feat: Add findConfigFile() method to FlatESLint

lib/eslint/flat-eslint.js

@@ -321,26 +321,15 @@ async function loadFlatConfigFile(filePath) {
 }
 
 /**
- * Calculates the config array for this run based on inputs.
- * @param {FlatESLint} eslint The instance to create the config array for.
+ * Determines which config file to use. This is determined by seeing if an
+ * override config file was passed, and if so, using it; otherwise, as long
+ * as override config file is not explicitly set to `false`, it will search
+ * upwards from the cwd for a file named `eslint.config.js`.
  * @param {import("./eslint").ESLintOptions} options The ESLint instance options.
- * @returns {FlatConfigArray} The config array for `eslint``.
+ * @returns {{configFilePath:string,basePath:string}} Location information for
+ *      the config file.
  */
-async function calculateConfigArray(eslint, {
-    cwd,
-    baseConfig,
-    overrideConfig,
-    configFile,
-    ignore: shouldIgnore,
-    ignorePatterns
-}) {
-
-    // check for cached instance
-    const slots = privateMembers.get(eslint);
-
-    if (slots.configs) {
-        return slots.configs;
-    }
+async function locateConfigFileToUse({ configFile, cwd }) {
 
     // determine where to load config file from
     let configFilePath;
@@ -360,7 +349,36 @@ async function calculateConfigArray(eslint, {
         basePath = path.resolve(path.dirname(configFilePath));
     }
 
+    return {
+        configFilePath,
+        basePath
+    };
+
+}
 
+/**
+ * Calculates the config array for this run based on inputs.
+ * @param {FlatESLint} eslint The instance to create the config array for.
+ * @param {import("./eslint").ESLintOptions} options The ESLint instance options.
+ * @returns {FlatConfigArray} The config array for `eslint``.
+ */
+async function calculateConfigArray(eslint, {
+    cwd,
+    baseConfig,
+    overrideConfig,
+    configFile,
+    ignore: shouldIgnore,
+    ignorePatterns
+}) {
+
+    // check for cached instance
+    const slots = privateMembers.get(eslint);
+
+    if (slots.configs) {
+        return slots.configs;
+    }
+
+    const { configFilePath, basePath } = await locateConfigFileToUse({ configFile, cwd });
     const configs = new FlatConfigArray(baseConfig || [], { basePath, shouldIgnore });
 
     // load config file
@@ -1160,6 +1178,19 @@ class FlatESLint {
         return configs.getConfig(absolutePath);
     }
 
+    /**
+     * Finds the config file being used by this instance based on the options
+     * passed to the constructor.
+     * @returns {string|undefined} The path to the config file being used or
+     *      `undefined` if no config file is being used.
+     */
+    async findConfigFile() {
+        const options = privateMembers.get(this).options;
+        const { configFilePath } = await locateConfigFileToUse(options);
+
+        return configFilePath;
+    }
+
     /**
      * Checks if a given path is ignored by ESLint.
      * @param {string} filePath The path of the file to check.

tests/lib/eslint/flat-eslint.js

@@ -3954,6 +3954,43 @@ describe("FlatESLint", () => {
         });
     });
 
+    describe("findConfigFile()", () => {
+
+        it("should return null when overrideConfigFile is true", async () => {
+            const engine = new FlatESLint({
+                overrideConfigFile: true
+            });
+
+            assert.strictEqual(await engine.findConfigFile(), void 0);
+        });
+
+        it("should return custom config file path when overrideConfigFile is a nonempty string", async () => {
+            const engine = new FlatESLint({
+                overrideConfigFile: "my-config.js"
+            });
+            const configFilePath = path.resolve(__dirname, "../../../my-config.js");
+
+            assert.strictEqual(await engine.findConfigFile(), configFilePath);
+        });
+
+        it("should return root level eslint.config.js when overrideConfigFile is null", async () => {
+            const engine = new FlatESLint({
+                overrideConfigFile: null
+            });
+            const configFilePath = path.resolve(__dirname, "../../../eslint.config.js");
+
+            assert.strictEqual(await engine.findConfigFile(), configFilePath);
+        });
+
+        it("should return root level eslint.config.js when overrideConfigFile is not specified", async () => {
+            const engine = new FlatESLint();
+            const configFilePath = path.resolve(__dirname, "../../../eslint.config.js");
+
+            assert.strictEqual(await engine.findConfigFile(), configFilePath);
+        });
+
+    });
+
     describe("getRulesMetaForResults()", () => {
 
         it("should throw an error when this instance did not lint any files", async () => {