chore: improve merging & return types

.vscode/dictionary.txt

@@ -17,6 +17,7 @@ dtsx
 entrypoints
 heroicons
 lockb
+Mergeable
 openweb
 outdir
 pausable

src/index.ts

@@ -34,3 +34,4 @@ export async function loadConfig<T>({ name, cwd, defaultConfig }: Config<T>): Pr
 }
 
 export * from './types'
+export * from './utils'

src/types.ts

@@ -10,3 +10,23 @@ export interface Config<T> {
   cwd?: string
   defaultConfig: T
 }
+
+export type SimplifyDeep<T> = T extends object
+  ? { [P in keyof T]: SimplifyDeep<T[P]> }
+  : T
+
+export type DeepMerge<T, S> = {
+  [P in keyof (T & S)]: P extends keyof T
+    ? P extends keyof S
+      ? DeepMergeable<T[P], S[P]>
+      : T[P]
+    : P extends keyof S
+      ? S[P]
+      : never
+}
+
+export type DeepMergeable<T, S> = T extends object
+  ? S extends object
+    ? DeepMerge<T, S>
+    : S
+  : S

src/utils.ts

@@ -1,39 +1,56 @@
+import type { DeepMerge, SimplifyDeep } from './types'
+
 /**
- * Deep merge objects or arrays
+ * Deep Merge
+ *
+ * Merges arrays if both configs are arrays, otherwise does object deep merge.
  *
- * @param target - The target object or array
- * @param sources - The source objects or arrays
- * @returns The merged result
+ * @param target - The target object.
+ * @param sources - The source objects.
+ * @returns The merged object.
+ * @example ```ts
+ * deepMerge({ foo: 'bar' }, { bar: 'baz' })
+ * deepMerge([{ foo: 'bar' }], [{ bar: 'baz' }])
+ * deepMerge({ foo: 'bar' }, [{ foo: 'baz' }])
+ * ```
  */
-export function deepMerge<T>(target: T, ...sources: Partial<T>[]): T {
+export function deepMerge<T, S>(target: T, ...sources: S[]): T extends object
+  ? S extends any[]
+    ? S
+    : S extends object
+      ? SimplifyDeep<DeepMerge<T, S>>
+      : T
+  : T extends any[]
+    ? S extends any[]
+      ? T
+      : T
+    : T {
   if (!sources.length)
-    return target
+    return target as any
 
   const source = sources.shift()
   if (!source)
-    return target
+    return target as any
+
+  if (Array.isArray(source) !== Array.isArray(target)
+    || isObject(source) !== isObject(target)) {
+    return source as any
+  }
 
   if (Array.isArray(target) && Array.isArray(source)) {
-    // If both are arrays, concatenate them
-    return [...target, ...source] as T
+    return [...target, ...source] as any
   }
 
   if (isObject(target) && isObject(source)) {
     for (const key in source) {
       if (Object.prototype.hasOwnProperty.call(source, key)) {
         const sourceValue = source[key]
-        if (Array.isArray(sourceValue) && Array.isArray((target as any)[key])) {
-          // Merge arrays within objects
-          (target as any)[key] = [...(target as any)[key], ...sourceValue]
-        }
-        else if (isObject(sourceValue) && isObject((target as any)[key])) {
-          // Deep merge nested objects
-          (target as any)[key] = deepMerge((target as any)[key], sourceValue)
-        }
-        else {
-          // Replace primitive values and objects/arrays that don't match in type
+        if (!Object.prototype.hasOwnProperty.call(target, key)) {
           (target as any)[key] = sourceValue
+          continue
         }
+
+        (target as any)[key] = deepMerge((target as any)[key], sourceValue)
       }
     }
   }