feat(no-export): new rule for no-export

README.md

@@ -114,6 +114,7 @@ installations requiring long-term consistency.
 | [no-commented-out-tests][]   | Disallow commented out tests                                      |                  |                     |
 | [no-duplicate-hooks][]       | Disallow duplicate hooks within a `describe` block                |                  |                     |
 | [no-empty-title][]           | Disallow empty titles                                             |                  |                     |
+| [no-export][]                | Disallow export from test files                                   | ![recommended][] |                     |
 | [no-focused-tests][]         | Disallow focused tests                                            | ![recommended][] |                     |
 | [no-hooks][]                 | Disallow setup and teardown hooks                                 |                  |                     |
 | [no-identical-title][]       | Disallow identical titles                                         | ![recommended][] |                     |
@@ -163,6 +164,7 @@ https://github.com/dangreenisrael/eslint-plugin-jest-formatting
 [no-duplicate-hooks]: docs/rules/no-duplicate-hooks.md
 [no-commented-out-tests]: docs/rules/no-commented-out-tests.md
 [no-empty-title]: docs/rules/no-empty-title.md
+[no-export]: docs/rules/no-export.md
 [no-focused-tests]: docs/rules/no-focused-tests.md
 [no-hooks]: docs/rules/no-hooks.md
 [no-identical-title]: docs/rules/no-identical-title.md

docs/rules/no-export.md

@@ -0,0 +1,46 @@
+# no export from test file (no-export)
+
+Prevents exports from test files. If a file has at least 1 test in it, then this
+rule will prevent exports.
+
+## Rule Details
+
+This rule aims to eliminate duplicate runs of tests by exporting things from
+test files. If you import from a test file, then all the tests in that file will
+be run in each imported instance, so bottom line, don't export from a test, but
+instead move helper functions into a seperate file when they need to be shared
+across tests.
+
+Examples of **incorrect** code for this rule:
+
+```js
+export function myHelper() {}
+
+module.exports = function() {};
+
+module.exports = {
+  something: 'that should be moved to a non-test file',
+};
+
+describe('a test', () => {
+  expect(1).toBe(1);
+});
+```
+
+Examples of **correct** code for this rule:
+
+```js
+function myHelper() {}
+
+const myThing = {
+  something: 'that can live here',
+};
+
+describe('a test', () => {
+  expect(1).toBe(1);
+});
+```
+
+## When Not To Use It
+
+Don't use this rule on non-jest test files.

src/__tests__/rules.test.js

@@ -3,7 +3,7 @@ import { resolve } from 'path';
 import { rules } from '../';
 
 const ruleNames = Object.keys(rules);
-const numberOfRules = 34;
+const numberOfRules = 35;
 
 describe('rules', () => {
   it('should have a corresponding doc for each rule', () => {

src/rules/__tests__/no-export.test.js

@@ -0,0 +1,50 @@
+import { RuleTester } from 'eslint';
+import rule from '../no-export';
+
+const ruleTester = new RuleTester({
+  parserOptions: {
+    ecmaVersion: 2015,
+    sourceType: 'module',
+  },
+});
+
+ruleTester.run('no-export', rule, {
+  valid: [
+    'describe("a test", () => { expect(1).toBe(1); })',
+    'window.location = "valid"',
+    'module.somethingElse = "foo";',
+    'export const myThing = "valid"',
+    'export default function () {}',
+    'module.exports = function(){}',
+    'module.exports.myThing = "valid";',
+  ],
+  invalid: [
+    {
+      code:
+        'export const myThing = "invalid";  test("a test", () => { expect(1).toBe(1);});',
+      parserOptions: { sourceType: 'module' },
+      errors: [{ endColumn: 34, column: 1, messageId: 'unexpectedExport' }],
+    },
+    {
+      code:
+        'export default function() {};  test("a test", () => { expect(1).toBe(1);});',
+      parserOptions: { sourceType: 'module' },
+      errors: [{ endColumn: 29, column: 1, messageId: 'unexpectedExport' }],
+    },
+    {
+      code:
+        'module.exports["invalid"] = function() {};  test("a test", () => { expect(1).toBe(1);});',
+      errors: [{ endColumn: 26, column: 1, messageId: 'unexpectedExport' }],
+    },
+    {
+      code:
+        'module.exports = function() {}; ;  test("a test", () => { expect(1).toBe(1);});',
+      errors: [{ endColumn: 15, column: 1, messageId: 'unexpectedExport' }],
+    },
+    {
+      code:
+        'module.export.invalid = function() {}; ;  test("a test", () => { expect(1).toBe(1);});',
+      errors: [{ endColumn: 22, column: 1, messageId: 'unexpectedExport' }],
+    },
+  ],
+});

src/rules/no-export.js

@@ -0,0 +1,49 @@
+import { getDocsUrl, isTestCase } from './util';
+
+let exportNodes = [];
+let hasTestCase = false;
+const messageId = 'unexpectedExport';
+export default {
+  meta: {
+    docs: {
+      url: getDocsUrl(__filename),
+    },
+    messages: {
+      [messageId]: `Do not export from a test file.`,
+    },
+    schema: [],
+  },
+  create(context) {
+    return {
+      'Program:exit'() {
+        if (hasTestCase && exportNodes.length > 0) {
+          for (let node of exportNodes) {
+            context.report({ node, messageId });
+          }
+        }
+        exportNodes = [];
+        hasTestCase = false;
+      },
+
+      CallExpression(node) {
+        if (isTestCase(node)) {
+          hasTestCase = true;
+        }
+      },
+      'ExportNamedDeclaration, ExportDefaultDeclaration'(node) {
+        exportNodes.push(node);
+      },
+      'AssignmentExpression > MemberExpression'(node) {
+        let { object, property } = node;
+
+        if (object.type === 'MemberExpression') {
+          ({ object, property } = object);
+        }
+
+        if (object.name === 'module' && !!property.name.match(/^exports?$/)) {
+          exportNodes.push(node);
+        }
+      },
+    };
+  },
+};