feat: Use ESLINT_USE_FLAT_CONFIG environment variable for flat config

lib/cli.js

@@ -278,6 +278,31 @@ async function printResults(engine, results, format, outputFile) {
     return true;
 }
 
+/**
+ * Returns whether flat config should be used.
+ * @param {boolean} [allowFlatConfig] Whether or not to allow flat config.
+ * @returns {Promise<boolean>} Where flat config should be used.
+ */
+async function shouldUseFlatConfig(allowFlatConfig) {
+    if (!allowFlatConfig) {
+        return false;
+    }
+
+    switch (process.env.ESLINT_USE_FLAT_CONFIG) {
+        case "true":
+            return true;
+        case "false":
+            return false;
+        default:
+
+            /*
+             * If neither explicitly enabled nor disabled, then use the presence
+             * of a flat config file to determine enablement.
+             */
+            return !!(await findFlatConfigFile(process.cwd()));
+    }
+}
+
 //------------------------------------------------------------------------------
 // Public Interface
 //------------------------------------------------------------------------------
@@ -307,7 +332,7 @@ const cli = {
          * switch to flat config we can remove this logic.
          */
 
-        const usingFlatConfig = allowFlatConfig && !!(await findFlatConfigFile(process.cwd()));
+        const usingFlatConfig = await shouldUseFlatConfig(allowFlatConfig);
 
         debug("Using flat config?", usingFlatConfig);
 

tests/lib/cli.js

@@ -137,6 +137,43 @@ describe("cli", () => {
 
         });
 
+        describe("flat config", () => {
+            const originalCwd = process.cwd;
+            const originalEnv = { ...process.env };
+
+            afterEach(() => {
+                process.cwd = originalCwd;
+                process.env = { ...originalEnv };
+            });
+
+            it(`should use it when an eslint.config.js is present and useFlatConfig is true:${configType}`, async () => {
+                process.cwd = getFixturePath;
+
+                const exitCode = await cli.execute(`--no-ignore --ext .js ${getFixturePath("files")}`, null, useFlatConfig);
+
+                // When flat config is used, we get an exit code of 2 because the --ext option is unrecognized.
+                assert.strictEqual(exitCode, useFlatConfig ? 2 : 0);
+            });
+
+            it(`should not use it when ESLINT_USE_FLAT_CONFIG=false even if an eslint.config.js is present:${configType}`, async () => {
+                process.cwd = getFixturePath;
+                process.env.ESLINT_USE_FLAT_CONFIG = "false";
+
+                const exitCode = await cli.execute(`--no-ignore --ext .js ${getFixturePath("files")}`, null, useFlatConfig);
+
+                assert.strictEqual(exitCode, 0);
+            });
+
+            it(`should use it when ESLINT_USE_FLAT_CONFIG=true and useFlatConfig is true even if an eslint.config.js is not present:${configType}`, async () => {
+                process.env.ESLINT_USE_FLAT_CONFIG = "true";
+
+                const exitCode = await cli.execute(`--no-ignore --ext .js ${getFixturePath("files")}`, null, useFlatConfig);
+
+                // When flat config is used, we get an exit code of 2 because the --ext option is unrecognized.
+                assert.strictEqual(exitCode, useFlatConfig ? 2 : 0);
+            });
+        });
+
         describe("when given a config with rules with options and severity level set to error", () => {
 
             const originalCwd = process.cwd;