refactor(core): optimize config merging and object checks

packages/core/src/defaultConfig.ts

@@ -322,11 +322,11 @@ export const normalizeConfig = (
     config.server.publicDir,
   );
 
+  const defaultConfig = createDefaultConfig();
+  defaultConfig.mode = getMode();
+
   const mergedConfig = mergeRsbuildConfig(
-    {
-      ...createDefaultConfig(),
-      mode: getMode(),
-    },
+    defaultConfig,
     config,
   ) as NormalizedConfig;
 

packages/core/src/helpers/index.ts

@@ -24,12 +24,11 @@ export const isObject = (obj: unknown): obj is Record<string, any> =>
 
 // Cache Object.prototype reference for better performance in hot paths
 const objectPrototype = Object.prototype;
+const getProto = Object.getPrototypeOf;
 
 export const isPlainObject = (obj: unknown): obj is Record<string, any> => {
   return (
-    obj !== null &&
-    typeof obj === 'object' &&
-    Object.getPrototypeOf(obj) === objectPrototype
+    obj !== null && typeof obj === 'object' && getProto(obj) === objectPrototype
   );
 };
 

packages/core/src/mergeConfig.ts

@@ -1,7 +1,7 @@
-import { castArray, cloneDeep, isFunction, isPlainObject } from './helpers';
+import { cloneDeep, isPlainObject } from './helpers';
 import type { RsbuildConfig } from './types';
 
-const OVERRIDE_PATHS = [
+const OVERRIDE_PATHS = new Set([
   'performance.removeConsole',
   'output.inlineScripts',
   'output.inlineStyles',
@@ -13,7 +13,7 @@ const OVERRIDE_PATHS = [
   'resolve.conditionNames',
   'resolve.mainFields',
   'provider',
-];
+]);
 
 /**
  * When merging configs, some properties prefer `override` over `merge to array`
@@ -22,10 +22,10 @@ const isOverridePath = (key: string) => {
   // ignore environments name prefix, such as `environments.web`
   if (key.startsWith('environments.')) {
     const realKey = key.split('.').slice(2).join('.');
-    return OVERRIDE_PATHS.includes(realKey);
+    return OVERRIDE_PATHS.has(realKey);
   }
   return (
-    OVERRIDE_PATHS.includes(key) ||
+    OVERRIDE_PATHS.has(key) ||
     // output.filename.* supports function but we should not merge them as array
     key.startsWith('output.filename.')
   );
@@ -45,21 +45,28 @@ const merge = (x: unknown, y: unknown, path = ''): unknown => {
     return isPlainObject(x) ? cloneDeep(x) : x;
   }
 
+  const typeX = typeof x;
+  const typeY = typeof y;
+
   // no need to merge boolean with object or function, such as `tools.htmlPlugin`
-  if (typeof x === 'boolean' || typeof y === 'boolean') {
+  if (typeX === 'boolean' || typeY === 'boolean') {
     return y;
   }
 
-  const pair = [x, y];
-
   // combine array
-  if (pair.some(Array.isArray)) {
-    return [...castArray(x), ...castArray(y)];
+  const isArrayX = Array.isArray(x);
+  const isArrayY = Array.isArray(y);
+  if (isArrayX && isArrayY) {
+    return x.concat(y);
+  } else if (isArrayX) {
+    return [...x, y];
+  } else if (isArrayY) {
+    return [x, ...y];
   }
 
   // convert function to chained function
-  if (pair.some(isFunction)) {
-    return pair;
+  if (typeX === 'function' || typeY === 'function') {
+    return [x, y];
   }
 
   if (!isPlainObject(x) || !isPlainObject(y)) {