diff --git a/src/defu.ts b/src/defu.ts
index df34a1d..b5f91bf 100644
--- a/src/defu.ts
+++ b/src/defu.ts
@@ -5,9 +5,9 @@ function isObject (val: any) {
 }
 
 // Base function to apply defaults
-function _defu<T> (baseObj: T, defaults: any, merger?: Merger): T {
+function _defu<T> (baseObj: T, defaults: any, namespace: string = '.', merger?: Merger): T {
   if (!isObject(defaults)) {
-    return _defu(baseObj, {}, merger)
+    return _defu(baseObj, {}, namespace, merger)
   }
 
   const obj = Object.assign({}, defaults)
@@ -23,14 +23,14 @@ function _defu<T> (baseObj: T, defaults: any, merger?: Merger): T {
       continue
     }
 
-    if (merger && merger(obj, key, val)) {
+    if (merger && merger(obj, key, val, namespace)) {
       continue
     }
 
     if (Array.isArray(val) && Array.isArray(obj[key])) {
       obj[key] = obj[key].concat(val)
     } else if (isObject(val) && isObject(obj[key])) {
-      obj[key] = _defu(val, obj[key], merger)
+      obj[key] = _defu(val, obj[key], (namespace ? `${namespace}.` : '') + key.toString(), merger)
     } else {
       obj[key] = val
     }
@@ -41,14 +41,14 @@ function _defu<T> (baseObj: T, defaults: any, merger?: Merger): T {
 
 // Create defu wrapper with optional merger and multi arg support
 function extend (merger?: Merger): DefuFn {
-  return (...args) => args.reduce((p, c) => _defu(p, c, merger), {} as any)
+  return (...args) => args.reduce((p, c) => _defu(p, c, '', merger), {} as any)
 }
 
 // Basic version
 const defu = extend() as Defu
 
 // Custom version with function merge support
-defu.fn = extend((obj, key, currentValue) => {
+defu.fn = extend((obj, key, currentValue, _namespace) => {
   if (typeof obj[key] !== 'undefined' && typeof currentValue === 'function') {
     obj[key] = currentValue(obj[key])
     return true
@@ -56,7 +56,7 @@ defu.fn = extend((obj, key, currentValue) => {
 })
 
 // Custom version with function merge support only for defined arrays
-defu.arrayFn = extend((obj, key, currentValue) => {
+defu.arrayFn = extend((obj, key, currentValue, _namespace) => {
   if (Array.isArray(obj[key]) && typeof currentValue === 'function') {
     obj[key] = currentValue(obj[key])
     return true
diff --git a/src/types.ts b/src/types.ts
index e5cd347..b75ef1d 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -1,7 +1,8 @@
 export type Merger = <T extends Input, K extends keyof T>(
   obj: T,
   key: keyof T,
-  value: T[K]
+  value: T[K],
+  namespace: string
 ) => any;
 
 export type DefuFn = <Source extends Input, Defaults extends Input>(
diff --git a/test/defu.test.ts b/test/defu.test.ts
index 46e0fc9..258d864 100644
--- a/test/defu.test.ts
+++ b/test/defu.test.ts
@@ -25,7 +25,7 @@ describe('defu', () => {
   it('should copy nested values', () => {
     const result = defu({ a: { b: 'c' } }, { a: { d: 'e' } })
     expect(result).toEqual({
-      a: { b: 'c', d: 'e' },
+      a: { b: 'c', d: 'e' }
     })
     expectTypeOf(result).toEqualTypeOf<{ a: { b: string, d: string } }>()
   })
@@ -33,7 +33,7 @@ describe('defu', () => {
   it('should concat array values by default', () => {
     const result = defu({ array: ['b', 'c'] }, { array: ['a'] })
     expect(result).toEqual({
-      array: ['a', 'b', 'c'],
+      array: ['a', 'b', 'c']
     })
     expectTypeOf(result).toEqualTypeOf<{ array: string[] }>()
   })
@@ -52,7 +52,7 @@ describe('defu', () => {
 
     const result = defu({ a: fn }, { a: re })
     expect(result).toEqual({ a: fn })
-    expectTypeOf(result).toEqualTypeOf<{ a: (() => number) | RegExp }>()
+    expectTypeOf(result).toEqualTypeOf<{ a:(() => number) | RegExp }>()
   })
 
   it('should handle non object first param', () => {
@@ -74,7 +74,7 @@ describe('defu', () => {
     expect(result).toEqual({
       a: 1,
       b: 2,
-      c: 3,
+      c: 3
     })
     expectTypeOf(result).toEqualTypeOf<{ a: string | number, b: string | number, c?: number }>()
   })
@@ -94,7 +94,7 @@ describe('defu', () => {
     // @ts-expect-error
     expect(defu(null, { foo: 1 }, false, 123, { bar: 2 })).toEqual({
       foo: 1,
-      bar: 2,
+      bar: 2
     })
   })
 
@@ -113,7 +113,7 @@ describe('defu', () => {
     expect(
       defu.fn(
         {
-          ignore: (val) => val.filter((i) => i !== 'dist'),
+          ignore: val => val.filter(i => i !== 'dist'),
           num,
           ignored: num
         },
@@ -142,4 +142,20 @@ describe('defu', () => {
       num
     })
   })
+
+  it('custom merger with namespace', () => {
+    const ext = defu.extend((obj, key, val, namespace) => {
+      // console.log({ obj, key, val, namespace })
+      if (key === 'modules') {
+        // TODO: It is not possible to override types with extend()
+        // @ts-ignore
+        obj[key] = namespace + ':' + [...val, ...obj[key]].sort().join(',')
+        return true
+      }
+    })
+
+    const obj1 = { modules: ['A'], foo: { bar: { modules: ['X'] } } }
+    const obj2 = { modules: ['B'], foo: { bar: { modules: ['Y'] } } }
+    expect(ext(obj1, obj2)).toEqual({ modules: ':A,B', foo: { bar: { modules: 'foo.bar:X,Y' } } })
+  })
 })