Add no-anonymous-default-export rule (#712)

* Add `no-anonymous-default-export` rule

CHANGELOG.md

@@ -4,6 +4,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 This change log adheres to standards from [Keep a CHANGELOG](http://keepachangelog.com).
 
 ## [Unreleased]
+### Added
+- [`no-anonymous-default-export`] rule: report anonymous default exports ([#712], thanks [@duncanbeevers]).
+
 ### Changed
 - [`no-extraneous-dependencies`]: use `read-pkg-up` to simplify finding + loading `package.json` ([#680], thanks [@wtgtybhertgeghgtwtg])
 
@@ -377,6 +380,7 @@ for info on changes for earlier releases.
 [`no-unassigned-import`]: ./docs/rules/no-unassigned-import.md
 [`unambiguous`]: ./docs/rules/unambiguous.md
 
+[#712]: https://github.com/benmosher/eslint-plugin-import/pull/712
 [#680]: https://github.com/benmosher/eslint-plugin-import/pull/680
 [#654]: https://github.com/benmosher/eslint-plugin-import/pull/654
 [#639]: https://github.com/benmosher/eslint-plugin-import/pull/639
@@ -561,3 +565,4 @@ for info on changes for earlier releases.
 [@ntdb]: https://github.com/ntdb
 [@jakubsta]: https://github.com/jakubsta
 [@wtgtybhertgeghgtwtg]: https://github.com/wtgtybhertgeghgtwtg
+[@duncanbeevers]: https://github.com/duncanbeevers

README.md

@@ -76,6 +76,7 @@ This plugin intends to support linting of ES2015+ (ES6+) import/export syntax, a
 * Limit the maximum number of dependencies a module can have ([`max-dependencies`])
 * Forbid unassigned imports ([`no-unassigned-import`])
 * Forbid named default exports ([`no-named-default`])
+* Forbid anonymous values as default exports ([`no-anonymous-default-export`])
 
 [`first`]: ./docs/rules/first.md
 [`no-duplicates`]: ./docs/rules/no-duplicates.md
@@ -87,6 +88,7 @@ This plugin intends to support linting of ES2015+ (ES6+) import/export syntax, a
 [`max-dependencies`]: ./docs/rules/max-dependencies.md
 [`no-unassigned-import`]: ./docs/rules/no-unassigned-import.md
 [`no-named-default`]: ./docs/rules/no-named-default.md
+[`no-anonymous-default-export`]: ./docs/rules/no-anonymous-default-export.md
 
 ## Installation
 

docs/rules/no-anonymous-default-export.md

@@ -0,0 +1,67 @@
+# no-anonymous-default-export
+
+Reports if a module's default export is unnamed. This includes several types of unnamed data types; literals, object expressions, arrays, anonymous functions, arrow functions, and anonymous class declarations.
+
+Ensuring that default exports are named helps improve the grepability of the codebase by encouraging the re-use of the same identifier for the module's default export at its declaration site and at its import sites.
+
+## Options
+
+By default, all types of anonymous default exports are forbidden, but any types can be selectively allowed by toggling them on in the options.
+
+The complete default configuration looks like this.
+
+```js
+"import/no-anonymous-default-export": ["error", {
+  "allowArray": false,
+  "allowArrowFunction": false,
+  "allowAnonymousClass": false,
+  "allowAnonymousFunction": false,
+  "allowLiteral": false,
+  "allowObject": false
+}]
+```
+
+## Rule Details
+
+### Fail
+```js
+export default []
+
+export default () => {}
+
+export default class {}
+
+export default function () {}
+
+export default 123
+
+export default {}
+```
+
+### Pass
+```js
+const foo = 123
+export default foo
+
+export default class MyClass() {}
+
+export default function foo() {}
+
+/* eslint import/no-anonymous-default-export: [2, {"allowArray": true}] */
+export default []
+
+/* eslint import/no-anonymous-default-export: [2, {"allowArrowFunction": true}] */
+export default () => {}
+
+/* eslint import/no-anonymous-default-export: [2, {"allowAnonymousClass": true}] */
+export default class {}
+
+/* eslint import/no-anonymous-default-export: [2, {"allowAnonymousFunction": true}] */
+export default function () {}
+
+/* eslint import/no-anonymous-default-export: [2, {"allowLiteral": true}] */
+export default 123
+
+/* eslint import/no-anonymous-default-export: [2, {"allowObject": true}] */
+export default {}
+```

src/index.js

@@ -13,6 +13,7 @@ export const rules = {
   'no-named-default': require('./rules/no-named-default'),
   'no-named-as-default': require('./rules/no-named-as-default'),
   'no-named-as-default-member': require('./rules/no-named-as-default-member'),
+  'no-anonymous-default-export': require('./rules/no-anonymous-default-export'),
 
   'no-commonjs': require('./rules/no-commonjs'),
   'no-amd': require('./rules/no-amd'),

src/rules/no-anonymous-default-export.js

@@ -0,0 +1,84 @@
+/**
+ * @fileoverview Rule to disallow anonymous default exports.
+ * @author Duncan Beevers
+ */
+
+const defs = {
+  ArrayExpression: {
+    option: 'allowArray',
+    description: 'If `false`, will report default export of an array',
+    message: 'Assign array to a variable before exporting as module default',
+  },
+  ArrowFunctionExpression: {
+    option: 'allowArrowFunction',
+    description: 'If `false`, will report default export of an arrow function',
+    message: 'Assign arrow function to a variable before exporting as module default',
+  },
+  ClassDeclaration: {
+    option: 'allowAnonymousClass',
+    description: 'If `false`, will report default export of an anonymous class',
+    message: 'Unexpected default export of anonymous class',
+    forbid: (node) => !node.declaration.id,
+  },
+  FunctionDeclaration: {
+    option: 'allowAnonymousFunction',
+    description: 'If `false`, will report default export of an anonymous function',
+    message: 'Unexpected default export of anonymous function',
+    forbid: (node) => !node.declaration.id,
+  },
+  Literal: {
+    option: 'allowLiteral',
+    description: 'If `false`, will report default export of a literal',
+    message: 'Assign literal to a variable before exporting as module default',
+  },
+  ObjectExpression: {
+    option: 'allowObject',
+    description: 'If `false`, will report default export of an object expression',
+    message: 'Assign object to a variable before exporting as module default',
+  },
+  TemplateLiteral: {
+    option: 'allowLiteral',
+    description: 'If `false`, will report default export of a literal',
+    message: 'Assign literal to a variable before exporting as module default',
+  },
+}
+
+const schemaProperties = Object.keys(defs).
+  map((key) => defs[key]).
+  reduce((acc, def) => {
+    acc[def.option] = {
+      description: def.description,
+      type: 'boolean',
+      default: false,
+    }
+
+    return acc
+  }, {})
+
+module.exports = {
+  meta: {
+    schema: [
+      {
+        type: 'object',
+        properties: schemaProperties,
+        'additionalProperties': false,
+      },
+    ],
+  },
+
+  create: function (context) {
+    const options = Object.assign({}, context.options[0])
+
+    return {
+      'ExportDefaultDeclaration': (node) => {
+        const def = defs[node.declaration.type]
+
+        // Recognized node type and allowed by configuration,
+        //   and has no forbid check, or forbid check return value is truthy
+        if (def && !options[def.option] && (!def.forbid || def.forbid(node))) {
+          context.report({ node, message: def.message })
+        }
+      },
+    }
+  },
+}

tests/src/rules/no-anonymous-default-export.js

@@ -0,0 +1,50 @@
+import { test, SYNTAX_CASES } from '../utils'
+
+import { RuleTester } from 'eslint'
+
+const ruleTester = new RuleTester()
+const rule = require('rules/no-anonymous-default-export')
+
+ruleTester.run('no-anonymous-default-export', rule, {
+    valid: [
+      // Exports with identifiers are valid
+      test({ code: 'const foo = 123\nexport default foo' }),
+      test({ code: 'export default function foo() {}'}),
+      test({ code: 'export default class MyClass {}'}),
+
+      // Allow each forbidden type with appropriate option
+      test({ code: 'export default []', options: [{ allowArray: true }] }),
+      test({ code: 'export default () => {}', options: [{ allowArrowFunction: true }] }),
+      test({ code: 'export default class {}', options: [{ allowAnonymousClass: true }] }),
+      test({ code: 'export default function() {}', options: [{ allowAnonymousFunction: true }] }),
+      test({ code: 'export default 123', options: [{ allowLiteral: true }] }),
+      test({ code: 'export default \'foo\'', options: [{ allowLiteral: true }] }),
+      test({ code: 'export default `foo`', options: [{ allowLiteral: true }] }),
+      test({ code: 'export default {}', options: [{ allowObject: true }] }),
+
+      // Allow forbidden types with multiple options
+      test({ code: 'export default 123', options: [{ allowLiteral: true, allowObject: true }] }),
+      test({ code: 'export default {}', options: [{ allowLiteral: true, allowObject: true }] }),
+
+      // Sanity check unrelated export syntaxes
+      test({ code: 'export * from \'foo\'' }),
+      test({ code: 'const foo = 123\nexport { foo }' }),
+      test({ code: 'const foo = 123\nexport { foo as default }' }),
+
+      ...SYNTAX_CASES,
+    ],
+
+    invalid: [
+      test({ code: 'export default []', errors: [{ message: 'Assign array to a variable before exporting as module default' }] }),
+      test({ code: 'export default () => {}', errors: [{ message: 'Assign arrow function to a variable before exporting as module default' }] }),
+      test({ code: 'export default class {}', errors: [{ message: 'Unexpected default export of anonymous class' }] }),
+      test({ code: 'export default function() {}', errors: [{ message: 'Unexpected default export of anonymous function' }] }),
+      test({ code: 'export default 123', errors: [{ message: 'Assign literal to a variable before exporting as module default' }] }),
+      test({ code: 'export default \'foo\'', errors: [{ message: 'Assign literal to a variable before exporting as module default' }] }),
+      test({ code: 'export default `foo`', errors: [{ message: 'Assign literal to a variable before exporting as module default' }] }),
+      test({ code: 'export default {}', errors: [{ message: 'Assign object to a variable before exporting as module default' }] }),
+
+      // Test failure with non-covering exception
+      test({ code: 'export default 123', options: [{ allowObject: true }], errors: [{ message: 'Assign literal to a variable before exporting as module default' }] }),
+    ],
+})