Function composition with TypeScript

functions.ts

console.log("hello world");

const data = {
  projects: [
    { technology: "Java", name: "Project1", budget: 1000, finishedInDays: 10 },
    { technology: "Java", name: "Project1", budget: 2000, finishedInDays: 15 },
    { technology: "Ruby", name: "Test", budget: 800, finishedInDays: 3 },
    { technology: "Ruby", name: "Backend", budget: 1200, finishedInDays: 10 },
    { technology: "Python", name: "Backend", budget: 1200, finishedInDays: 9 },
    {
      technology: "React",
      name: "Expensive",
      budget: 3800,
      finishedInDays: 40,
    },
  ],
  employees: [
    { firstName: "Alice", lastName: "Cooper", age: 25 },
    { firstName: "Bob", lastName: "Sinclair", age: 31 },
    { firstName: "Claire", lastName: "Unknown", age: 52 },
  ],
};

const query = <DataCollection extends Record<string, any[]>>(
  dataCollection: DataCollection
) => {
  type Source = keyof DataCollection;
  type FieldsOfType<Record extends {}, Type> = {
    [R in keyof Record]: Record[R] extends Type ? R : never;
  }[keyof Record];

  return {
    from: <S extends Source>(source: S) => {
      type RowType<List> = List extends Array<infer Item> ? Item : List;

      type Row = RowType<DataCollection[S]>;
      type Condition = (row: Row) => boolean;

      type StringFields = FieldsOfType<Row, string>;
      type NumberFields = FieldsOfType<Row, number>;

      return {
        where: (condition: Condition): Row[] =>
          dataCollection[source].filter<Row>((r): r is Row => condition(r)),

        startsWith:
          (field: StringFields, value: string): Condition =>
          (row) =>
            row[field].startsWith(value),

        greaterThan:
          (field: NumberFields, value: number): Condition =>
          (row) =>
            row[field] > value,

        lessThan:
          (field: NumberFields, value: number): Condition =>
          (row) =>
            row[field] < value,

        or:
          (a: Condition, b: Condition): Condition =>
          (r) =>
            a(r) || b(r),
        and:
          (a: Condition, b: Condition): Condition =>
          (r) =>
            a(r) && b(r),
      };
    },
  };
};

const { where, and, greaterThan, startsWith } = query(data).from("employees");

console.log(where(and(greaterThan("age", 20), startsWith("firstName", "Bo"))));