diff --git a/src/frontend.ts b/src/frontend.ts
index 7798a08..adc72d1 100644
--- a/src/frontend.ts
+++ b/src/frontend.ts
@@ -13,6 +13,7 @@
 // limitations under the License.
 
 import { Enforcer } from './enforcer';
+import { deepCopy } from './util';
 
 /**
  * Experiment!
@@ -41,7 +42,7 @@ export async function casbinJsGetPermissionForUser(e: Enforcer, user: string): P
   s += '[matchers]\n';
   s += `m = ${m.get('m')?.get('m')?.value.replace(/_/g, '.')}`;
   obj['m'] = s;
-  obj['p'] = await e.getPolicy();
+  obj['p'] = deepCopy(await e.getPolicy());
   for (const arr of obj['p']) {
     arr.splice(0, 0, 'p');
   }
diff --git a/src/util/util.ts b/src/util/util.ts
index 42c7380..2d55149 100644
--- a/src/util/util.ts
+++ b/src/util/util.ts
@@ -159,6 +159,17 @@ async function generatorRunAsync(iterator: Generator<any>): Promise<any> {
   }
 }
 
+function deepCopy(obj: Array<any> | any): any {
+  if (typeof obj !== 'object') return;
+  const newObj: any = obj instanceof Array ? [] : {};
+  for (const key in obj) {
+    if (obj.hasOwnProperty(key)) {
+      newObj[key] = typeof obj[key] === 'object' ? deepCopy(obj[key]) : obj[key];
+    }
+  }
+  return newObj;
+}
+
 export {
   escapeAssertion,
   removeComments,
@@ -175,4 +186,5 @@ export {
   getEvalValue,
   generatorRunSync,
   generatorRunAsync,
+  deepCopy,
 };
diff --git a/test/frontend.test.ts b/test/frontend.test.ts
index 54c82d0..3d2bc9a 100644
--- a/test/frontend.test.ts
+++ b/test/frontend.test.ts
@@ -18,6 +18,11 @@ import { casbinJsGetPermissionForUser } from '../src/frontend';
 
 test('TestCasbinJsGetPermissionForUser', async () => {
   const e = await newEnforcer('examples/rbac_model.conf', 'examples/rbac_with_hierarchy_policy.csv');
+  const a = await casbinJsGetPermissionForUser(e, 'alice');
+  const b = await casbinJsGetPermissionForUser(e, 'alice');
+  if (a !== b) {
+    throw new Error('Unexpected side affect.');
+  }
   const received = JSON.parse(await casbinJsGetPermissionForUser(e, 'alice'));
   const expectedModelStr = readFileSync('examples/rbac_model.conf').toString();
   expect(received['m']).toBe(expectedModelStr.replace(/\n\n/g, '\n'));