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

docs/rules/no-export.md

@@ -0,0 +1,51 @@
+# no export from test file (no-export)
+
+I ran into this issue when we had a set of helper functions in 1 test file that
+got exported directly from a `-test.js` file. The test that was importing this
+file then executes both it's own tests as well as the tests in the imported
+file. We started seeing phantom snapshots show up that we weren't able to find
+the origin of until we finally tracked it down and realized that a test file was
+exporting shared functions. We fixed this by moving those functions out into
+seperate files, but that was the inspiration of this rule.
+
+## 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 +1,43 @@
+import { RuleTester } from 'eslint';
+import rule from '../no-export';
+
+const ruleTester = new RuleTester({
+  parserOptions: {
+    ecmaVersion: 2015,
+  },
+});
+
+ruleTester.run('no-export', rule, {
+  valid: [
+    'describe("a test", () => { expect(1).toBe(1); })',
+    'window.location = "hello"',
+    'module.somethingElse = "foo";',
+  ],
+  invalid: [
+    {
+      code:
+        'export const myThing = "hello";  describe("a test", () => { expect(1).toBe(1);});',
+      parserOptions: { sourceType: 'module' },
+      errors: [{ endColumn: 32, column: 1, messageId: 'unexpectedExport' }],
+    },
+    {
+      code:
+        'export default function() {};  describe("a test", () => { expect(1).toBe(1);});',
+      parserOptions: { sourceType: 'module' },
+      errors: [{ endColumn: 29, column: 1, messageId: 'unexpectedExport' }],
+    },
+    {
+      code:
+        'module.exports["foo"] = function() {};  describe("a test", () => { expect(1).toBe(1);});',
+      errors: [{ endColumn: 22, column: 1, messageId: 'unexpectedExport' }],
+    },
+    {
+      code: 'module.exports = function() {};',
+      errors: [{ endColumn: 15, column: 1, messageId: 'unexpectedExport' }],
+    },
+    {
+      code: 'module.export.thing = function() {};',
+      errors: [{ endColumn: 20, column: 1, messageId: 'unexpectedExport' }],
+    },
+  ],
+});

src/rules/no-export.js

@@ -0,0 +1,31 @@
+import { getDocsUrl } from './util';
+
+export default {
+  meta: {
+    docs: {
+      url: getDocsUrl(__filename),
+    },
+    messages: {
+      unexpectedExport: `Do not export from a test file.`,
+    },
+    schema: [],
+  },
+  create(context) {
+    return {
+      'ExportNamedDeclaration, ExportDefaultDeclaration'(node) {
+        context.report({ node, messageId: 'unexpectedExport' });
+      },
+      'AssignmentExpression > MemberExpression'(node) {
+        let { object, property } = node;
+
+        if (object.type === 'MemberExpression') {
+          ({ object, property } = object);
+        }
+
+        if (object.name === 'module' && !!property.name.match(/^exports?$/)) {
+          context.report({ node, messageId: 'unexpectedExport' });
+        }
+      },
+    };
+  },
+};