Viktor,

Here is a refactored service that IMO is more clear:

class PurchaseService {
  inProgress = false;

  constructor(
    private backend: PurchaseBackend, 
    private messages: Messages) { }

  order(sku: string, qty: number): void {
    if (this.inProgress) {
      this.messages.error("Already purchasing an item");
      return;
    }

    this.backend
      .checkAvailability(sku, qty)
      .then(this.orderFromBackEnd.bind(this))
      .then(this.announcePurchase.bind(this));
  }
  
  orderFromBackEnd(enoughQuantity:boolean) {
    let finishProgress = () => this.inProgress = false;
    this.inProgress = enoughQuantity;
    return !enoughQuantity ? false :
           this.backend.order(sku,qty).then(finishProgress);
  }

  announcePurchase((success: boolean) => {
    if (success) this.messages.success("Your purchase succeeded!");
    else         this.messages.error("Your purchase failed!");
  }
}