diff --git a/index.js b/index.js
index 4265c69..9b0fe6c 100644
--- a/index.js
+++ b/index.js
@@ -14,10 +14,8 @@
  */
 
 exports = module.exports = function(a, b){
-  if (a && b) {
-    for (var key in b) {
-      a[key] = b[key];
-    }
-  }
-  return a;
+    return {
+        ...(a ?? {}),
+        ...(b ?? {})
+    };
 };
diff --git a/test/index.test.js b/test/index.test.js
index 7241855..0202d3b 100644
--- a/test/index.test.js
+++ b/test/index.test.js
@@ -47,5 +47,23 @@ describe('merge', function() {
       expect(o).to.be.equal(a);
     });
   });
-  
+
+   it('should not mutate first parameter', function() {
+    const foo = { foo: "foo" };
+    const bar = { bar: "bar" };
+    const result = merge(foo, bar);
+    expect(Object.keys(foo)).to.have.length(1);
+    expect(result.foo).to.be.equal('foo');
+    expect(result.bar).to.be.equal('bar');
+  });
+
+    
+  it('should not mutate second parameter', function() {
+    const foo = { foo: "foo" };
+    const bar = { bar: "bar" };
+    const result = merge(foo, bar);
+    expect(Object.keys(bar)).to.have.length(1);
+    expect(result.foo).to.be.equal('foo');
+    expect(result.bar).to.be.equal('bar');
+  });
 });