Angular TypeError: is not a function when trying to call an object.object[].method

I am getting the following error when I tried to call a method of an object.object[].method in angular 4:

OrderComponent.html:30 ERROR TypeError: item.increaseQuantity is not a function at OrderComponent.increaseQuantity (order.component.ts:87)

The typescript code I have is below. When I call the OrderComponent.IncreaseQuantity(itemLoc: number) method, it generates the error above when I try to call this.orderItems[itemLoc].increaseQuantity() method within the method. I don't know why it does not know the OrderItem.increaseQuantity method:

import { Component, Inject } from '@angular/core';
import { Http } from '@angular/http';

class OrderItem {
    id: number;
    name: string;
    unitPrice: number;
    quantity: number;
    amount: number;
    comment: string;

    increaseQuantity(): boolean {
        console.log("In the Order Item itself - worked.");
        this.quantity += 1;
        this.amount = this.quantity * this.unitPrice;
        return false;
    }
}

class Order {
    id: number;
    createdTime: Date;
    orderType: string;
    tableName: string;
    orderItems: OrderItem[];
}

@Component({
    selector: 'order',
    templateUrl: './order.component.html'
})
export class OrderComponent {
    public order: Order;
    public total: number = 0;


    constructor(http: Http, @Inject('ORIGIN_URL') originUrl: string) {
        http.get(originUrl + '/api/Order/2').subscribe(result => {
            this.order = result.json() as Order;
            this.updateTotal();
        });

    }

    updateTotal(): void {
        this.total = 0;
        //console.log("all: " + JSON.stringify(this.order.orderItems));
        this.order.orderItems.forEach(s => this.total += s.amount);
    }

    increaseQuantity(itemLoc: number): boolean {
        //item.increaseQuantity();
        let item = this.order.orderItems[itemLoc] as OrderItem;

        console.log("This increaseQuantity work." + JSON.stringify(item));

        return item.increaseQuantity();
        //return false;
    }

}