};
+
 // Parses a comma-separated list of module exports.
 
 pp.parseExportSpecifiers = function () {

src/tokenizer/state.js

@@ -43,6 +43,8 @@ export default class State {
     this.containsEsc = this.containsOctal = false;
     this.octalPosition = null;
 
+    this.exportedIdentifiers = {};
+
     return this;
   }
 
@@ -119,6 +121,10 @@ export default class State {
   containsOctal: boolean;
   octalPosition: ?number;
 
+  // Names of exports store. `default` is stored as a name for both
+  // `export default foo;` and `export { foo as default };`.
+  exportedIdentifiers: {[id:string]: boolean};
+
   curPosition() {
     return new Position(this.curLine, this.pos - this.lineStart);
   }

test/fixtures/es2015/modules/duplicate-export-default-and-export-as-default/actual.js

@@ -0,0 +1,2 @@
+export default function() {};
+export { foo as default };

test/fixtures/es2015/modules/duplicate-export-default-and-export-as-default/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Only one default export allowed per module. (2:9)"
+}

test/fixtures/es2015/modules/duplicate-export-default/actual.js

@@ -0,0 +1,2 @@
+export default {};
+export default function() {};

test/fixtures/es2015/modules/duplicate-export-default/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Only one default export allowed per module. (2:0)"
+}

test/fixtures/es2015/modules/duplicate-named-export-class-declaration/actual.js

@@ -0,0 +1,2 @@
+export { Foo };
+export class Foo {};

test/fixtures/es2015/modules/duplicate-named-export-class-declaration/options.json

@@ -0,0 +1,4 @@
+{
+  "throws": "`Foo` has already been exported. Exported identifiers must be unique. (2:0)"
+}
+

test/fixtures/es2015/modules/duplicate-named-export-function-declaration/actual.js

@@ -0,0 +1,2 @@
+export { foo };
+export function foo() {};

test/fixtures/es2015/modules/duplicate-named-export-function-declaration/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "`foo` has already been exported. Exported identifiers must be unique. (2:0)"
+}

test/fixtures/es2015/modules/duplicate-named-export-variable-declaration/actual.js

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

test/fixtures/es2015/modules/duplicate-named-export-variable-declaration/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "`foo` has already been exported. Exported identifiers must be unique. (2:13)"
+}

test/fixtures/es2015/modules/duplicate-named-export/actual.js

@@ -0,0 +1,2 @@
+export { foo };
+export { bar as foo };

test/fixtures/es2015/modules/duplicate-named-export/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "`foo` has already been exported. Exported identifiers must be unique. (2:9)"
+}

test/fixtures/flow/type-exports/interface/actual.js

@@ -1,2 +1,2 @@
 export interface foo { p: number };
-export interface foo<T> { p: T };
+export interface bar<T> { p: T };

test/fixtures/flow/type-exports/interface/expected.json

@@ -206,7 +206,7 @@
                 "column": 20
               }
             },
-            "name": "foo"
+            "name": "bar"
           },
           "typeParameters": {
             "type": "TypeParameterDeclaration",
@@ -346,4 +346,4 @@
     ],
     "directives": []
   }
-}
+}