Jest Test Fails: TypeError: Cannot read property 'fxaa' of undefined

I have a line in my code to enable anti-aliasing:

viewer.scene.postProcessStages.fxaa.enabled = true;

Could someone please let me know why my test fails with the following error?

TypeError: Cannot read property ‘fxaa’ of undefined

In other words, I actually do not have a mock test for this. How would I write a Jest test script to cover this case?