Angular unit test : spy a function with jest spectator

Question

Starting with Jest spectator, I try to test a function that is called in another. The purpose of the test is to know whether or not the function was called correctly.

I tried to follow the documentation here: https://jestjs.io/docs/expect#tohavebeencalled

For information, the function to be called manages the emission of an EventEmitter, and this method is private.

Here's the .ts code:

  private emitPiecesJointes(): void {
    this.piecesJointes.emit(this.pieceJointesUploaded);
  }

emitPiecesJointes() is called like this:

  public fileDeleted(index: number): void {
    this._piecesJointes.splice(index, 1);
    this.emitPiecesJointes();
  }

Here's the test I try to run with the wrong code:

describe('BlablaComponent', () => {
  let spectator: Spectator<BlablaContratComponent>;
  const createComponent = createComponentFactory({
    component: BlablaContratComponent,
    schemas: [NO_ERRORS_SCHEMA],
  });

  it('it should issue an attachment', () => {
    const emitPiecesJointes = jest.fn();
    spectator.component.fileDeleted(1);
    expect(emitPiecesJointes).toHaveBeenCalled();
  });

Thanks for help

Answer 1

There problem you are having is that you are building a mock into a local variable and not doing anything with it. const emitPiecesJointes = jest.fn(); creates a mock, but doesn't actually do anything else. This would be appropriate to replace a public function like this: spectator.component.fileDeleted = jest.fn();, but won't work for a private function.
To mock a private function, replace that line with the following:
const emitPiecesJointes = jest.spyOn(BlablaContratComponent.prototype as any, "emitPiecesJointes");