AG-Grid: While writing unit test with Jasmine getting "TypeError: Cannot read properties of undefined (reading 'refreshCells')"

Question

I am working with AG-Grid and I have to write unit test case for the createDef method. I have something like this.

createColDef(){

    const colDef = {

    //all the coldef's goes here
    }

    this.gridApi.refreshCells();

    return colDef;
}
    

Now when I write unit test case for the createColDef method I get error saying "TypeError: Cannot read properties of undefined (reading 'refreshCells')" . Also, I have all these codes in grid.service.ts

Below is sample of how my spec.ts file looks

   it('should return the correct columndef', () => {
      const result = service.createColumnDef();
      const expectedColDef = [//expected coldef];
      expect(JSON.stringify(result)).toEqual(JSON.stringify(expectedColDef));
    });

How do I overcome this gridApi issue. Please help!