Fix flatMap for tests using old Node versions

Author: jportner

Diff for: tests/src/rules/no-duplicates.js

@@ -5,6 +5,7 @@ import jsxConfig from '../../../config/react';
 import { RuleTester } from 'eslint';
 import eslintPkg from 'eslint/package.json';
 import semver from 'semver';
+import flatMap from 'array.prototype.flatmap';
 
 const ruleTester = new RuleTester();
 const rule = require('rules/no-duplicates');
@@ -131,31 +132,34 @@ ruleTester.run('no-duplicates', rule, {
     }),
 
     // These test cases use duplicate import identifiers, which causes a fatal parsing error using ESPREE (default) and TS_OLD.
-    ...[parsers.BABEL_OLD, parsers.TS_NEW].flatMap((parser => !parser ? [] : [
-      // #2347: duplicate identifiers should be removed
-      test({
-        code: "import {a} from './foo'; import { a } from './foo'",
-        output: "import {a} from './foo'; ",
-        errors: ['\'./foo\' imported multiple times.', '\'./foo\' imported multiple times.'],
-        parser,
-      }),
+    ...flatMap([parsers.BABEL_OLD, parsers.TS_NEW], parser => {
+      if (!parser) return []; // TS_NEW is not always available
+      return [
+        // #2347: duplicate identifiers should be removed
+        test({
+          code: "import {a} from './foo'; import { a } from './foo'",
+          output: "import {a} from './foo'; ",
+          errors: ['\'./foo\' imported multiple times.', '\'./foo\' imported multiple times.'],
+          parser,
+        }),
 
-      // #2347: duplicate identifiers should be removed
-      test({
-        code: "import {a,b} from './foo'; import { b, c } from './foo'; import {b,c,d} from './foo'",
-        output: "import {a,b, c ,d} from './foo';  ",
-        errors: ['\'./foo\' imported multiple times.', '\'./foo\' imported multiple times.', '\'./foo\' imported multiple times.'],
-        parser,
-      }),
+        // #2347: duplicate identifiers should be removed
+        test({
+          code: "import {a,b} from './foo'; import { b, c } from './foo'; import {b,c,d} from './foo'",
+          output: "import {a,b, c ,d} from './foo';  ",
+          errors: ['\'./foo\' imported multiple times.', '\'./foo\' imported multiple times.', '\'./foo\' imported multiple times.'],
+          parser,
+        }),
 
-      // #2347: duplicate identifiers should be removed, but not if they are adjacent to comments
-      test({
-        code: "import {a} from './foo'; import { a/*,b*/ } from './foo'",
-        output: "import {a, a/*,b*/ } from './foo'; ",
-        errors: ['\'./foo\' imported multiple times.', '\'./foo\' imported multiple times.'],
-        parser,
-      }),
-    ])),
+        // #2347: duplicate identifiers should be removed, but not if they are adjacent to comments
+        test({
+          code: "import {a} from './foo'; import { a/*,b*/ } from './foo'",
+          output: "import {a, a/*,b*/ } from './foo'; ",
+          errors: ['\'./foo\' imported multiple times.', '\'./foo\' imported multiple times.'],
+          parser,
+        }),
+      ];
+    }),
 
     test({
       code: "import {x} from './foo'; import {} from './foo'; import {/*c*/} from './foo'; import {y} from './foo'",