json-serializer-typescript.ts

// Don't ask about this one. I have no idea what this magic is.
// Copied from https://github.com/microsoft/TypeScript/issues/13298#issuecomment-724542300.
type UnionToIntersection<U> = (
  U extends any ? (arg: U) => any : never
) extends (arg: infer I) => void
  ? I
  : never;

// Continuation of above.
type UnionToTuple<T> = UnionToIntersection<(T extends any ? (t: T) => T : never)> extends (_: any) => infer W
  ? [...UnionToTuple<Exclude<T, W>>, W]
  : [];

// The rest below is mine.

type JSONValue = boolean | number | string | JSONValue[] | { [K: string]: JSONValue };

type EscapedChar<C extends string> =
  C extends '\u0000' ? '\\\u0000' :
  C extends '\u0001' ? '\\\u0001' :
  C extends '\u0002' ? '\\\u0002' :
  C extends '\u0003' ? '\\\u0003' :
  C extends '\u0004' ? '\\\u0004' :
  C extends '\u0005' ? '\\\u0005' :
  C extends '\u0006' ? '\\\u0006' :
  C extends '\u0007' ? '\\\u0007' :
  C extends '\b' ? '\\\b' :
  C extends '\t' ? '\\\t' :
  C extends '\n' ? '\\\n' :
  C extends '\u000b' ? '\\\u000b' :
  C extends '\f' ? '\\\f' :
  C extends '\r' ? '\\\r' :
  C extends '\u000e' ? '\\\u000e' :
  C extends '\u000f' ? '\\\u000f' :
  C extends '\u0010' ? '\\\u0010' :
  C extends '\u0011' ? '\\\u0011' :
  C extends '\u0012' ? '\\\u0012' :
  C extends '\u0013' ? '\\\u0013' :
  C extends '\u0014' ? '\\\u0014' :
  C extends '\u0015' ? '\\\u0015' :
  C extends '\u0016' ? '\\\u0016' :
  C extends '\u0017' ? '\\\u0017' :
  C extends '\u0018' ? '\\\u0018' :
  C extends '\u0019' ? '\\\u0019' :
  C extends '\u001a' ? '\\\u001a' :
  C extends '\u001b' ? '\\\u001b' :
  C extends '\u001c' ? '\\\u001c' :
  C extends '\u001d' ? '\\\u001d' :
  C extends '\u001e' ? '\\\u001e' :
  C extends '\u001f' ? '\\\u001f' :
  C extends '"' | '\\' ? `\\${C}` :
  C;

type EscapedString<S extends string> = S extends `${infer C}${infer Rest}` ? `${EscapedChar<C>}${EscapedString<Rest>}` : '';

type Join<A> =
    A extends [string, string, ...infer Rest] ? `${A[0]},${Join<[A[1], ...Rest]>}` :
    A extends [string] ? A[0] :
    A extends [] ? '' :
    never;

type StringifiedArray<A extends JSONValue[]> = {
    [K in keyof A]: Stringified<A[K]>
};

type StringifiedObject<O> = {
    [K in keyof O]: `${Stringified<K>}:${Stringified<O[K]>}`
};

type Stringified<T> =
    T extends (number | boolean) ? `${T}` :
    T extends string ? `"${EscapedString<T>}"` :
    T extends Array<any> ? `[${Join<StringifiedArray<T>>}]` :
    T extends object ? `{${Join<ObjectValues<StringifiedObject<T>>>}}` :
    string;

type MapObjectValues<O, Keys> = {
  [K in keyof Keys]: (Keys[K] extends keyof O ? O[Keys[K]] : never)
};

type ObjectValues<O> = MapObjectValues<O, UnionToTuple<keyof O>>;

interface JSON {
  stringify<V>(value: V): Stringified<V>;
}

let s = JSON.stringify({
  x: true,
  y: 123.456,
  z: 'foo"bar\\baz'
} as const);