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feat(eslint-plugin): add no-useless-empty-export rule (#4380)
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‎packages/eslint-plugin/README.md‎

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@@ -151,6 +151,7 @@ Pro Tip: For larger codebases you may want to consider splitting our linting int
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| [`@typescript-eslint/no-unsafe-call`](./docs/rules/no-unsafe-call.md) | Disallows calling an any type value | :white_check_mark: | | :thought_balloon: |
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| [`@typescript-eslint/no-unsafe-member-access`](./docs/rules/no-unsafe-member-access.md) | Disallows member access on any typed variables | :white_check_mark: | | :thought_balloon: |
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| [`@typescript-eslint/no-unsafe-return`](./docs/rules/no-unsafe-return.md) | Disallows returning any from a function | :white_check_mark: | | :thought_balloon: |
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| [`@typescript-eslint/no-useless-empty-export`](./docs/rules/no-useless-empty-export.md) | Disallow empty exports that don't change anything in a module file | | :wrench: | |
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| [`@typescript-eslint/no-var-requires`](./docs/rules/no-var-requires.md) | Disallows the use of require statements except in import statements | :white_check_mark: | | |
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| [`@typescript-eslint/non-nullable-type-assertion-style`](./docs/rules/non-nullable-type-assertion-style.md) | Prefers a non-null assertion over explicit type cast when possible | | :wrench: | :thought_balloon: |
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| [`@typescript-eslint/prefer-as-const`](./docs/rules/prefer-as-const.md) | Prefer usage of `as const` over literal type | :white_check_mark: | :wrench: | |

‎packages/eslint-plugin/docs/rules/no-useless-empty-export.md‎

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# `no-useless-empty-export`
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Disallow empty exports that don't change anything in a module file.
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## Rule Details
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An empty `export {}` statement is sometimes useful in TypeScript code to turn a file that would otherwise be a script file into a module file.
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Per the TypeScript Handbook [Modules](https://www.typescriptlang.org/docs/handbook/modules.html) page:
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> In TypeScript, just as in ECMAScript 2015, any file containing a top-level import or export is considered a module.
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> Conversely, a file without any top-level import or export declarations is treated as a script whose contents are available in the global scope (and therefore to modules as well).
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However, an `export {}` statement does nothing if there are any other top-level import or export statements in a file.
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Examples of code for this rule:
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<!--tabs-->
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### ❌ Incorrect
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```ts
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export const value = 'Hello, world!';
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export {};
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```
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```ts
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import 'some-other-module';
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export {};
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```
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### ✅ Correct
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```ts
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export const value = 'Hello, world!';
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```
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```ts
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import 'some-other-module';
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```
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## Attributes
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- [ ] ✅ Recommended
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- [x] 🔧 Fixable
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- [ ] 💭 Requires type information

‎packages/eslint-plugin/src/configs/all.ts‎

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'@typescript-eslint/no-unused-vars': 'error',
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'no-use-before-define': 'off',
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'@typescript-eslint/no-use-before-define': 'error',
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'@typescript-eslint/no-useless-empty-export': 'error',
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'no-useless-constructor': 'off',
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'@typescript-eslint/no-useless-constructor': 'error',
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'@typescript-eslint/no-var-requires': 'error',

‎packages/eslint-plugin/src/rules/index.ts‎

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@@ -82,6 +82,7 @@ import noUnusedExpressions from './no-unused-expressions';
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import noUnusedVars from './no-unused-vars';
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import noUseBeforeDefine from './no-use-before-define';
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import noUselessConstructor from './no-useless-constructor';
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import noUselessEmptyExport from './no-useless-empty-export';
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import noVarRequires from './no-var-requires';
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import nonNullableTypeAssertionStyle from './non-nullable-type-assertion-style';
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import objectCurlySpacing from './object-curly-spacing';
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'no-unused-vars': noUnusedVars,
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'no-use-before-define': noUseBeforeDefine,
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'no-useless-constructor': noUselessConstructor,
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'no-useless-empty-export': noUselessEmptyExport,
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'no-var-requires': noVarRequires,
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'non-nullable-type-assertion-style': nonNullableTypeAssertionStyle,
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'object-curly-spacing': objectCurlySpacing,

‎packages/eslint-plugin/src/rules/no-useless-empty-export.ts‎

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import { AST_NODE_TYPES, TSESTree } from '@typescript-eslint/utils';
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import * as util from '../util';
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function isEmptyExport(
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node: TSESTree.Node,
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): node is TSESTree.ExportNamedDeclaration {
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return (
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node.type === AST_NODE_TYPES.ExportNamedDeclaration &&
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node.specifiers.length === 0 &&
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!node.declaration
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);
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}
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const exportOrImportNodeTypes = new Set([
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AST_NODE_TYPES.ExportAllDeclaration,
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AST_NODE_TYPES.ExportDefaultDeclaration,
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AST_NODE_TYPES.ExportNamedDeclaration,
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AST_NODE_TYPES.ExportSpecifier,
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AST_NODE_TYPES.ImportDeclaration,
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AST_NODE_TYPES.TSExportAssignment,
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AST_NODE_TYPES.TSImportEqualsDeclaration,
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]);
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export default util.createRule({
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name: 'no-useless-empty-export',
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meta: {
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docs: {
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description:
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"Disallow empty exports that don't change anything in a module file",
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recommended: false,
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suggestion: true,
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},
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fixable: 'code',
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hasSuggestions: true,
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messages: {
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uselessExport: 'Empty export does nothing and can be removed.',
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},
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schema: [],
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type: 'suggestion',
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},
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defaultOptions: [],
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create(context) {
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function checkNode(
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node: TSESTree.Program | TSESTree.TSModuleDeclaration,
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): void {
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if (!Array.isArray(node.body)) {
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return;
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}
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let emptyExport: TSESTree.ExportNamedDeclaration | undefined;
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let foundOtherExport = false;
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for (const statement of node.body) {
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if (isEmptyExport(statement)) {
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emptyExport = statement;
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if (foundOtherExport) {
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break;
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}
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} else if (exportOrImportNodeTypes.has(statement.type)) {
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foundOtherExport = true;
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}
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}
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if (emptyExport && foundOtherExport) {
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context.report({
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fix: fixer => fixer.remove(emptyExport!),
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messageId: 'uselessExport',
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node: emptyExport,
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});
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}
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}
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return {
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Program: checkNode,
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TSModuleDeclaration: checkNode,
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};
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},
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});

‎packages/eslint-plugin/tests/rules/no-useless-empty-export.test.ts‎

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/* eslint-disable eslint-comments/no-use */
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// this rule tests the spacing, which prettier will want to fix and break the tests
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/* eslint "@typescript-eslint/internal/plugin-test-formatting": ["error", { formatWithPrettier: false }] */
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/* eslint-enable eslint-comments/no-use */
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import rule from '../../src/rules/no-useless-empty-export';
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import { RuleTester } from '../RuleTester';
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const ruleTester = new RuleTester({
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parserOptions: {
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ecmaVersion: 2020,
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sourceType: 'module',
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},
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parser: '@typescript-eslint/parser',
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});
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const error = {
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messageId: 'uselessExport',
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} as const;
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ruleTester.run('no-useless-empty-export', rule, {
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valid: [
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"declare module '_'",
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"import {} from '_';",
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"import * as _ from '_';",
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'export = {};',
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'export = 3;',
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'export const _ = {};',
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`
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const _ = {};
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export default _;
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`,
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`
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export * from '_';
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export = {};
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`,
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`
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export {};
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`,
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],
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invalid: [
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{
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code: `
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export const _ = {};
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export {};
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`,
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errors: [error],
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output: `
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export const _ = {};
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`,
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},
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{
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code: `
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export * from '_';
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export {};
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`,
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errors: [error],
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output: `
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export * from '_';
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`,
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},
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{
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code: `
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export {};
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export * from '_';
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`,
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errors: [error],
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output: `
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export * from '_';
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`,
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},
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{
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code: `
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const _ = {};
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export default _;
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export {};
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`,
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errors: [error],
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output: `
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const _ = {};
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export default _;
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`,
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},
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{
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code: `
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export {};
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const _ = {};
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export default _;
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`,
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errors: [error],
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output: `
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const _ = {};
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export default _;
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`,
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},
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{
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code: `
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const _ = {};
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export { _ };
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export {};
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`,
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errors: [error],
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output: `
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const _ = {};
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export { _ };
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`,
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},
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{
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code: `
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import _ = require('_');
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export {};
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`,
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errors: [error],
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output: `
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import _ = require('_');
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`,
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},
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],
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});

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