Real World Example

Example

type Product = { id: number; name: string; price: number; stock: number; category: "audio" | "kitchen" };

class Query<T, S extends keyof T = keyof T> {
  constructor(private rows: T[], private cols?: S[], private filters: ((r: T) => boolean)[] = [], private order?: [keyof T, "asc" | "desc"]) {}

  select<K extends keyof T>(...cols: K[]): Query<T, K> {
    return new Query<T, K>(this.rows, cols, this.filters, this.order);
  }
  where<K extends keyof T>(col: K, op: "=" | ">" | "<", value: T[K]): Query<T, S> {
    const test = (r: T) => (op === "=" ? r[col] === value : op === ">" ? r[col] > value : r[col] < value);
    return new Query<T, S>(this.rows, this.cols, [...this.filters, test], this.order);
  }
  orderBy(col: keyof T, dir: "asc" | "desc" = "asc"): Query<T, S> {
    return new Query<T, S>(this.rows, this.cols, this.filters, [col, dir]);
  }
  run(): Pick<T, S>[] {
    let out = this.rows.filter((r) => this.filters.every((f) => f(r)));
    if (this.order) {
      const [c, d] = this.order;
      out = [...out].sort((a, b) => (a[c] < b[c] ? -1 : 1) * (d === "asc" ? 1 : -1));
    }
    const cols = this.cols;
    return cols ? out.map((r) => Object.fromEntries(cols.map((c) => [c, r[c]])) as Pick<T, S>) : out;
  }
}

const products: Product[] = [
  { id: 1, name: "Earbuds", price: 1799, stock: 12, category: "audio" },
  { id: 2, name: "Speaker", price: 2999, stock: 0, category: "audio" },
  { id: 3, name: "Headphones", price: 4999, stock: 4, category: "audio" },
  { id: 4, name: "Kettle", price: 1299, stock: 9, category: "kitchen" },
];

const result = new Query(products).select("name", "price").where("category", "=", "audio").where("stock", ">", 0).orderBy("price", "desc").run();
// result: { name: string; price: number }[]
result.forEach((r) => console.log(r.name.padEnd(11), r.price));

Output

Headphones  4999
Earbuds     1799