diff --git a/test/unit/utils.js b/test/unit/utils.js
index ecaec44..70b365e 100644
--- a/test/unit/utils.js
+++ b/test/unit/utils.js
@@ -94,11 +94,22 @@ test('map', (t) => {
   const result = map([1,2], (value) => value+2);
   t.is(result[0], 3);
   t.is(result[1], 4);
+  const original = Array.prototype.map;
+  delete Array.prototype.map;
+  const polyfillResult = map([1,2], (value) => value+2);
+  Array.prototype.map = original;
+  t.is(polyfillResult[0], 3);
+  t.is(polyfillResult[1], 4);
 });
 
 test('find', (t) => {
   const result = find([1,2], (value) => value==2);
   t.is(result, 2);
+  const original = Array.prototype.find;
+  delete Array.prototype.find;
+  const polyfillResult = find([1,2], (value) => value==2);
+  Array.prototype.find = original;
+  t.is(polyfillResult, 2);
 });
 
 test('assign', (t) => {
@@ -106,6 +117,13 @@ test('assign', (t) => {
   t.is(result['a'], 1);
   t.is(result['b'], 2);
   t.is(result['c'], 3);
+  const original = Object.assign;
+  delete Object.assign;
+  const polyfillResult = assign({}, { a: 1 }, { b: 1 }, { b: 2, c: 3 });
+  Object.assign = original;
+  t.is(polyfillResult['a'], 1);
+  t.is(polyfillResult['b'], 2);
+  t.is(polyfillResult['c'], 3);
 });
 
 test('getBrowserAlias', (t) => {