Fix: init error in extending recommended config (fixes #12707)

lib/init/autoconfig.js

@@ -316,10 +316,10 @@ class Registry {
 /**
  * Extract rule configuration into eslint:recommended where possible.
  *
- * This will return a new config with `"extends": "eslint:recommended"` and
+ * This will return a new config with `["extends": [ ..., "eslint:recommended"]` and
  * only the rules which have configurations different from the recommended config.
  * @param   {Object} config config object
- * @returns {Object}        config object using `"extends": "eslint:recommended"`
+ * @returns {Object}        config object using `"extends": ["eslint:recommended"]`
  */
 function extendFromRecommended(config) {
     const newConfig = Object.assign({}, config);
@@ -333,7 +333,7 @@ function extendFromRecommended(config) {
             delete newConfig.rules[ruleId];
         }
     });
-    newConfig.extends = RECOMMENDED_CONFIG_NAME;
+    newConfig.extends.unshift(RECOMMENDED_CONFIG_NAME);
     return newConfig;
 }
 

tests/lib/init/autoconfig.js

@@ -12,7 +12,8 @@
 const assert = require("chai").assert,
     autoconfig = require("../../../lib/init/autoconfig"),
     sourceCodeUtils = require("../../../lib/init/source-code-utils"),
-    baseDefaultOptions = require("../../../conf/default-cli-options");
+    baseDefaultOptions = require("../../../conf/default-cli-options"),
+    recommendedConfig = require("../../conf/eslint-recommended");
 
 const defaultOptions = Object.assign({}, baseDefaultOptions, { cwd: process.cwd() });
 
@@ -328,4 +329,43 @@ describe("autoconfig", () => {
             });
         });
     });
+
+    describe("extendFromRecommended()", () => {
+        it("should return a configuration which has `extends` key with Array type value", () => {
+            const oldConfig = { extends: [], rules: {} };
+            const newConfig = autoconfig.extendFromRecommended(oldConfig);
+
+            assert.exists(newConfig.extends);
+            assert.isArray(newConfig.extends);
+        });
+
+        it("should return a configuration which has array property `extends`", () => {
+            const oldConfig = { extends: [], rules: {} };
+            const newConfig = autoconfig.extendFromRecommended(oldConfig);
+
+            assert.include(newConfig.extends, "eslint:recommended");
+        });
+
+        it("should return a configuration which preserves the previous extending configurations", () => {
+            const oldConfig = { extends: ["previous:configuration1", "previous:configuration2"], rules: {} };
+            const newConfig = autoconfig.extendFromRecommended(oldConfig);
+
+            assert.includeMembers(newConfig.extends, oldConfig.extends);
+        });
+
+        it("should return a configuration which has `eslint:recommended` at the first of `extends`", () => {
+            const oldConfig = { extends: ["previous:configuration1", "previous:configuration2"], rules: {} };
+            const newConfig = autoconfig.extendFromRecommended(oldConfig);
+            const [firstExtendInNewConfig] = newConfig.extends;
+
+            assert.strictEqual(firstExtendInNewConfig, "eslint:recommended");
+        });
+
+        it("should return a configuration which not includes rules configured in `eslint:recommended`", () => {
+            const oldConfig = { extends: [], rules: { ...recommendedConfig.rules } };
+            const newConfig = autoconfig.extendFromRecommended(oldConfig);
+
+            assert.notInclude(newConfig.rules, oldConfig.rules);
+        });
+    });
 });