From f076e54ecdb0fae70d9b43ad6888606097beef97 Mon Sep 17 00:00:00 2001
From: "Nicholas C. Zakas" <nicholas@humanwhocodes.com>
Date: Fri, 5 May 2023 12:57:33 -0700
Subject: [PATCH] fix: Ensure FlatESLint#findConfigFile() doesn't throw.
 (#17151)

If findConfigFile() can't find eslint.config.js, it now returns
undefined instead of throwing an error.

Fixes #17150
---
 lib/eslint/flat-eslint.js       | 21 +++++++++++++++------
 tests/lib/eslint/flat-eslint.js | 10 +++++++++-
 2 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/lib/eslint/flat-eslint.js b/lib/eslint/flat-eslint.js
index 49228f16e05..faf055b265e 100644
--- a/lib/eslint/flat-eslint.js
+++ b/lib/eslint/flat-eslint.js
@@ -326,7 +326,7 @@ async function loadFlatConfigFile(filePath) {
  * 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 {{configFilePath:string,basePath:string}} Location information for
+ * @returns {{configFilePath:string,basePath:string,error:boolean}} Location information for
  *      the config file.
  */
 async function locateConfigFileToUse({ configFile, cwd }) {
@@ -334,6 +334,7 @@ async function locateConfigFileToUse({ configFile, cwd }) {
     // determine where to load config file from
     let configFilePath;
     let basePath = cwd;
+    let error = false;
 
     if (typeof configFile === "string") {
         debug(`Override config file path is ${configFile}`);
@@ -342,16 +343,18 @@ async function locateConfigFileToUse({ configFile, cwd }) {
         debug("Searching for eslint.config.js");
         configFilePath = await findFlatConfigFile(cwd);
 
-        if (!configFilePath) {
-            throw new Error("Could not find config file.");
+        if (configFilePath) {
+            basePath = path.resolve(path.dirname(configFilePath));
+        } else {
+            error = true;
         }
 
-        basePath = path.resolve(path.dirname(configFilePath));
     }
 
     return {
         configFilePath,
-        basePath
+        basePath,
+        error
     };
 
 }
@@ -378,7 +381,13 @@ async function calculateConfigArray(eslint, {
         return slots.configs;
     }
 
-    const { configFilePath, basePath } = await locateConfigFileToUse({ configFile, cwd });
+    const { configFilePath, basePath, error } = await locateConfigFileToUse({ configFile, cwd });
+
+    // config file is required to calculate config
+    if (error) {
+        throw new Error("Could not find config file.");
+    }
+
     const configs = new FlatConfigArray(baseConfig || [], { basePath, shouldIgnore });
 
     // load config file
diff --git a/tests/lib/eslint/flat-eslint.js b/tests/lib/eslint/flat-eslint.js
index 0eea528a12c..ce79693af1b 100644
--- a/tests/lib/eslint/flat-eslint.js
+++ b/tests/lib/eslint/flat-eslint.js
@@ -3956,7 +3956,7 @@ describe("FlatESLint", () => {
 
     describe("findConfigFile()", () => {
 
-        it("should return null when overrideConfigFile is true", async () => {
+        it("should return undefined when overrideConfigFile is true", async () => {
             const engine = new FlatESLint({
                 overrideConfigFile: true
             });
@@ -3964,6 +3964,14 @@ describe("FlatESLint", () => {
             assert.strictEqual(await engine.findConfigFile(), void 0);
         });
 
+        it("should return undefined when a config file isn't found", async () => {
+            const engine = new FlatESLint({
+                cwd: path.resolve(__dirname, "../../../../")
+            });
+
+            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"