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How to mock JavaScript functions

How to test whether a built-in JavaScript function is being called


Event, Promise


I want to verify that a JS function of an object is invoked after a certain action is taken. The function does not raise any event that can be listened to.

For example, I wanted to test that when the done button is clicked, the window.close() is called to close the app.

   * Close the app when the user clicks the done button.
   * @protected
  handleDoneButtonClicked_() {


One simple way is to replace the function definition with a mock function. The mock function contains code that:

Key Steps:

  1. Import PromiseResolver and flushTasks.
import {PromiseResolver} from 'chrome://resources/js/promise_resolver.m.js';
import {flushTasks} from '../../test_util.js';
  1. Add a new test
// Test clicking the done button should close the window.
test('ClickDoneButtonShouldCloseWindow', async () => {
  // Initialize the page by adding the polymer element to the page.
  await initializePage();

  // Create a  promise resolver.
  const resolver = new PromiseResolver();

  // Initialize a counter.
  let windowCloseCalled = 0;

  // Define a mock close function to replace the window.close().
  const closeMock = () => {
    return resolver.promise;

  // Replace the close function with the mock one.
  window.close = closeMock;

  // Click the done button.
  const doneButton = getElement(page, '#buttonDone');;

  // Wait for the event to be processed.
  await flushTasks();
  // Verify the expectation. Counter value of 1 indicates the window.close()
  // has  been called once.
  assertEquals(1, windowCloseCalled);

If the function being tested is a private function, simply use the set method in the element's constructor:

const mockFunction = () => {
  return resolver.promise;

element.set('privateMethodToCall_', mockFunction);

###Testing methods to fire a custom event### One way to verify that a method successfully fire an event is adding temporary event listeners to the element in the browser test.

In the following example, I want to test that the select element will observe changing of selectedValue and fire a custom event button-remapping-changed:

In the initialize state, add a event listener to the select element being tested:

select = document.createElement (;

const mockFunction = () => {
  return resolver.promise;

select.addEventListener('button-remapping-changed', mockFunction);

Then in the testing stage, simply update the observed selectedValue and assert buttonRemappingChangedEventCount:

// Update select to another remapping action.
select.selectedValue = 'acceleratorAction1';
await flushTasks();

// Verify that event is fired and button remapping is updated.
assertEquals(buttonRemappingChangedEventCount, 1);

Example CL


Tip: This technique can also be used to return fake data in the function.