restore current logic

lib/config/flat-config-schema.js

@@ -61,20 +61,23 @@ function isUndefined(value) {
     return typeof value === "undefined";
 }
 
+// A unique empty object to be used internally as a mapping key in `deepMerge`.
+const EMPTY_OBJECT = {};
+
 /**
  * Deeply merges two objects.
  * @param {Object} first The base object.
- * @param {Object} second The overrides object.
+ * @param {any} second The overrides value.
  * @param {Map<string, Map<string, Object>>} [mergeMap] Maps the combination of first and second arguments to a merged result.
  * @returns {Object} An object with properties from both first and second.
  */
-function deepMerge(first, second, mergeMap = new Map()) {
+function deepMerge(first, second = {}, mergeMap = new Map()) {
 
     /*
-     * If either argument is an array, just return the second one. We don't merge
+     * If the second value is an array, just return it. We don't merge
      * arrays because order matters and we can't know the correct order.
      */
-    if (Array.isArray(first) || Array.isArray(second)) {
+    if (Array.isArray(second)) {
         return second;
     }
 
@@ -94,7 +97,7 @@ function deepMerge(first, second, mergeMap = new Map()) {
     }
 
     /*
-     * First create a result object where properties from the second object
+     * First create a result object where properties from the second value
      * overwrite properties from the first. This sets up a baseline to use
      * later rather than needing to inspect and change every property
      * individually.
@@ -117,10 +120,14 @@ function deepMerge(first, second, mergeMap = new Map()) {
         const firstValue = first[key];
         const secondValue = second[key];
 
-        if (isNonNullObject(firstValue) && isNonNullObject(secondValue)) {
+        if (isNonNullObject(firstValue)) {
             result[key] = deepMerge(firstValue, secondValue, mergeMap);
-        } else if (isUndefined(secondValue)) {
-            result[key] = firstValue;
+        } else if (isUndefined(firstValue)) {
+            if (isNonNullObject(secondValue)) {
+                result[key] = deepMerge(EMPTY_OBJECT, secondValue, mergeMap);
+            } else if (!isUndefined(secondValue)) {
+                result[key] = secondValue;
+            }
         }
     }
 

tests/lib/config/flat-config-schema.js

@@ -20,28 +20,28 @@ describe("merge", () => {
         assert.deepStrictEqual(result, { ...first, ...second });
     });
 
-    it("does not merge an object and an array", () => {
+    it("overrides an object with an array", () => {
         const first = { foo: 42 };
         const second = ["bar", "baz"];
         const result = merge(first, second);
 
         assert.strictEqual(result, second);
     });
 
-    it("does not merge an array with an object", () => {
+    it("merges an array with an object", () => {
         const first = ["foo", "bar"];
         const second = { baz: 42 };
         const result = merge(first, second);
 
-        assert.deepStrictEqual(result, second);
+        assert.deepStrictEqual(result, { 0: "foo", 1: "bar", baz: 42 });
     });
 
-    it("does not merge two arrays", () => {
+    it("overrides an array with another array", () => {
         const first = ["foo", "bar"];
         const second = ["baz", "qux"];
         const result = merge(first, second);
 
-        assert.deepStrictEqual(result, second);
+        assert.strictEqual(result, second);
     });
 
     it("returns an emtpy object if both values are undefined", () => {
@@ -74,7 +74,7 @@ describe("merge", () => {
         assert.deepStrictEqual(result, { foo: { bar: "baz", qux: 42 } });
     });
 
-    it("does not overwrite a value in the first object with undefined in the second one", () => {
+    it("does not override a value in a property with undefined", () => {
         const first = { foo: { bar: "baz" } };
         const second = { foo: void 0 };
         const result = merge(first, second);
@@ -100,22 +100,28 @@ describe("merge", () => {
         assert.notStrictEqual(result, second);
     });
 
-    it("overwrites a value in the first object with null in the second one", () => {
+    it("throws an error if a value in a property is overriden with null", () => {
         const first = { foo: { bar: "baz" } };
         const second = { foo: null };
-        const result = merge(first, second);
 
-        assert.deepStrictEqual(result, second);
-        assert.notStrictEqual(result, second);
+        assert.throws(() => merge(first, second), TypeError);
     });
 
-    it("overwrites a value in the first object with a primitive in the second one", () => {
+    it("does not override a value in a property with a primitive", () => {
         const first = { foo: { bar: "baz" } };
         const second = { foo: 42 };
         const result = merge(first, second);
 
-        assert.deepStrictEqual(result, second);
-        assert.notStrictEqual(result, second);
+        assert.deepStrictEqual(result, first);
+        assert.notStrictEqual(result, first);
+    });
+
+    it("merges an object in a property with a string", () => {
+        const first = { foo: { bar: "baz" } };
+        const second = { foo: "qux" };
+        const result = merge(first, second);
+
+        assert.deepStrictEqual(result, { foo: { 0: "q", 1: "u", 2: "x", bar: "baz" } });
     });
 
     it("merges objects with self-references", () => {
@@ -128,7 +134,7 @@ describe("merge", () => {
         const result = merge(first, second);
 
         assert.strictEqual(result.first, first);
-        assert.strictEqual(result.second, second);
+        assert.deepStrictEqual(result.second, second);
 
         const expected = { foo: 42, bar: "baz" };
 
@@ -164,7 +170,7 @@ describe("merge", () => {
 
         const result = merge(first, second);
 
-        assert.strictEqual(result.first, first);
+        assert.deepStrictEqual(result.first, first);
         assert.strictEqual(result.second, second);
 
         const expected = { foo: 42, bar: "baz" };
@@ -208,7 +214,7 @@ describe("merge", () => {
         };
         const result = merge(first, second);
 
-        assert.strictEqual(result.a, result.d);
+        assert.deepStrictEqual(result.a, result.d);
 
         const expected = {
             a: { foo: 42, bar: "baz" },

tests/lib/eslint/flat-eslint.js

@@ -6065,7 +6065,7 @@ describe("FlatESLint", () => {
 
             await eslint.lintText("debugger;");
 
-            assert.strictEqual(resolvedSettings.sharedData, circular);
+            assert.deepStrictEqual(resolvedSettings.sharedData, circular);
         });
 
         it("in 'parserOptions'", async () => {
@@ -6093,7 +6093,7 @@ describe("FlatESLint", () => {
 
             await eslint.lintText("debugger;");
 
-            assert.strictEqual(resolvedParserOptions.testOption, circular);
+            assert.deepStrictEqual(resolvedParserOptions.testOption, circular);
         });
     });