javascript reflection. How to inject value to not required variable

What is the best way to inject value to not required variable? Please see following code.

export interface IApprove {
  approve():boolean;
}

export class FirstImplementation implements IApprove {
  approve():boolean {
    return true;
  }
}

export class SecondImplementation implements IApprove {
  private data:number;

  approve():boolean {
    return this.data > 1;
  }
}

function approve(guardsClasses:Array<any>, data:number):boolean {
  guardsClaases.forEach((guardClass) => {
    let guard = injector.get(guardClass); // this is some abstract injector that create instance from class
    // here I would like to check whether guard instance have "data" variable. If so - pass data value.
    let result = guard.approve();
    if (!result) {
      return false
    }
  });
  return true;
}

approve([FirstImplementation, SecondImplementation], 2);

1) I could use Object.hasOwnProperty, but it seems to work only when property is initialized. In other case it returns null

2) I could force setting value on each guard, but type script compiler will throw error since it will not fit IApprove interface.