really-naive-filtering-implementation.ts

class Filter<T> {
    static greaterThan(threshold: number, orEqualTo: boolean = false) {
        return (input: number) => orEqualTo ? input >= threshold : input > threshold;
    }

    static equalTo(compare: number | string) {
        return (input: number | string) => input === compare;
    }

    constructor(private readonly collection: Array<T>) { }

    where<V>(prop: keyof T, predicate: (item: T[keyof T]) => V) {
        const filtered = this.collection.filter(item => predicate(item[prop]));
        if (!filtered.length) {
            throw new RangeError(`No more items in collection! :(`);
        }
        return new Filter(filtered);
    }

    // finally, let's be able access our initial array
    toArray() {
        return this.collection;
    }
}

class Person {
    constructor(public age: number, public firstName: string, public lastName: string) { }
}

const myPersons = [
    new Person(15, 'Alice', 'Cage'),
    new Person(21, 'Bob', 'Johnson'),
    new Person(32, 'Johnny', 'Appleseed'),
];

const myFilter = new Filter(myPersons);

const myResult = myFilter
    .where('age', Filter.greaterThan(20) as any)
    .where('firstName', Filter.equalTo('Johnny'))
    .toArray();

console.log(myResult.map(person => `${person.firstName} ${person.lastName}`));
// -> Logs [ 'Johnny Appleseed' ]