Agregate Root demo with invariants.

BillAndLine.ts

/**
 * Entity
 */
class Line {
  constructor(private description: string, private amount: number, private vat: number) { }

  public isEmpty(): boolean {
    return this.description === '' && this.amount === 0 && this.vat === 0;
  }
}

/**
 * Aggregate Root
 */
class Bill {
  private lines: Line[] = [];

  public addLine(): void {
    const previousLinesProperlyFilled = this.lines.every(line => !line.isEmpty());

    if (previousLinesProperlyFilled) {
      this.lines.push(new Line('', 0, 0));
    } else {
      throw new Error('You must fill all the previous lines before adding a new one');
    }
  }
}

const bill = new Bill();
bill.addLine();
bill.addLine(); // "Executed JavaScript Failed:" You must fill all the previous lines before adding a new one