adds a test for #990

tests/jsPsych/default-parameters.test.js

@@ -27,4 +27,32 @@ describe('nested defaults', function(){
         expect(display.querySelector('input').placeholder).toBe("")
         expect(display.querySelector('input').size).toBe(40)
     });
+
+    test('safe against extending the array.prototype (issue #989)', function(){
+        Array.prototype.qq = jest.fn();
+        const spy = jest.spyOn(console, 'error').mockImplementation();
+
+        var t = {
+            type: 'survey-text',
+            questions: [
+                {
+                    prompt: 'Question 1.'
+                },
+                {
+                    prompt: 'Question 2.'
+                }
+            ]
+        }
+
+        jsPsych.init({timeline: [t]})
+
+        var display = jsPsych.getDisplayElement();
+
+        expect(display.querySelector('input').placeholder).toBe("")
+        expect(display.querySelector('input').size).toBe(40)
+
+        expect(spy).not.toHaveBeenCalled();
+
+        spy.mockRestore();
+    });
 })