@merry-solutions/type-filter

Well, what is it?

Type filter is a package, the purpose of which is to provide proper typing to filtered results. For example, given an array of numbers:

const numbers = [1, 2, 3, 4, 5];
const filtered = numbers.filter((n) => n === 2);

The type of filtered variable is still Array<number>, while in reality after the filter has been applied, it sould be Array<2>. And this is exactly what this package does: provides propert typing to the filtered array items.

Installation

npm i @merry-solutions/type-filter

Usage

Supposed you have a user interface and an array of users:

interface User {
  name: string;
  address: {
    geo: {
      lat: string;
      lng: string;
    };
  };
}

const USERS: User[] = [
  {
    address: {
      geo: {
        lat: '11',
        lng: '22',
      },
    },
    name: 'Bob',
  },
  {
    address: {
      geo: {
        lat: '33',
        lng: '33',
      },
    },
    name: 'Mary',
  },
];

To filter out users and get an array in which every user has specific name, i.e. Bob, we pass the interface, along with property path and the filtering value:

import { typeFilter } from '@merry-solutions/type-filter';

const results = typeFilter<User, 'name', 'Bob'>(USERS, 'name', 'Bob');

results[0].name;
// name is no longer of 'string' type, it is 'Bob' now

Nested paths are also supported, the nested level is the 4th parameter in the generics list, by default the max level is 99, so filtering by lng would look the following way:

import { typeFilter } from '@merry-solutions/type-filter';

const results = typeFilter<User, 'address.geo.lng', '33'>(
  USERS,
  'address.geo.lng',
  '33'
);
// results[0].address.geo.lng is now of type '33' instead of 'string'

For filtering of primitives there is primitiveTypeFilter which is even more simple:

import { primitiveTypeFilter } from '@merry-solutions/type-filter';

const arrayOf2s = primitiveTypeFilter<number, 2>([1, 2, 3, 4, 5], 2);
// arrayOf2s type is Array<2>