filter-collection.ts

export const filterCollection = <TOriginal>(originalCollection: TOriginal[], search: (item: TOriginal) => boolean) => {
    const result = originalCollection.filter(search);
    return {
        anyMatches: result.length > 0,
        originalCollection,
        filteredCollection: result
    };
};

import { filterCollection } from './filter-collection';

describe('Primitive types', () => {
    test('should return the original collection of numbers', () => {
        const original = [1, 2, 3, 4, 5];
        const result = filterCollection<number>(original, (item => false));

        expect(result.originalCollection).toEqual(original);
    });

    test('should return no number results with a noop search function', () => {
        const result = filterCollection<number>([1, 2, 3, 4, 5], (item => false));

        expect(result.anyMatches).toBe(false);
        expect(result.filteredCollection).toHaveLength(0);
    });

    test('should return with two matching numbers', () => {
        const result = filterCollection<number>([1, 2, 3, 4, 5], (item => item % 2 === 0));

        expect(result.anyMatches).toBe(true);
        expect(result.filteredCollection).toHaveLength(2);
    });
});

describe('Objects', () => {
    interface Customer {
        name: string,
        getsDiscount: boolean,
    };

    const customers: Customer[] = [
        { name: 'Lloyd 1', getsDiscount: true },
        { name: 'Bob', getsDiscount: false },
        { name: 'Tom', getsDiscount: true },
        { name: 'Tim', getsDiscount: false },
    ];

    test('should return the original collection of customers', () => {
        const result = filterCollection<Customer>(customers, (item => false));

        expect(result.originalCollection).toEqual(customers);
    });

    test('should return no customer results with a noop search function', () => {
        const result = filterCollection<Customer>(customers, (item => false));

        expect(result.anyMatches).toBe(false);
        expect(result.filteredCollection).toHaveLength(0);
    });

    test('should return customers with discount', () => {
        const result = filterCollection<Customer>(customers, (item => item.getsDiscount));

        expect(result.anyMatches).toBe(true);
        expect(result.filteredCollection).toHaveLength(2);


        expect(result.filteredCollection).toContainEqual(customers[0]);
        expect(result.filteredCollection).toContainEqual(customers[2]);
    });
});