Advanced Typescript Cheatsheet

cheatsheet.ts

export type Await<T> = T extends PromiseLike<infer U> ? Await<U> : T;
export type IsPromise<T> = PromiseLike<infer U> ? true : false;

export type Length<T> = T extends { length: infer L } ? L : never;

export type KeysOfType<O, T> = {
    [K in keyof O]: O[K] extends T ? K : never;
}[keyof O];

// ConvertLiterals would convert literal types like `1337` to their base type like `number` if set to true 
export type PickByType<O, T, ConvertLiterals extends boolean = false> = {
    [K in KeysOfType<O, T>]: ConvertLiterals extends true ? T : O[K]
};

export type OneOf<T, Strict extends boolean = true> = {
    [OuterKey in keyof T]: Strict extends false
        ? { [K in OuterKey]: T[K] }
        : { [InnerKey in OuterKey|keyof T]?: InnerKey extends OuterKey ? T[OuterKey] : never } & { [TheKey in OuterKey]: T[OuterKey] } 
}[keyof T];

type Push<T extends unknown[], U> = T extends [...infer R] ? [...T, U] : never;
type PushFront<T extends unknown[], U> = T extends [...infer R] ? [U, ...T] : never;
type Pop<T extends unknown[]> = T extends [...infer R, infer U] ? U : never;
type PopFront<T extends unknown[]> = T extends [infer U, ...infer R] ? U : never;
type Shift<T extends unknown[]> = T extends [infer U, ...infer R] ? R : never;
type ShiftRight<T extends unknown[]> = T extends [...infer R, infer U] ? R : never;

type Reverse<T extends unknown[], U extends unknown[] = []> = Length<T> extends 1 ? Push<U, Pop<T>> : Reverse<ShiftRight<T>, Push<U, Pop<T>>>;

type Filter<T extends unknown[], U> = T extends [] ? [] : T extends [infer F, ...infer R] ? F extends U ? Filter<R, U> : [F, ...Filter<R, U>] : never
type TupleIncludes<T extends unknown[], U> = Length<Filter<T, U>> extends Length<T> ? false : true

type StringIncludes<S extends string, D extends string> = S extends `${infer T}${D}${infer U}` ? true : false;

type Includes<T extends unknown[]|string, U> = T extends unknown[] ? TupleIncludes<T, U> : T extends string ? U extends string ? StringIncludes<T, U> : never : never;

type Split<S extends string, D extends string> =
    string extends S ? string[] :
        S extends '' ? [] :
            S extends `${infer T}${D}${infer U}` ?  [T, ...Split<U, D>] : [S];

type Join<T extends unknown[], D extends string> = string[] extends T ? string : T extends string[]
  ? PopFront<T> extends string ? Length<T> extends 1 ? `${PopFront<T>}` : `${PopFront<T>}${D}${Join<Shift<T>, D>}` : never
  : never;
  
type ValidPaths<T> = keyof T extends never ? never : ({
     [K in keyof T]: T[K] extends never ? never : T[K] extends Record<string|number|symbol, unknown>
       ? K extends string ? `${K}.${ValidPaths<T[K]>}` | K : never
       : K
   })[keyof T] & string;

type ValidPathTuples<T> = keyof T extends never ? never : ({
     [K in keyof T]: T[K] extends never ? never : T[K] extends Record<string|number|symbol, unknown>
       ? [K, ...ValidPathTuples<T[K]>] | [K]
       : [K]
   })[keyof T];

type NestedType<T, P extends string> = (
  Includes<P, '.'> extends true
    ? PopFront<Split<P, '.'>> extends keyof T
      ? NestedType<T[PopFront<Split<P, '.'>>], Join<Shift<Split<P, '.'>>, '.'>>
      : never
    : P extends keyof T ? T[P] : never
);

type NestedTypeByTuple<T, P extends string[]> = (
  Length<P> extends 1
    ? Pop<P> extends keyof T ? T[Pop<P>] : never
    : PopFront<P> extends keyof T ? Shift<P> extends string[] 
      ? NestedTypeByTuple<T[PopFront<P>], Shift<P>>
      : never : never
);

type NestedTypeUsingTuplesAgain<T, P extends ValidPaths<T>> = NestedTypeByTuple<T, Split<P, '.'>>;

Quick question, how could I make this work with interfaces where a key may be undefined. For example.

interface X {
  [path: string]: {
    a: string;
    b?: {
      c: number;
    };
  };
}

const a = getByPath({} as X, 'rest.b.c');  // a = never

Is there any way I can ignore undefined? @Alia5

You need to wrap your type with optional keys in the Required util-type (builtin)

interface X {
  [path: string]: {
    a: string;
    b?: {
      c: number;
    };
  };
}

const a = GetByPath({} as Required<{[K in keyof X]: Required<X[K]>}>, 'rest.b.c'); // a = number

Thanks for taking the time to comment. Any way I can do this recursively throughout the whole object if it was nested arbitrarily deep?

https://stackoverflow.com/a/57837897/5106063

I ran into some strange behaviour where ValidPaths treated const enum as an object and iterated over its properties (charCodeAt etc.). Getting to the bottom of this issue is above my pay grade but I was able to create an alternative version of ValidPaths that doesn't suffer from this problem.

export type ValidPaths<T> = T extends string | number | boolean | null | undefined
  ? never
  : {
      [K in keyof T]: K extends string ? `${K}.${ValidPaths<T[K]>}` | K : never;
    }[keyof T];

🤔