Implement extensions rule

This fixes #209 and fixes #204.

Author: lo1tuma

Diff for: README.md

@@ -50,10 +50,12 @@ This plugin intends to support linting of ES2015+ (ES6+) import/export syntax, a
 * Ensure all imports appear before other statements ([`imports-first`])
 * Report repeated import of the same module in multiple places ([`no-duplicates`])
 * Report namespace imports ([`no-namespace`])
+* Ensure consistent use of file extension within the import path ([`extensions`])
 
 [`imports-first`]: ./docs/rules/imports-first.md
 [`no-duplicates`]: ./docs/rules/no-duplicates.md
 [`no-namespace`]: ./docs/rules/no-namespace.md
+[`extensions`]: ./docs/rules/extensions.md
 
 
 ## Installation

Diff for: docs/rules/extensions.md

@@ -0,0 +1,85 @@
+# extensions - Ensure consistent use of file extension within the import path
+
+Some file resolve algorithms allow you to omit the file extension within the import source path. For example the `node` resolver can resolve `./foo/bar` to the absolute path `/User/someone/foo/bar.js` because the `.js` extension is resolved automatically by default. Depending on the resolver you can configure more extensions to get resolved automatically.
+
+In order to provide a consistent use of file extensions across your code base, this rule can enforce or disallow the use of certain file extensions.
+
+## Rule Details
+
+This rule has one option which could be either a string or an object. If it is `"never"` (the default value) the rule forbids the use for any extension. If `"always"` then the rule enforces the use of extensions for all import statements.
+
+By providing an object you can configure each extension separately, so for example `{ "js": "always", "json": "never" }` would always enforce the use of the `.js` extension but never allow the use of the `.json` extension.
+
+### Exception
+
+When disallowing the use of certain extensions this rule makes an exception and allows the use of extension when the file would not be resolvable without extension.
+
+For example, given the following folder structure:
+
+```
+├── foo
+│   ├── bar.js
+│   ├── bar.json
+```
+
+and this import statement:
+
+```js
+import bar from './foo/bar.json';
+```
+
+then the extension can’t be omitted because it would then resolve to `./foo/bar.js`.
+
+### Examples
+
+The following patterns are considered problems when configuration set to "never":
+
+```js
+import foo from './foo.js';
+
+import bar from './bar.json';
+
+import Component from './Component.jsx'
+
+import express from 'express/index.js';
+```
+
+The following patterns are not considered problems when configuration set to "never":
+
+```js
+import foo from './foo';
+
+import bar from './bar';
+
+import Component from './Component'
+
+import express from 'express/index';
+```
+
+The following patterns are considered problems when configuration set to "always":
+
+```js
+import foo from './foo';
+
+import bar from './bar';
+
+import Component from './Component'
+
+import express from 'express';
+```
+
+The following patterns are not considered problems when configuration set to "always":
+
+```js
+import foo from './foo.js';
+
+import bar from './bar.json';
+
+import Component from './Component.jsx'
+
+import express from 'express/index.js';
+```
+
+## When Not To Use It
+
+If you are not concerned about a consistent usage of file extension.

Diff for: package.json

@@ -72,6 +72,7 @@
     "es6-symbol": "*",
     "eslint-import-resolver-node": "^0.2.0",
     "lodash.cond": "^4.3.0",
+    "lodash.endswith": "^4.0.1",
     "object-assign": "^4.0.1",
     "pkg-up": "^1.0.0"
   }

Diff for: src/index.js

@@ -5,6 +5,7 @@ export const rules = {
   'namespace': require('./rules/namespace'),
   'no-namespace': require('./rules/no-namespace'),
   'export': require('./rules/export'),
+  'extensions': require('./rules/extensions'),
 
   'no-named-as-default': require('./rules/no-named-as-default'),
   'no-named-as-default-member': require('./rules/no-named-as-default-member'),

Diff for: src/rules/extensions.js

@@ -0,0 +1,66 @@
+import path from 'path'
+import resolve from '../core/resolve'
+import endsWith from 'lodash.endsWith'
+
+module.exports = function (context) {
+  const configuration = context.options[0] || 'never'
+
+  function isUseOfExtensionEnforced(extension) {
+    if (typeof configuration === 'object') {
+      return configuration[extension] === 'always'
+    }
+
+    return configuration === 'always'
+  }
+
+  function isResolvableWithoutExtension(file) {
+    const extension = path.extname(file)
+    const fileWithoutExtension = file.slice(0, -extension.length)
+    const resolvedFileWithoutExtension = resolve(fileWithoutExtension, context)
+
+    return resolvedFileWithoutExtension === resolve(file, context)
+  }
+
+  function checkFileExtension(node) {
+    const { source } = node
+    const importPath = source.value
+    const resolvedPath = resolve(importPath, context)
+    const extension = path.extname(resolvedPath).substring(1)
+
+    if (!endsWith(importPath, extension)) {
+      if (isUseOfExtensionEnforced(extension)) {
+        context.report({
+          node: source,
+          message: `Missing file extension "${extension}" for "${importPath}"`,
+        })
+      }
+    } else {
+      if (!isUseOfExtensionEnforced(extension) && isResolvableWithoutExtension(importPath)) {
+        context.report({
+          node: source,
+          message: `Unexpected use of file extension "${extension}" for "${importPath}"`,
+        })
+      }
+    }
+  }
+
+  return {
+    ImportDeclaration: checkFileExtension,
+  }
+}
+
+module.exports.schema = [
+  {
+    oneOf: [
+      {
+        enum: [ 'always', 'never' ],
+      },
+      {
+        type: 'object',
+        patternProperties: {
+          '.*': { enum: [ 'always', 'never' ] },
+        },
+      },
+    ],
+  },
+]

Diff for: tests/files/bar.json

@@ -0,0 +1 @@
+{}

Diff for: tests/files/bar.jsx

@@ -0,0 +1 @@
+export default null

Diff for: tests/files/file.with.dot.js

@@ -0,0 +1 @@
+export default null

Diff for: tests/src/rules/extensions.js

@@ -0,0 +1,121 @@
+import { RuleTester } from 'eslint'
+import rule from 'rules/extensions';
+import { test } from '../utils';
+
+const ruleTester = new RuleTester()
+
+ruleTester.run('extensions', rule, {
+  valid: [
+    test({ code: 'import a from "a"' }),
+    test({ code: 'import dot from "./file.with.dot"' }),
+    test({
+      code: 'import a from "a/index.js"',
+      options: [ 'always' ]
+    }),
+    test({
+      code: 'import dot from "./file.with.dot.js"',
+      options: [ 'always' ]
+    }),
+    test({
+      code: [
+        'import a from "a"',
+        'import packageConfig from "./package.json"',
+      ].join('\n'),
+      options: [ { json: 'always', js: 'never' } ]
+    }),
+    test({
+      code: [
+        'import lib from "./bar"',
+        'import component from "./bar.jsx"',
+        'import data from "./bar.json"'
+      ].join('\n'),
+      options: [ 'never' ],
+      settings: { 'import/resolve': { 'extensions': [ '.js', '.jsx', '.json' ] } }
+    })
+  ],
+
+  invalid: [
+    test({
+      code: 'import a from "a/index.js"',
+      errors: [ {
+        message: 'Unexpected use of file extension "js" for "a/index.js"',
+        line: 1,
+        column: 15
+      } ]
+    }),
+    test({
+      code: 'import a from "a"',
+      options: [ 'always' ],
+      errors: [ {
+        message: 'Missing file extension "js" for "a"',
+        line: 1,
+        column: 15
+      } ]
+    }),
+    test({
+      code: 'import dot from "./file.with.dot"',
+      options: [ "always" ],
+      errors: [
+        {
+          message: 'Missing file extension "js" for "./file.with.dot"',
+          line: 1,
+          column: 17
+        }
+      ]
+    }),
+    test({
+      code: [
+        'import a from "a/index.js"',
+        'import packageConfig from "./package"',
+      ].join('\n'),
+      options: [ { json: 'always', js: 'never' } ],
+      settings: { 'import/resolve': { 'extensions': [ '.js', '.json' ] } },
+      errors: [
+        {
+          message: 'Unexpected use of file extension "js" for "a/index.js"',
+          line: 1,
+          column: 15
+        },
+        {
+          message: 'Missing file extension "json" for "./package"',
+          line: 2,
+          column: 27
+        }
+      ]
+    }),
+    test({
+      code: [
+        'import lib from "./bar.js"',
+        'import component from "./bar.jsx"',
+        'import data from "./bar.json"'
+      ].join('\n'),
+      options: [ 'never' ],
+      settings: { 'import/resolve': { 'extensions': [ '.js', '.jsx', '.json' ] } },
+      errors: [
+        {
+            message: 'Unexpected use of file extension "js" for "./bar.js"',
+            line: 1,
+            column: 17
+        }
+      ]
+    }),
+    test({
+      code: [
+        'import lib from "./bar.js"',
+        'import component from "./bar.jsx"',
+        'import data from "./bar.json"'
+      ].join('\n'),
+      options: [ { json: 'always', js: 'never', jsx: 'never' } ],
+      settings: { 'import/resolve': { 'extensions': [ '.js', '.jsx', '.json' ] } },
+      errors: [
+        {
+            message: 'Unexpected use of file extension "js" for "./bar.js"',
+            line: 1,
+            column: 17
+        }
+      ]
+    })
+
+  ]
+})
+