feat: Add --ext CLI option (#19405)

* feat: Add `--ext` CLI option

Fixes #19361

* fix typos in test descriptions

* check for empty input

Author: mdjermanovic

docs/src/use/command-line-interface.md

@@ -93,6 +93,7 @@ Basic configuration:
   -c, --config path::String       Use this configuration instead of eslint.config.js, eslint.config.mjs, or
                                   eslint.config.cjs
   --inspect-config                Open the config inspector with the current configuration
+  --ext [String]                  Specify additional file extensions to lint
   --global [String]               Define global variables
   --parser String                 Specify the parser to be used
   --parser-options Object         Specify parser options
@@ -219,34 +220,32 @@ Details about the global variables defined by each environment are available in
 
 #### `--ext`
 
-**eslintrc Mode Only.** If you are using flat config (`eslint.config.js`), please see [migration guide](./configure/migration-guide#--ext).
-
-This option allows you to specify which file extensions ESLint uses when searching for target files in the directories you specify.
+This option allows you to specify additional file extensions to lint.
 
 * **Argument Type**: String. File extension.
 * **Multiple Arguments**: Yes
-* **Default Value**: `.js` and the files that match the `overrides` entries of your configuration.
+* **Default Value**: By default, ESLint lints files with extensions `.js`, `.mjs`, `.cjs`, and additional extensions [specified in the configuration file](configure/configuration-files#specifying-files-with-arbitrary-extensions).
 
-`--ext` is only used when the patterns to lint are directories. If you use glob patterns or file names, then `--ext` is ignored. For example, `npx eslint "lib/*" --ext .js` matches all files within the `lib/` directory, regardless of extension.
+This option is primarely intended for use in combination with the `--no-config-lookup` option, since in that case there is no configuration file in which the additional extensions would be specified.
 
 ##### `--ext` example
 
 {{ npx_tabs ({
     package: "eslint",
     args: [".", "--ext", ".ts"],
-    comment: "Use only .ts extension"
+    comment: "Include .ts files"
 }) }}
 
 {{ npx_tabs ({
     package: "eslint",
-    args: [".", "--ext", ".js", "--ext", ".ts"],
-    comment: "Use both .js and .ts"
+    args: [".", "--ext", ".ts", "--ext", ".tsx"],
+    comment: "Include .ts and .tsx files"
 }) }}
 
 {{ npx_tabs ({
     package: "eslint",
-    args: [".", "--ext", ".js,.ts"],
-    comment: "Also use both .js and .ts"
+    args: [".", "--ext", ".ts,.tsx"],
+    comment: "Also include .ts and .tsx files"
 }) }}
 
 #### `--global`

lib/cli.js

@@ -192,6 +192,12 @@ async function translateOptions({
             overrideConfig[0].plugins = await loadPlugins(importer, plugin);
         }
 
+        if (ext) {
+            overrideConfig.push({
+                files: ext.map(extension => `**/*${extension.startsWith(".") ? "" : "."}${extension}`)
+            });
+        }
+
     } else {
         overrideConfigFile = config;
 
@@ -489,6 +495,23 @@ const cli = {
             return 2;
         }
 
+        if (usingFlatConfig && options.ext) {
+
+            // Passing `--ext ""` results in `options.ext` being an empty array.
+            if (options.ext.length === 0) {
+                log.error("The --ext option value cannot be empty.");
+                return 2;
+            }
+
+            // Passing `--ext ,ts` results in an empty string at index 0. Passing `--ext ts,,tsx` results in an empty string at index 1.
+            const emptyStringIndex = options.ext.indexOf("");
+
+            if (emptyStringIndex >= 0) {
+                log.error(`The --ext option arguments cannot be empty strings. Found an empty string at index ${emptyStringIndex}.`);
+                return 2;
+            }
+        }
+
         const ActiveESLint = usingFlatConfig ? ESLint : LegacyESLint;
         const eslintOptions = await translateOptions(options, usingFlatConfig ? "flat" : "eslintrc");
         const engine = new ActiveESLint(eslintOptions);

lib/options.js

@@ -123,6 +123,12 @@ module.exports = function(usingFlatConfig) {
             type: "[String]",
             description: "Specify JavaScript file extensions"
         };
+    } else {
+        extFlag = {
+            option: "ext",
+            type: "[String]",
+            description: "Specify additional file extensions to lint"
+        };
     }
 
     let resolvePluginsFlag;

tests/fixtures/file-extensions/a.js

@@ -0,0 +1 @@
+// empty

tests/fixtures/file-extensions/b.mjs

@@ -0,0 +1 @@
+// empty

tests/fixtures/file-extensions/c.cjs

@@ -0,0 +1 @@
+// empty

tests/fixtures/file-extensions/d.jsx

@@ -0,0 +1 @@
+// empty

tests/fixtures/file-extensions/eslint.config.js

@@ -0,0 +1 @@
+module.exports = [{ files: ["**/*.jsx"] }];

tests/fixtures/file-extensions/f.ts

@@ -0,0 +1 @@
+// empty

tests/fixtures/file-extensions/foots

@@ -0,0 +1 @@
+// this file should not be linted with --ext ts