Aron Schüler on 14. December 2022

Mocking IntersectionObserver in Jest

Today, I tried to test a component in React which uses the HeadlessUI Dialog. Unfortunately, this test turned out to be rather hard, because the window object in jest tests gets (most commonly) generated by JSDOM and this in turn does not support layout utilities such as IntersectionObserver. The error message/stacktrace i got was the following:

Error: Uncaught [ReferenceError: IntersectionObserver is not defined]

Fixing the error

To do so, all that is really necessary is creating a new class and assign it both to window.IntersectionObserver and global.IntersectionObserver, so that node knows how to create an instance for this.

I did this by inserting the following snippet into my setupTests.ts file, but you could also do this on a test-level directly:

// Mock the IntersectionObserver, see
export class IntersectionObserver {
  root = null;
  rootMargin = "";
  thresholds = [];

  disconnect() {
    return null;

  observe() {
    return null;

  takeRecords() {
    return [];

  unobserve() {
    return null;
window.IntersectionObserver = IntersectionObserver;
global.IntersectionObserver = IntersectionObserver;


With this snippet inserted, I was able to run my test without any problems. Hope this helped, let me know if it didn’t! 😊

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