151 - Query String Parser

[akcyp]

151 - Query String Parser

Code

type QPair = [string, string | true];
type MarkAs<A, T> = A extends T ? A : never;
// MarkAs<4, number> => 4, MarkAs<"4", number> => never

type ToQPair<T extends string[]> = MarkAs<[T[0], T[1] extends undefined ? true : T[1]], QPair>;
// ToQPair<['a', 'b']> => ['a', 'b'], ToQPair<['a']> => ['a', true]

type Split<T extends string, S extends string> =
  T extends `${infer X}${S}${infer Y}` ? [X, ...Split<Y, S>] : T extends '' ? [] : [T];
// Split<'a,b,c'> => ['a', 'b', 'c']

type SplitQueryString<T extends string, R extends string[] = Split<T, '&'>> =
  R extends [infer A, ...infer B] ?
    [ToQPair<Split<MarkAs<A, string>, '='>>, ...SplitQueryString<T, MarkAs<B, string[]>>]
    : [];
// SplitQueryString<'a=b&c'> => [['a', 'b'], ['c', true]]

type FindByKey<K extends string, T extends QPair[]> =
  T extends [infer A, ...infer B] ? MarkAs<A, QPair>[0] extends K ?
    [MarkAs<A, QPair>[1], ...FindByKey<K, MarkAs<B, QPair[]>>] : FindByKey<K, MarkAs<B, QPair[]>> : [];
// FindByKey<'key', [['key', 'value'], ['test', 'data'], ['key', 'val']]> => ['value', 'val']

type RemoveDuplicates<
  T extends any[],
  R extends any[] = [],
  V extends unknown[] = [],
  L extends number = V['length']
> =
  L extends T['length'] ? R :
    T[L] extends R[number] ?
      RemoveDuplicates<T, R, [...V, unknown]> :
      RemoveDuplicates<T, [...R, T[L]], [...V, unknown]>;
// RemoveDuplicates<[1, 2, 1, 3]> => [1, 2, 3]

type ArrayOrFirstElement<T extends any[]> = T['length'] extends 1 ? T[0] : T;
// ArrayOrFirstElement<[1, 2]> => [1, 2], ArrayOrFirstElement<[1]> => 1

type ParseQueryString<T extends string, P extends QPair[] = SplitQueryString<T>> = {
  [K in P[number][0]]: ArrayOrFirstElement<RemoveDuplicates<FindByKey<K, P>>>
};

Test cases

/* _____________ Test Cases _____________ */
import { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Equal<ParseQueryString<''>, {}>>,
  Expect<Equal<ParseQueryString<'k1'>, { k1: true }>>,
  Expect<Equal<ParseQueryString<'k1&k1'>, { k1: true }>>,
  Expect<Equal<ParseQueryString<'k1&k2'>, { k1: true, k2: true }>>,
  Expect<Equal<ParseQueryString<'k1=v1'>, { k1: 'v1' }>>,
  Expect<Equal<ParseQueryString<'k1=v1&k1=v2'>, { k1: ['v1', 'v2'] }>>,
  Expect<Equal<ParseQueryString<'k1=v1&k2=v2'>, { k1: 'v1', k2: 'v2' }>>,
  Expect<Equal<ParseQueryString<'k1=v1&k2=v2&k1=v2'>, { k1: ['v1', 'v2'], k2: 'v2' }>>,
  Expect<Equal<ParseQueryString<'k1=v1&k2'>, { k1: 'v1', k2: true }>>,
  Expect<Equal<ParseQueryString<'k1=v1&k1=v1'>, { k1: 'v1' }>>
]