I am trying to test Angular components with jest. One component initializes an inherited (previously undefined) variable in its constructor. When running the application it works as expected. When I run my test and call MockRender(MyComponent), the variable is initialized in the constructor but when the ngOnInit-Block is reached, it is null (only for testing, not when running the application).

I tried replicating this error with a minimal example, but an initialized variable keeps its value between constructor call and ngOnInit as expected.

  constructor() {
    super()
    this.myVariable = "i am initialized"
    //successfully set in minimum example and test
    console.log("constructed " + this.myVariable) 
  }

  ngOnInit(): void {
    //successfully set in minimum example (including test for minimum example)
    //  and running application but null in test for original component
    console.log("ngOnInit " + this.myVariable) 
    super.ngOnInit()
  }

When I completely remove the initialization of the variable from my constructor, it is still changed from undefined to null between constructor-call and ngOnInit.

From my understanding, when a component is set up, the constructor is called and then ngOnInit, so nothing should be able to interfere between those two calls. In addition, I can’t find anything in the code that might set the variable to null.

What might be the cause for a variable being reset to null between constructor-call and ngOnInit only when testing, not when running the application?