TypeDrop

2026-02-20 Challenge

2026-02-20 Easy

Typed Inventory Aggregator

You're building a dashboard for a small e-commerce warehouse. Given a flat list of product entries (each with a category, SKU, price, and stock count), aggregate them into a per-category summary — the hardest part is getting the TypeScript types exactly right.

Goals

Define a branded `Sku` type and implement `toSku` without using `any` or a type assertion.
Build the `InventoryItem` and `CategorySummary` interfaces with all required fields typed correctly.
Implement `aggregateInventory` to produce a correct per-category summary in a single pass over the items array.
Implement `topCategories` returning category names sorted by total inventory value descending, capped at n results.

          
          challenge.ts

// Key types & main function signature

type Sku = string & { readonly _brand: "Sku" };

interface InventoryItem {
  sku:          Sku;
  name:         string;
  category:     string;
  priceUsd:     number;
  stock:        number;
}

interface CategorySummary {
  totalItems:     number;
  totalStock:     number;
  totalValueUsd:  number;
  cheapestSku:    Sku;
  mostStockedSku: Sku;
}

type InventorySummary = Record<string, CategorySummary>;

function aggregateInventory(items: InventoryItem[]): InventorySummary { ... }
function topCategories(summary: InventorySummary, n: number): string[] { ... }

Hints (click to reveal)

Hints

For `toSku`, look at how TypeScript's `satisfies` operator or a simple return-type annotation can coerce a `string` to a branded type without writing `as Sku`.
Use a `for...of` loop or `Array.reduce` over the items and build up the `InventorySummary` object incrementally — initialise each category bucket on first encounter.
For `topCategories`, `Object.entries` gives you `[category, summary]` pairs you can sort by `totalValueUsd` before slicing to `n`.

Useful resources

StackBlitz

CodeSandbox

Or clone locally

git clone -b challenge/2026-02-20 https://github.com/niltonheck/typedrop.git