Type-Function to Parse JSON into TypeScript types

type-from-json.ts

type Whitespace = " " | "\n" | "\r" | "\t";
type TrimStart<T extends string> = T extends `${Whitespace}${infer R}` ? TrimStart<R> : T;
type TrimEnd<T extends string> = T extends `${infer R}${Whitespace}` ? TrimEnd<R> : T;
type Trim<T extends string> = TrimStart<TrimEnd<T>>;

type BoolLiteral = "true" | "false";
type Digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9";

type InferJsonScalarType<T extends string> = Trim<T> extends `"${infer _}"` ? string
  : Trim<T> extends `${BoolLiteral}` ? boolean
  : Trim<T> extends `${Digit}${infer _}` ? number
  : Trim<T> extends `null` ? null
  : never;

type HeadingJsonScalar<T extends string> = Trim<T> extends `"${infer Str}",${infer _}` ? `"${Str}"`
  : Trim<T> extends `${infer Token},${infer _}` ? Token
  : T;
type TrailingJsonEntries<T extends string> = Trim<T> extends `"${infer _}",${infer Tail}` ? Tail
  : Trim<T> extends `${infer _},${infer Tail}` ? Tail
  : T;

type DecodeJsonStr<T extends string> = T extends `"${infer R}"` ? R : never;

type InfoerJsonObjectEntries<T extends string> = Trim<T> extends `${infer K}:${infer Tail}`
  ? { [key in DecodeJsonStr<Trim<K>>]: InferJsonScalarType<HeadingJsonScalar<Tail>> } & InfoerJsonObjectEntries<TrailingJsonEntries<Tail>>
  : {};

export type TypeFromJson<T extends string> = Trim<T> extends `{${infer InnerJsonObject}}`
  ? InfoerJsonObjectEntries<InnerJsonObject>
  : never;

const json = `
{
  "name": "foo",
  "answer": 42,
  "ok": true
}
`;

type ResultType =  TypeFromJson<typeof json>;
// ResultType is almost { name: string; answer: number; ok: boolean }