Invoking a function variable instead of a function in JavaScript/TypeScript

Question

class component()
{
    Service _service = new Service();
    let serviceMethod = isUpdate ? this._service.update : 
    this._service.create;

    serviceMethod(param);
}

class Service
{
    update()
    {
        // Other stuff
        // Cannot access this here
        this.privatelocalfunc();
    }

    create()
    {
       // Other stuff
       // Cannot access this here
       this.privatelocalfunc();
    }
}

I have to conditionally switch between methods as described in the code above. However when I do this, I cannot access the 'this' variable for the service method. Whats the best way to achieve this?