Using TypeScript to create order summary.

orderSummary.ts

/**
 * Wed, 15 Nov 2017 at 0:56:13 MYT
 * "Using TypeScript to create order summary."
 * 
 * The purpose of this snippet is to demonstrate the
 * use of TypeScript in our code.
 * You can run this code online here:
 * https://www.typescriptlang.org/play/
 * 
 * PS:
 * The code is based on Fun Fun Function (@mpjme),
 * Unit testing in JavaScript Part 1
 * https://www.youtube.com/watch?v=Eu35xM76kKY
 * 
 **/

interface IOrderItem {
  name: string;
  price: number;
  shipppingCost?: number; // TS: use `?` to indicate optional property
}

interface IOrder {
  // [TRY THIS]
  // using VSCode, hover on `IOrderItem` below
  // and hold `option` key (OSX) to display symbol definition
  items: Array<IOrderItem>;
}

interface IOrderSummary {
  itemsCount: number;
  totalPrice: number;
}

const orderSummary = (order: IOrder): IOrderSummary => {
  const initialValue: number = 0; // ← [TRY THIS] change `number` to `string` and see the error
  const totalPrice: number = order.items.reduce(
    (prev: number, current: IOrderItem): number => {
      // [CHALLENGE] convert this return statement into ES6 "Implicit Return Shorthand"
      return prev + current.price + (current.shipppingCost || 0);
    },
    initialValue
  );

  return {
    itemsCount: order.items.length,
    totalPrice // ← use ES6 property-value shorthand
  };
};

// [TRY THIS] change `IOrder` to other type and see the error
const sampleOrder: IOrder = {
  items: [
    {
      name: "Penetration Tester's Open Source Toolkit",
      price: 100,
      shipppingCost: 21
    },
    {
      name: "Gray Hat Hacking Second Edition",
      price: 250

      // notice that it's *safe* to not put `shippingCost` here
      // because we declared it as optional in interface `IOrderItem`
    }
  ]
};

const summary = orderSummary(sampleOrder);

// sample of test case
console.log(summary, summary.totalPrice == 371 ? "✔ PASSED" : "✘ FAILED");
console.log(summary, summary.totalPrice != 370 ? "✔ PASSED" : "✘ FAILED");

// ----------------------------
//
// Bonuses:
// 1. command to re-run this file on save:
//    → `nodemon --watch '*.ts' --exec 'ts-node' orderSummary.ts`
// 2. Article about JS shorthand techniques:
//    → https://www.sitepoint.com/shorthand-javascript-techniques/
//
// ----------------------------