Typescript Helper Types

types.ts

// when T is any|unknown, Y is returned, otherwise N
type IsAnyUnknown<T, Y, N> = unknown extends T ? Y : N;
  
// when T is never, Y is returned, otherwise N
type IsNever<T, Y = true, N = false> = [T] extends [never] ? Y : N;

// empty object
type EmptyObject = { [key: string]: never };
  
// is a type an empty object?
type IsEmpty<T, Y = true, N = false> = T extends EmptyObject ? Y : N;

// returns the value type for T[K] without having to do "K extends keyof T ? T[K] : never"
type Lookup<T, K, N = never> = T extends any ? K extends keyof T ? T[K] : N : N;

// sometimes you have a type with never values, this removes those keys from T
type StripNever<T> = Pick<T, { [K in keyof T]: IsNever<T[K], never, K> }[keyof T]>;

// sometimes something is an expected type, but TypeScript has problem recognizing it.
// This can ensure the expected type is being used.
type Cast<T, AS> = T extends AS ? T : never;

// Returns the result of { ...A, ...B }
type MergeObjects<A, B> = {
  [K in (keyof B | keyof A)]: K extends keyof B 
    ? B[K] 
    : K extends keyof A 
      ? A[K]
      : never
};
           
// Which keys in T are undefined
type UndefinedKeys<T> = { 
  [P in keyof T]-?: undefined extends T[P] ? P : never 
}[keyof T];

// When a type is really deep and has retained an unnessecary amount of type information, 
// this flattens it to a single array/object/value.
type Simplify<T> = 
  T extends (object | any[])
    ? { [K in keyof T]: T[K] }
    : T;

// Converts { x: string | undefined } to { x?: string | undefined }
type UndefinedToOptional<T> = Simplify<
  Pick<T, Exclude<keyof T, UndefinedKeys<T>>> &
  Partial<Pick<T, UndefinedKeys<T>>>
>;

// Converts { x: string } | { y: number } to { x: string, y: number }
type UnionToIntersection<T> = 
  (T extends any ? (x: T) => any : never) extends 
  (x: infer R) => any ? R : never;

// Converts string | number | boolean to [string, number, boolean]
type UnionToTuple<T> = (
(
  (
    T extends any
      ? (t: T) => T
      : never
  ) extends infer U
    ? (U extends any
      ? (u: U) => any
      : never
    ) extends (v: infer V) => any
      ? V
      : never
    : never
) extends (_: any) => infer W
  ? [...UnionToTuple<Exclude<T, W>>, W]
  : []
);

// Converts (string | number)[] to [string, number]
type ArrayToTuple<T> =
  T extends Array<infer E> 
    ? UnionToTuple<E>
    : T
;

// Merges objects and changes undefined to optional if any exist.
type AppendObjects<A, B> = 
  UndefinedToOptional<MergeObjects<A, B>>
;

// AppendTuples<[string, boolean], [string, number]> = [string, boolean, string, number]
type AppendTuples<A extends any[], B extends any[]> = 
  Simplify<[...A, ...B]>
;

// JSON
type JsonScalar = number | string | boolean | null;
type JsonObject = { [key: string]: JsonScalar | JsonObject | JsonArray; };
type JsonArray = Array<JsonScalar | JsonObject | JsonArray>;
type Json = JsonScalar | JsonObject | JsonArray;
             
// ObjectKeys<{ x: string, y: number }> = ["x", "y"]
type ObjectKeys<T> = 
  UnionToTuple<keyof T> extends Array<keyof T>
    ? UnionToTuple<keyof T> 
    : never;

// The properties of T should be Partialed
type PartialChildren<T> = {
  [K in keyof T]: Partial<T[K]>
};

// JoinTuples<[[string], [], [boolean, number], [string, Date]]> = [string, boolean, number, string, Date]
type JoinTuples<T extends any[]> =
  T extends [infer A]
    ? ToTuple<A>
    : T extends [infer B, ...infer C]
      ? [...ToTuple<B>, ...JoinTuples<C>]
      : [];

// Converts anything to a tuple or array, unless it aleady is.
type ToTuple<T extends any> = T extends any[] ? T : [T];

// A extends B is not good enough sometimes, especially when never is involved.
// Also order matters when doing extends, so this does both directions.
type Extends<A, B, T = true, F = false> = [A] extends [B] ? [B] extends [A] ? T : F : F;

// Testing return types. 
// If the input doesn't match T there will be a TS error.
//
//  expectType<string>('hello')
function expectType<Expected>(type: Expected) {}

// Testing types against each other (used to unit test the above types)
// If the types don't match there will be a TS error.
// 
//  expectTypeMatch<["x", "y"], ObjectKeys<{ x: string, y: number }>>(true);
//
// If you want to test they are NOT equivalent, pass false to function.
function expectTypeMatch<A, B>(truthy: Extends<A, B>) {}