replace deepmerge-ts with defu

code/core/package.json

@@ -257,7 +257,7 @@
     "@types/express": "^4.17.21",
     "@types/node": "^18.0.0",
     "browser-assert": "^1.2.1",
-    "deepmerge-ts": "^7.0.3",
+    "defu": "^6.1.4",
     "esbuild": "^0.18.0 || ^0.19.0 || ^0.20.0 || ^0.21.0 || ^0.22.0 || ^0.23.0",
     "esbuild-register": "^3.5.0",
     "express": "^4.19.2",

code/core/src/manager-api/lib/merge.test.ts

@@ -0,0 +1,15 @@
+import { test, describe } from "vitest";
+
+import merge from "./merge";
+
+describe(merge.name, () => {
+  test("applies entries not in the source", ({ expect }) => {
+    const merged = merge({ a: 1 }, { a: 2, b: 3, c: 4 });
+    console.log({ merged });
+    expect(merged).toStrictEqual({
+      a: 1,
+      b: 3,
+      c: 4
+    });
+  });
+});

code/core/src/manager-api/lib/merge.ts

@@ -1,12 +1,19 @@
-import { deepmerge } from "deepmerge-ts";
+import { defu, defuArrayFn } from 'defu';
 
 import { logger } from '@storybook/core/client-logger';
 
-export default <TObj = any>(a: TObj, b: Partial<TObj>) => {
-  if (Array.isArray(a) && !Array.isArray(b)) {
+export default <TObj = any>(a: TObj, b: Partial<TObj> & any) => {
+  if (Array.isArray(a)) {
+    if (Array.isArray(b)) {
+      return defuArrayFn(a, b);
+    }
+
     logger.log(['the types mismatch, picking', a]);
     return a;
   }
 
-  return deepmerge(a, b);
+  // TODO: Ask for what is prefered:
+  // 1. type guard (safe at runtime, but potential performance slow-down)
+  // 2. or type assertion (unsafe at runtime)
+  return defu(a as object, b);
 };