Allow mapping of expected extensions in import/extension rule

src/rules/extensions.js

@@ -24,6 +24,7 @@ function buildProperties(context) {
     defaultConfig: 'never',
     pattern: {},
     ignorePackages: false,
+    extensionMap: [],
   };
 
   context.options.forEach(obj => {
@@ -34,6 +35,11 @@ function buildProperties(context) {
       return;
     }
 
+    if (obj.extensionMap !== undefined) {
+      result.extensionMap = obj.extensionMap;
+      return;
+    }
+
     // If this is not the new structure, transfer all props to result.pattern
     if (obj.pattern === undefined && obj.ignorePackages === undefined) {
       Object.assign(result.pattern, obj);
@@ -49,6 +55,7 @@ function buildProperties(context) {
     if (obj.ignorePackages !== undefined) {
       result.ignorePackages = obj.ignorePackages;
     }
+
   });
 
   if (result.defaultConfig === 'ignorePackages') {
@@ -135,6 +142,10 @@ module.exports = {
       return false;
     }
 
+    function mapExtension(extension, extensionMap) {
+      return extensionMap && extensionMap[extension] ? extensionMap[extension] : extension;
+    }
+
     function checkFileExtension(source) {
       // bail if the declaration doesn't have a source, e.g. "export { foo };"
       if (!source) return;
@@ -154,7 +165,9 @@ module.exports = {
 
       // get extension from resolved path, if possible.
       // for unresolved, use source value.
-      const extension = path.extname(resolvedPath || importPath).substring(1);
+      let extension = path.extname(resolvedPath || importPath).substring(1);
+
+      extension = mapExtension(extension, props.extensionMap);
 
       // determine if this is a module
       const isPackage = isExternalModule(

tests/files/typescript-import-js-extension/file.ts

@@ -0,0 +1 @@
+export const helloWorld = 'helloWorld';

tests/files/typescript-import-js-extension/tsconfig.json

@@ -0,0 +1,5 @@
+{
+    "compilerOptions": {
+        "baseUrl": "."
+    }
+}

tests/src/rules/extensions.js

@@ -1,6 +1,7 @@
 import { RuleTester } from 'eslint';
+import path from 'path';
 import rule from 'rules/extensions';
-import { test, testFilePath } from '../utils';
+import { getTSParsers, test, testFilePath } from '../utils';
 
 const ruleTester = new RuleTester();
 
@@ -587,3 +588,67 @@ ruleTester.run('extensions', rule, {
     }),
   ],
 });
+
+
+context('TypeScript', function () {
+  getTSParsers().forEach((parser) => {
+    ruleTester.run(`extensions`, rule, {
+      valid: [
+        test({
+          code: 'import { helloWorld } from "./typescript-import-js-extension/file.js"',
+          parser: parser,
+          options: ['always', { 'extensionMap': { 'ts': 'js' } }],
+          settings: {
+            'import/parsers': { [parser]: ['.ts'] },
+            'typescript': { 'extensions': ['.ts'], 'directory': path.resolve(__dirname, '../../files/typescript-import-js-extension/') },
+          },
+        }),
+        test({
+          code: 'import { helloWorld } from "./typescript-import-js-extension/file.ts"',
+          options: ['always', { 'extensionMap': { 'json': 'jsonx' } }],
+          settings: {
+            'import/resolver': {
+              'typescript': { 'extensions': ['.ts'], 'directory': path.resolve(__dirname, '../../files/typescript-import-js-extension/') },
+            },
+          },
+        }),
+      ],
+      invalid: [
+        test({
+          code: 'import { helloWorld } from "./typescript-import-js-extension/file.ts"',
+          options: ['always', { 'extensionMap': { 'ts': 'js' } }],
+          settings: {
+            'import/resolver': {
+              'typescript': { 'extensions': ['.ts'], 'directory': path.resolve(__dirname, '../../files/typescript-import-js-extension/') },
+            },
+          },
+          errors: [
+            {
+              message: 'Missing file extension "js" for "./typescript-import-js-extension/file.ts"',
+              line: 1,
+              column: 28,
+            },
+          ],
+        }),
+
+        test({
+          code: 'import { helloWorld } from "./typescript-import-js-extension/file"',
+          filename: testFilePath('./any/typescript/file.ts'),
+          options: ['always', { 'extensionMap': { 'ts': 'js' } }],
+          settings: {
+            'import/resolver': {
+              'typescript': { 'extensions': ['.ts'], 'directory': path.resolve(__dirname, '../../files/typescript-import-js-extension/') },
+            },
+          },
+          errors: [
+            {
+              message: 'Missing file extension for "./typescript-import-js-extension/file"',
+              line: 1,
+              column: 28,
+            },
+          ],
+        }),
+      ],
+    });
+  });
+});