feat!: undefined will no longer replace defined values by default

src/defaults/into.ts

@@ -115,11 +115,17 @@ export function mergeMaps<
 }
 
 /**
- * Set the target to the last value.
+ * Set the target to the last non-undefined value.
  */
 export function mergeOthers<Ts extends ReadonlyArray<unknown>>(
   m_target: Reference<unknown>,
   values: Ts,
 ) {
-  m_target.value = values.at(-1);
+  for (let i = values.length - 1; i >= 0; i--) {
+    if (values[i] !== undefined) {
+      m_target.value = values[i];
+      return;
+    }
+  }
+  m_target.value = undefined;
 }

src/defaults/vanilla.ts

@@ -119,8 +119,13 @@ export function mergeMaps<
 }
 
 /**
- * Get the last value in the given array.
+ * Get the last non-undefined value in the given array.
  */
 export function mergeOthers<Ts extends ReadonlyArray<unknown>>(values: Ts) {
-  return values.at(-1);
+  for (let i = values.length - 1; i >= 0; i--) {
+    if (values[i] !== undefined) {
+      return values[i];
+    }
+  }
+  return undefined;
 }

src/types/merging.ts

@@ -9,8 +9,10 @@ import {
   type EveryIsMap,
   type EveryIsRecord,
   type EveryIsSet,
+  type Is,
   type IsNever,
   type IsTuple,
+  type Or,
 } from "./utils";
 
 /**
@@ -169,7 +171,7 @@ export type DeepMergeLeaf<Ts extends ReadonlyArray<unknown>> =
     : Ts extends readonly [infer T]
       ? T
       : Ts extends readonly [...infer Rest, infer Tail]
-        ? IsNever<Tail> extends true
+        ? Or<IsNever<Tail>, Is<Tail, undefined>> extends true
           ? Rest extends ReadonlyArray<unknown>
             ? DeepMergeLeaf<Rest>
             : never

tests/deepmerge-into.test.ts

@@ -266,11 +266,11 @@ describe("deepmergeInto", () => {
     expect(x).toStrictEqual(expected);
   });
 
-  it(`replaces records with undefined`, () => {
+  it(`doesn't replaces records with undefined`, () => {
     const x = { key1: { subkey: `one` } };
     const y = { key1: undefined };
 
-    const expected = { key1: undefined };
+    const expected = { key1: { subkey: `one` } };
 
     deepmergeInto(x, y);
 

tests/deepmerge.test-d.ts

@@ -163,7 +163,7 @@ const i = {
 };
 
 const test13 = deepmerge(a, i);
-expectType<{ foo: undefined; baz: { quux: string[] }; garply: number }>(test13);
+expectType<{ foo: string; baz: { quux: string[] }; garply: number }>(test13);
 
 const j = {
   foo: new Set([1, 2]),

tests/deepmerge.test.ts

@@ -300,18 +300,18 @@ describe("deepmerge", () => {
     const x = { key1: undefined };
     const y = { key1: { subkey: `one` } };
 
-    const expected = { key1: { subkey: `one` } };
+    const expected = { key1: y.key1 };
 
     const merged = deepmerge(x, y);
 
     expect(merged).toStrictEqual(expected);
   });
 
-  it(`replaces records with undefined`, () => {
+  it(`doesn't replaces records with undefined`, () => {
     const x = { key1: { subkey: `one` } };
     const y = { key1: undefined };
 
-    const expected = { key1: undefined };
+    const expected = { key1: x.key1 };
 
     const merged = deepmerge(x, y);