type S=string;type N=never;type C='"'|'\\'|'/';type Q<A,B extends S>=[A,B];type D<T>={[P in keyof T]:T[P]};type B={b:'\b',t:'\t',n:'\n',r:'\r',f:'\f'};type W<T extends S>=T extends`${' '|'\n'|'\t'|'\r'}${infer T}`?W<T>:T;type X<_ extends S,T extends S=W<_>>= T extends`${boolean|null}${infer R}`?T extends`${infer T extends boolean|null}${R}`?[T,R]:N:N;type T<I extends S,O extends S=''>=I extends`${infer K}${infer R}`?K extends B[keyof B]?N:K extends '\\'?R extends`${infer K extends C|keyof B}${infer R}`?K extends keyof B?T<R,`${O}${B[K]}`>:T<R,`${O}${K}`>:N:K extends'"'?[O,R]:T<R,`${O}${K}`>:N;type L<_ extends S,O={},R extends S=W<_>>=R extends`"${infer R}`?T<R> extends Q<infer K extends S,infer R>?W<R>extends`:${infer R}`?M<R> extends Q<infer V,infer R>?W<R>extends`${infer E extends','|'}'}${infer R}`?E extends'}'?[D<O&Record<K,V>>,R]:L<R,D<O&Record<K, V>>>:N:N:N:N:R extends`}${infer R}`?[{},R]:N;type K<_ extends S,O extends unknown[]=[],R extends S=W<_>>=R extends`]${infer R}`?[[],R]:M<R>extends Q<infer V,infer R>?W<R>extends`${infer E extends','|']'}${infer R}`?E extends']'?[[...O,V],R]:K<R,[...O,V]>:N:N;type M<_ extends S,I extends S=W<_>>=I extends`${infer F}${infer R}`?F extends'"'?T<R>:F extends'{'?L<R>:F extends'['?K<R>:X<I>:N;type Parse<T extends S>=M<T>extends[infer T,infer R extends S]?W<R>extends''?T:N:N;

Solution by tomoriny #35142

type Parse<T extends string>
  = T extends `${Space}${infer U}` ? Parse<U>
  : T extends `${infer U}${Space}` ? Parse<U>
  : T extends `${infer U extends boolean | null}` ? U
  : T extends `"${infer U}` ? ParseString<U>
  : T extends `[${infer U}` ? ParseSequence<U, ']'>
  : T extends `{${infer U}` ? ParseSequence<U, '}'> : never
type VaildString<T extends string, V = T> =
  T extends `${string}${Escapes[keyof Omit<Escapes, '/'>]}${string}` ? never : V
type ParseString<T extends string> = T extends `${infer L}\\${infer E}${infer R}`
  ? VaildString<L, E extends keyof Escapes ? `${L}${Escapes[E]}${ParseString<R>}` : never>
  : T extends `${infer U}"` ? VaildString<U> : never
type ParseSequence<T extends string, End extends ']' | '}', Last extends string = ''>
  = `${Last}${T}` extends End ? End extends ']' ? [] : {}
  : T extends `${infer First},${infer Rest}`
  ? (End extends ']' ? Parse<`${Last}${First}`> : ParseEntry<`${Last}${First}`>) extends never
  ? ParseSequence<Rest, End, `${Last}${First},`> : ParseSequence<Rest, End> extends never ? never
  : End extends ']' ? [Parse<`${Last}${First}`>, ...ParseSequence<Rest, End>]
  : Prettify<ParseEntry<`${Last}${First}`> & ParseSequence<Rest, End>>
  : `${Last}${T}` extends `${infer U}${End}`
  ? End extends ']' ? Parse<U> extends never ? never : [Parse<U>]
  : {} extends ParseEntry<U> ? never : Prettify<ParseEntry<U>> : never
type ParseEntry<T extends string> =
  T extends `${infer Key}:${infer Value}`
  ? Parse<Key> extends keyof any ? Parse<Value> extends never ? never
  : Record<Parse<Key>, Parse<Value>> : never : never
type Prettify<T> = { [K in keyof T]: T[K] } & {}
type Space = ' ' | '\t' | '\n' | '\r'
type Escapes = { '"': '"'; '\\': '\\'; '/': '/'; 'b': '\b'; 'f': '\f'; 'n': '\n'; 'r': '\r'; 't': '\t' }

Solution by montmorill #35125

Spaces and escaping quotes are not supported

// examples
type example1 = Parse<'"text"'> // string
type example2 = Parse<'123.322'> // number
type example3 = Parse<'null'> // null
type example4 = Parse<'true'> // boolean
type example5 = Parse<'false'> // boolean
type example6 = Parse<`{
"x":"y",
"foo":{
"bar":"baz",
"arr":[123.123,true,{"test":10}],
"nested":{"nested":{"nested":true}}
}
}`> /*
{
    x: string;
    foo: {
        bar: string;
        arr: [number, boolean, {
            test: number;
        }];
        nested: {
            nested: {
                nested: boolean;
            };
        };
    };
}
*/
type example7 = Parse<`[
true,
false,
null,
123.123,
[123,{},{"foo":"bar","baz":{"foo":1}}],
{"nested":{"nested":[]}}
]`> /*
[boolean, boolean, null, number, [number, {}, {
    foo: string;
    baz: {
        foo: number;
    };
}], {
    nested: {
        nested: [];
    };
}]
*/

type Parse<T extends string, JSON extends string = RemoveNewLines<T>> = (
    | ParseNumber<JSON> 
    | ParseNumber<JSON> 
    | ParseString<JSON> 
    | ParseBoolean<JSON> 
    | ParseArray<JSON> 
    | ParseObj<JSON>
    | ParseNull<JSON>
) extends [infer Parsed, any]
    ? ExpandRecursively<Parsed>
    : never

type ParseString<JSON extends string> = JSON extends `"${infer Str}"${string}`
    ? [string, `\"${Str}\"`]
    : never

type ParseStringEnum<JSON extends string> = JSON extends `"${infer Str}"${string}`
    ? [Str, `\"${Str}\"`]
    : never

type ParseNull<JSON extends string> = JSON extends `null${string}`
    ? [null, 'null']
    : never

type ParseNumber<JSON extends string, End extends string = never> = [End] extends [never]
    ? JSON extends `${infer Num extends number}`
        ? [number, `${Num}`]
        : never
    : JSON extends `${infer Num extends number}${End}${string}`
        ? [number, `${Num}`]
        : never

type ParseBoolean<JSON extends string> = JSON extends `true${string}`
    ? [boolean, 'true']
    : JSON extends `false${string}`
        ? [boolean, 'false']
        : never

type ParseArray<
    JSON extends string, 
    StringResult extends string = '[', 
    Result extends any[] = []
> = JSON extends `[${infer RestArray}`
    ? RestArray extends `]${string}`
        ? [Result,`${StringResult}]`]
        : ParseNull<RestArray> | ParseNumber<RestArray, ','> | ParseNumber<RestArray, ']'> | ParseString<RestArray> | ParseBoolean<RestArray> | ParseArray<RestArray> | ParseObj<RestArray> extends infer FirstElem extends [any, string]
            ? RestArray extends `${FirstElem[1]}${infer RestElements}`
                ? ParseArray<
                    `[${RemoveHeadComma<RestElements>}`,
                    `${StringResult extends '[' ? '[' : `${StringResult},`}${FirstElem[1]}`,
                    [...Result, FirstElem[0]]
                >
                : never
            : never
    : never

type ParseObj<
    JSON extends string,
    StringResult extends string = '{',
    Result extends Record<string, any> = {}
> = JSON extends `{${infer ObjFields}`
    ? ObjFields extends `}${string}`
        ? [Result, `${StringResult}}`]
        : ParseStringEnum<ObjFields> extends infer ObjKey extends [any, string]
            ? ObjFields extends `${ObjKey[1]}:${infer RestFields}`
                ? ParseNull<RestFields> | ParseNumber<RestFields, ','> | ParseNumber<RestFields, '}'> | ParseString<RestFields> | ParseBoolean<RestFields> | ParseArray<RestFields> | ParseObj<RestFields> extends infer ObjValue extends [any, string]
                    ? RestFields extends `${ObjValue[1]}${infer RestObject}`
                        ? ParseObj<
                            `{${RemoveHeadComma<RestObject>}`,
                            `${StringResult extends '{' ? '{' : `${StringResult},`}${ObjKey[1]}:${ObjValue[1]}`,
                            Result & Record<ObjKey[0], ObjValue[0]>
                        >
                        : never 
                    : never
                : never
            : never
    : never;

type RemoveHeadComma<Str extends string> = Str extends `,${infer Result}` ? Result : Str;

type RemoveNewLines<Str extends string, Result extends string = ''> = Str extends `${infer Before}\n${infer After}`
    ? `${Result}${Before}${RemoveNewLines<After>}`
    : `${Result}${Str}`

type ExpandRecursively<T> = T extends (...args: infer A) => infer R
    ? (...args: ExpandRecursively<A>) => ExpandRecursively<R>
        : T extends object
            ? T extends infer O
                ? { [K in keyof O]: ExpandRecursively<O[K]> }
                : never
    : T

Solution by AlexeyDuybo #34163

type Pure<T> = {
  [P in keyof T]: T[P] extends object ? Pure<T[P]> : T[P];
};

type SetProperty<T, K extends PropertyKey, V> = {
  [P in keyof T | K]: P extends K ? V : P extends keyof T ? T[P] : never;
};

type StaticToken = "{" | "}" | ":" | "," | "[" | "]";
type Token = StaticToken | "true" | "false" | "null" | { string: string };

type StaticValues = { true: true; false: false; null: null };

type SkipWhitespace<S extends string> = S extends `${
  | " "
  | "\n"
  | "\r"
  | "\t"}${infer Rest}`
  ? SkipWhitespace<Rest>
  : S;
type Escapes = {
  n: "\n";
  r: "\r";
  b: "\b";
  f: "\f";
};
type Unescape<S extends string> =
  S extends `${infer Prefix}\\${infer E extends keyof Escapes}${infer Suffix}`
    ? `${Prefix}${Escapes[E]}${Unescape<Suffix>}`
    : S;

type CollectString<
  S extends string,
  T extends Token[]
> = S extends `${infer Literal}"${infer Rest}`
  ? Tokenize<Rest, [...T, { string: Unescape<Literal> }]>
  : never;

type ParseObject<R, T extends Token[]> = T extends [
  "}",
  ...infer AfterDirectCloseRest
]
  ? [R, AfterDirectCloseRest]
  : T extends [
      infer P extends { string: string },
      ":",
      ...infer ObjectValueRest extends Token[]
    ]
  ? ParseResult<{}, ObjectValueRest> extends [
      infer V,
      infer AfterValueRest extends [] | Token[]
    ]
    ? AfterValueRest extends [",", ...infer ObjectRest extends Token[]]
      ? ParseObject<SetProperty<R, P["string"], V>, ObjectRest>
      : AfterValueRest extends ["}", ...infer AfterObjectRest]
      ? [SetProperty<R, P["string"], V>, AfterObjectRest]
      : never
    : never
  : never;

type ParseArray<R extends any[], T extends Token[]> = T extends [
  "]",
  ...infer AfterDirectCloseRest
]
  ? [R, AfterDirectCloseRest]
  : ParseResult<any, T> extends [infer V, infer ArrayRest]
  ? ArrayRest extends [",", ...infer ArrayRestCntd extends Token[]]
    ? ParseArray<[...R, V], ArrayRestCntd>
    : ArrayRest extends ["]", ...infer AfterArrayRest]
    ? [[...R, V], AfterArrayRest]
    : never
  : never;

type ParseResult<T, K extends Token[]> = K extends []
  ? [T, []]
  : K extends [
      infer CurrentToken extends Token,
      ...infer RestTokens extends Token[]
    ]
  ? CurrentToken extends keyof StaticValues
    ? [StaticValues[CurrentToken], RestTokens]
    : CurrentToken extends { string: infer S }
    ? [S, RestTokens]
    : CurrentToken extends "["
    ? ParseArray<[], RestTokens>
    : CurrentToken extends "{"
    ? ParseObject<{}, RestTokens>
    : never
  : never;

type Tokenize<
  T extends string,
  S extends Token[] = []
> = SkipWhitespace<T> extends ""
  ? S
  : SkipWhitespace<T> extends `true${infer Rest}`
  ? Tokenize<Rest, [...S, "true"]>
  : SkipWhitespace<T> extends `false${infer Rest}`
  ? Tokenize<Rest, [...S, "false"]>
  : SkipWhitespace<T> extends `null${infer Rest}`
  ? Tokenize<Rest, [...S, "null"]>
  : SkipWhitespace<T> extends `${infer Static extends StaticToken}${infer Rest}`
  ? Tokenize<Rest, [...S, Static]>
  : SkipWhitespace<T> extends `"${infer Rest}`
  ? CollectString<Rest, S>
  : never;

type ParseLiteral<T extends Token[]> = ParseResult<{}, T>;

type Parse<T extends string> = Pure<ParseLiteral<Tokenize<T>>[0]>;

Solution by dexmo007 #28742

// ===============================================
//  test file and complex test data parse example, see: https://github.com/Max10240/tiny-json5-parser/blob/main/packages/ts-type-json-parser/src/index.ts
// ===============================================

type TTokenType =
  'L_BRACE'
  | 'R_BRACE'
  | 'L_S_BRACE'
  | 'R_S_BRACE'
  | 'COMMA'
  | 'COLON'
  | 'TRUE'
  | 'FALSE'
  | 'NULL'
  | 'NUMBER'
  | 'STRING'
  | 'EOF';

type SimpleTokenMap = {
  '{': 'L_BRACE';
  '}': 'R_BRACE';
  '[': 'L_S_BRACE';
  ']': 'R_S_BRACE';
  ',': 'COMMA';
  ':': 'COLON';
  'true': 'TRUE';
  'false': 'FALSE';
  'null': 'NULL';
};

interface IToken {
  type: TTokenType;
  value: string;
}

type IsExtends<T, U> = T extends U ? true : false;
type Cast<T, To> = T extends To ? T : never;
type ReMap<T> = {[P in keyof T]: ReMap<T[P]>};
type StringToNumber<T extends string> = T extends `${infer N extends number}` ? N : never;
type SliceFrom<T extends unknown[], Start extends number = 0, Filtered extends unknown[] = []> = Filtered['length'] extends Start
  ? T
  : T extends [infer H, ...infer Rest]
    ? SliceFrom<Rest, Start, [...Filtered, H]>
    : [];
type SliceStrFrom<T extends string, Start extends number = 0, Counter extends 0[] = []> = Counter['length'] extends Start
  ? T
  : T extends `${infer H}${infer Rest}`
    ? SliceStrFrom<Rest, Start, [...Counter, 0]>
    : '';
type StartsWith<Start extends string, S extends string> = S extends `${Start}${infer R}` ? true : boolean;

type JsonNumber<T extends string> = number & { value: T };

type Match<Pattern extends string, S extends string> = S extends `${infer P extends Pattern}${infer R}` ? [true, P, R] : [false, '', S];
type MatchSequence<Pattern extends string, S extends string> = S extends `${Pattern}${infer R}` ? [true, Pattern, R] : [false, '', S];

type MatchPureNumber<S extends string, Result extends string = ''> = S extends `${infer H extends number}${infer R}`
  ? MatchPureNumber<R, `${Result}${H}`>
  : Result extends ''
    ? [false, '', `${Result}${S}`]
    : [true, Result, S];

type MatchFloatNum<S extends string, Result extends string = ''> = MatchPureNumber<S> extends infer IntR extends [boolean, string, string]
  ? IntR[0] extends true
    ? Match<'.', IntR[2]>[0] extends true
      ? MatchPureNumber<SliceStrFrom<IntR[2], 1>> extends infer DecimalR extends [boolean, string, string]
        ? DecimalR[0] extends true
          ? [true, `${IntR[1]}.${DecimalR[1]}`, DecimalR[2]]
          : [false, '', S, `expected number behind '.' at ${SliceStrFrom<IntR[2], 1>}`]
        : never
      : [true, IntR[1], IntR[2]]
    : [false, '', S, `expected number at ${S}`]
  : never;

type MatchNumber<S extends string> = MatchFloatNum<S> extends infer FloatR extends [boolean, string, string, string?]
  ? FloatR[0] extends true
    ? Match<'e', FloatR[2]>[0] extends true
      ? Match<'+' | '-', SliceStrFrom<FloatR[2], 1>> extends infer ExpoSignR extends [boolean, string, string]
        ? MatchPureNumber<ExpoSignR[2]> extends infer ExpoR extends [boolean, string, string]
          ? ExpoR[0] extends true
            ? [true, `${FloatR[1]}e${ExpoSignR[1]}${ExpoR[1]}`, ExpoR[2]]
            : [false, '', S, `expected expo behind 'e' at ${ExpoSignR[2]}`]
          : never
        : never
      : [true, FloatR[1], FloatR[2]]
    : [false, '', S, `expected number at ${S}`]
  : never;


type EscapeCharMap = {
  '"': '"';
  '\\': '\\';
  '/': '/';
  'b': '\b';
  'f': '\f';
  'n': '\n';
  'r': '\r';
  't': '\t';
};

type MatchStringContent<S extends string, Result extends string = ''> = Match<'"', S>[0] extends true
  ? [true, Result, S]
  : Match<'\r' | '\n', S>[0] extends true
    ? [false, '', `${Result}${S}`, `unexpected token after ${Result}`]
    : Match<'\\', S>[0] extends true
      ? Match<string, Match<'\\', S>[2]> extends infer EscapeCharR extends [boolean, string, string]
        ? [EscapeCharR[0], IsExtends<EscapeCharR[1], keyof EscapeCharMap>][number] extends true
          ? MatchStringContent<EscapeCharR[2], `${Result}${EscapeCharMap[EscapeCharR[1] & keyof EscapeCharMap]}`>
          : [false, '', `${Result}${S}`, `unexpected escape char after ${Result}`]
        : never
      : Match<string, S>[0] extends true
        ? MatchStringContent<Match<string, S>[2], `${Result}${Match<string, S>[1]}`>
        : [false, '', `${Result}${S}`, `unexpected EOF after ${Result}`];

type MatchString<S extends string> = Match<'"', S>[0] extends true
  ? MatchStringContent<Match<'"', S>[2]> extends infer MatchStringContentResult extends [boolean, string, string, string?]
    ? MatchStringContentResult[0] extends true
      ? [true, MatchStringContentResult[1], Match<string, MatchStringContentResult[2]>[2]]
      : [false, '', S, MatchStringContentResult[3]]
    : 1
  : [false, '', S, `expect '"' at start`];

type MatchSimpleToken<S extends string, MatchOptions extends keyof SimpleTokenMap = keyof SimpleTokenMap> = (MatchOptions extends unknown
  ? MatchSequence<MatchOptions, S> extends infer MatchResult extends [boolean, string, string, string?]
    ? MatchResult[0] extends true
      ? [true, { type: SimpleTokenMap[MatchOptions], value: MatchOptions }, MatchResult[2]]
      : never
    : never
  : never) extends infer R
  ? [R] extends [never]
    ? [false, never, S]
    : R
  : never;

type Lexer<S extends string, R extends IToken[] = []> = S extends ''
  ? [true, R]
  : Match<' ' | '\t' | '\r' | '\n', S> extends infer MatchWSResult extends [boolean, string, string]
    ? MatchWSResult[0] extends true
      ? Lexer<MatchWSResult[2], R>
      : MatchSimpleToken<S> extends infer MatchSTResult extends [boolean, IToken, string]
        ? MatchSTResult[0] extends true
          ? Lexer<MatchSTResult[2], [...R, MatchSTResult[1]]>
          : MatchNumber<S> extends infer MatchNumResult extends [boolean, string, string, string?]
            ? MatchNumResult[0] extends true
              ? Lexer<MatchNumResult[2], [...R, { type: 'NUMBER', value: MatchNumResult[1] }]>
              : MatchString<S> extends infer MatchStrResult extends [boolean, string, string, string?]
                ? MatchStrResult[0] extends true
                  ? Lexer<MatchStrResult[2], [...R, { type: 'STRING', value: MatchStrResult[1] }]>
                  : [false, R, `unexpected token at '${S}'`]
                : never
            : never
        : never
    : never;

type MatchToken<Types extends TTokenType[], T extends IToken[]> = Types extends [infer H extends TTokenType, ...infer Rest extends TTokenType[]]
  ? T extends [infer HT extends IToken, ...infer RestT extends IToken[]]
    ? HT['type'] extends H
      ? MatchToken<Rest, RestT>
      : [false]
    : [false]
  : [true, T];

type MatchAnyToken<Type extends TTokenType, T extends IToken[]> = (Type extends unknown
  ? MatchToken<[Type], T>[0] extends true
    ? [true, T[0], MatchToken<[Type], T>[1]]
    : never
  : 2) extends infer Result
  ? [Result] extends [never]
    ? [false, T]
    : Result
  : never;

type MatchKVPair<T extends IToken[], Result extends Record<string, unknown> = {}> =
  MatchToken<['STRING', 'COLON'], T> extends infer KeyColonResult extends ([true, IToken[]] | [false])
    ? KeyColonResult[0] extends true
      ? Parser<SliceFrom<T, 2>> extends infer ValueResult extends [boolean, unknown, IToken[], string?]
        ? ValueResult[0] extends true
          ? MatchToken<['COMMA'], ValueResult[2]>[0] extends true
            ? MatchKVPair<SliceFrom<ValueResult[2], 1>, Result & { [P in T[0]['value']]: ValueResult[1] }>
            : [true, Result & { [P in T[0]['value']]: ValueResult[1] }, ValueResult[2]]
          : [false, Result, [], `error while parsing value of KV pair, current: ${T[0]['value']}:`]
        : never
      : [false, Result, [], `error while parsing key of KV pair, current: ${T[0]['value']}:`]
    : never;

type ParseObject<T extends IToken[]> = MatchToken<['L_BRACE'], T>[0] extends true
  ? MatchToken<['L_BRACE', 'R_BRACE'], T>[0] extends true
    ? [true, {}, SliceFrom<T, 2>]
    : MatchKVPair<SliceFrom<T, 1>> extends infer KVPairResult extends [boolean, unknown, IToken[], string?]
      ? KVPairResult[0] extends true
        ? MatchToken<['R_BRACE'], KVPairResult[2]>[0] extends true
          ? [true, KVPairResult[1], SliceFrom<KVPairResult[2], 1>]
          : [false, `expected '}' at ${KVPairResult[2][0]['value']}`]
        : [false, T]
      : never
  : [false, `expected '{' at ${T[0]['value']}`];

type ParseArrayElems<T extends IToken[], Result extends unknown[] = []> =
  Parser<T> extends infer ElemR extends [boolean, unknown, IToken[], string?]
    ? ElemR[0] extends true
      ? MatchToken<['COMMA'], ElemR[2]>[0] extends true
        ? ParseArrayElems<SliceFrom<ElemR[2], 1>, [...Result, ElemR[1]]>
        : [true, [...Result, ElemR[1]], ElemR[2]]
      : [false, Result, [], `error while parsing array elem, current: ${T[0]['value']}:`]
    : never;

type ParseArray<T extends IToken[]> = MatchToken<['L_S_BRACE'], T>[0] extends true
  ? MatchToken<['L_S_BRACE', 'R_S_BRACE'], T>[0] extends true
    ? [true, [], SliceFrom<T, 2>]
    : ParseArrayElems<SliceFrom<T, 1>> extends infer ArrElemsR extends [boolean, unknown[], IToken[], string?]
      ? ArrElemsR[0] extends true
        ? MatchToken<['R_S_BRACE'], ArrElemsR[2]>[0] extends true
          ? [true, ArrElemsR[1], SliceFrom<ArrElemsR[2], 1>]
          : [false, never, T, `expected ']' behind elems`]
        : [false, never, T, ArrElemsR[3]]
      : never
  : [false, never, T, `expected '[' at start`];

type SimpleLiteralValueMap = {
  TRUE: true;
  FALSE: false;
  NULL: null;
};

type GetSimpleLiteralValue<T extends IToken> = T['type'] extends 'STRING'
  ? T['value']
  : T['type'] extends 'NUMBER'
    ? number extends StringToNumber<T['value']>
      ? JsonNumber<T['value']>
      : StringToNumber<T['value']>
    : SimpleLiteralValueMap[Cast<T['type'], keyof SimpleLiteralValueMap>];

type Parser<T extends IToken[]> =
  MatchAnyToken<'STRING' | 'NUMBER' | 'NULL' | 'TRUE' | 'FALSE', T> extends infer SimpleLiteralResult extends unknown[]
  ? SimpleLiteralResult[0] extends true
    ? [true, GetSimpleLiteralValue<T[0]>, SimpleLiteralResult[2]]
    : ParseObject<T> extends infer ObjectResult extends unknown[]
      ? ObjectResult[0] extends true
        ? [true, ObjectResult[1], ObjectResult[2]]
        : ParseArray<T> extends infer ArrayResult extends unknown[]
          ? ArrayResult[0] extends true
            ? [true, ArrayResult[1], ArrayResult[2]]
            : [false, never, T, `unexpected token: ${T[0]['value']}`]
          : never
      : never
  : never;

  type Parse<T extends string> = ReMap<Parser<Lexer<T>[1]>[1]>;

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

type cases = [
  Expect<Equal<(
    Parse<`
      {
        "a": "b", 
        "b": false, 
        "c": [true, false, "hello", {
          "a": "b", 
          "b": false
        }], 
        "nil": null
      }
    `>
  ), (
    {
      nil: null
      c: [true, false, 'hello', {
        a: 'b'
        b: false
      }]
      b: false
      a: 'b'
    }

  )>>,
  Expect<Equal<Parse<'{}'>, {}>>,

  Expect<Equal<Parse<'[]'>, []>>,


  Expect<Equal<Parse<'[1]'>, [1]>>,

  Expect<Equal<Parse<'true'>, true>>,

  Expect<Equal<
  Parse<'["Hello", true, false, null]'>,
  ['Hello', true, false, null]
  >>,

  Expect<Equal<
  (
    Parse<`
      {
        "hello\\r\\n\\b\\f": "world"
      }`>
  ), (
    {
      'hello\r\n\b\f': 'world'
    }
  )
  >>,

  Expect<Equal<Parse<'{ 1: "world" }'>, never>>,

  Expect<Equal<Parse<`{ "hello
  
  world": 123 }`>, never>>,
]

/* _____________ Further Steps _____________ */
/*
  > Share your solutions: https://tsch.js.org/6228/answer
  > View solutions: https://tsch.js.org/6228/solutions
  > More Challenges: https://tsch.js.org
*/

Solution by Max10240 #27235

type SpecialChar = {r: '\r', n: '\n', b: '\b', f: '\f'}

type Format<S extends string, Res extends string = ""> = 
S extends `\\${infer L extends keyof SpecialChar}${infer R}`
  ? Format<R, `${Res}${SpecialChar[L]}`>
  : S extends `${infer L}${infer R}`
    ? Format<R, `${Res}${
        L extends '}' | ']'
          ? `,${L}`
          : L extends ' ' | '\n' | '\t' ? '' : L
      }`>
    : Res

type Eval<V> = 
V extends `"${infer T}"`
  ? T 
  : V extends `${infer B extends boolean | null}`
    ? B 
    : V extends `${number}` ? never : V

type ParseObject<S extends string, Res extends object = {}> = 
S extends `${'}' | ',}'}${infer R}`
  ? [Omit<Res, never>, R extends `,${infer P}` ? P : R]
  : S extends `${infer K}:${infer R}`
    ? [Eval<K>] extends [never | boolean | null]
      ? never
      : ParseValue<R> extends [infer V, infer T extends string]
        ? ParseObject<T, Res & Record<Eval<K>, V>> 
        : never
    : never

type ParseArray<S extends string, Res extends any[] = []> = 
S extends `${']' | ',]'}${infer R}`
  ? [Res, R extends `,${infer P}` ? P : R]
  : ParseValue<S> extends [infer V, infer T extends string]
    ? ParseArray<T, [...Res, V]> 
    : never

type ParseValue<S extends string> = 
S extends `{${infer O}`
  ? ParseObject<O>
  : S extends `[${infer A}` 
    ? ParseArray<A> 
    : S extends `${infer L},${infer R}`
      ? [Eval<L>] extends [never] ? [] : [Eval<L>, R] 
      : [Eval<S>, ""]

type Parse<T extends string> = ParseValue<Format<T>>[0]

Solution by omittee #27130

Playground

// Helper =================================
type IgnoreChar = ' ' | '\n' | '\r' | '\b' | '\f'

type ExtractPattern<
  T extends string,
  _LeftChar extends string = '[',
  _RightChar extends string = ']',
  _LeftCharCounters extends 0[] = [],
  _RightCharCounters extends 0[] = [],
  _Result extends string = '',
> = T extends `${ infer F }${ infer Rest }`
  ? F extends _LeftChar
    ? ExtractPattern<Rest, _LeftChar, _RightChar, [ ..._LeftCharCounters, 0 ], _RightCharCounters, `${ _Result }${ F }`>
    : F extends _RightChar
      ? ExtractPattern<Rest, _LeftChar, _RightChar, _LeftCharCounters, [ ..._RightCharCounters, 0 ], `${ _Result }${ F }`>
      : _LeftCharCounters['length'] extends _RightCharCounters['length']
        ? _Result
        : ExtractPattern<Rest, _LeftChar, _RightChar, _LeftCharCounters, _RightCharCounters, `${ _Result }${ F }`>
  : _Result

type TrimBothEnds<T extends string, Chars extends string = IgnoreChar> =
  T extends `${ Chars }${ infer Rest }`
    ? TrimBothEnds<Rest, Chars>
    : T extends `${ infer Rest }${ Chars }`
      ? TrimBothEnds<Rest, Chars>
      : T

type ResolveSpecialChar<T extends string, _Map extends Record<'r' | 'n' | 'b' | 'f', string> = { n: '\n', r: '\r', b: '\b', f: '\f', }> =
  T extends `${ infer A }\\${ infer I extends 'r' | 'n' | 'b' | 'f' }${ infer B }`
    ? ResolveSpecialChar<`${ A }${ _Map[I] }${ B }`> : T
// Helper =================================

// ParseObject ============================
type ParseObject<T extends string, _Key extends string = '', _Result extends Record<PropertyKey, unknown> = {}> =
  T extends `${ infer F }${ infer Rest }`
    ? F extends IgnoreChar | ','
      ? ParseObject<Rest, _Key, _Result>
      : F extends '"'
        ? _Key extends ''
          ? Rest extends `${ infer Key }":${ infer Rest2 }`
            ? ParseObject<Rest2, ResolveSpecialChar<Key>, _Result> // extract key
            : never
          : Rest extends `${ infer StringValue }"${ infer Rest2 }`
            ? ParseObject<Rest2, '', _Result & Record<_Key, StringValue>> // extract string value
            : Rest
        : T extends `false${ infer Rest2 }`
          ? ParseObject<Rest2, '', _Result & Record<_Key, false>>
          : T extends `true${ infer Rest2 }`
            ? ParseObject<Rest2, '', _Result & Record<_Key, true>>
            : T extends `null${ infer Rest2 }`
              ? ParseObject<Rest2, '', _Result & Record<_Key, null>>
              : F extends '['
                ? ExtractPattern<T> extends infer I
                  ? T extends `${ I & string }${ infer Rest2 }`
                    ? ParseObject<Rest2, '', _Result & Record<_Key, Parse<I & string>>>
                    : never
                  : never
                : F extends '{'
                  ? ExtractPattern<T, '{', '}'> extends infer I
                    ? T extends `${ I & string }${ infer Rest2 }`
                      ? ParseObject<Rest2, '', _Result & Record<_Key, Parse<I & string>>>
                      : never
                    : never
                  : F extends `${ number }`
                    ? never
                    : _Result
    : { [K in keyof _Result]: _Result[K] }
// ParseObject ============================

// ParseArray =============================
type ParseArray<T extends string, _Result extends unknown[] = []> =
  T extends `${ infer F }${ infer Rest }`
    ? F extends IgnoreChar | ','
      ? ParseArray<Rest, _Result>
      : F extends '['
        ? ExtractPattern<T> extends infer I
          ? T extends `${ I & string }${ infer Rest2 }`
            ? ParseArray<Rest2, [ ..._Result, Parse<I & string> ]>
            : never
          : never
        : F extends '{'
          ? ExtractPattern<T, '{', '}'> extends infer I
            ? T extends `${ I & string }${ infer Rest2 }`
              ? ParseArray<Rest2, [ ..._Result, Parse<I & string> ]>
              : never
            : never
          : F extends '"'
            ? Rest extends `${ infer StringValue }"${ infer Rest2 }`
              ? ParseArray<Rest2, [ ..._Result, StringValue]>
              : never
            : T extends `false${ infer Rest2 }`
              ? ParseArray<Rest2, [ ..._Result, false]>
              : T extends `true${ infer Rest2 }`
                ? ParseArray<Rest2, [ ..._Result, true]>
                : T extends `null${ infer Rest2 }`
                  ? ParseArray<Rest2, [ ..._Result, null]>
                  : F extends `${ number }`
                    ? never
                    : _Result
    : _Result
// ParseArray =============================


type Parse<T extends string, _T extends string = TrimBothEnds<T>> =
  _T extends `{${ infer I }}`
    ? ParseObject<I>
    : _T extends `[${ infer I }]`
      ? ParseArray<I>
      : T extends `${ infer I extends boolean }`
        ? I
        : T

Solution by lvjiaxuan #23284

type Escape = {
  '"': '"',
  '\\': '\\',
  '/': '/',
  'b': '\b',
  'f': '\f',
  'n': '\n',
  'r': '\r',
  't': '\t'
}

type Space = ' ' | '\r' | '\t' | '\n'
type Symbols = '{' | '}' | '[' | ']' | ':' | ','
type SymbolToken<T extends Symbols> = [0, T]
type StringToken<T extends string> = [1, T]
type BooleanToken<T extends boolean> = [2, T]
type NullToken = [3, null]
type Token =
  | SymbolToken<Symbols>
  | StringToken<string>
  | BooleanToken<boolean>
  | NullToken
  | [number, unknown]

type EvalResult<Output, Tokens extends Token[]> = [Output, Tokens]
type TokenSplit<First extends Token, Rest extends Token[]> = [First, ...Rest]
type Tokenize<Input extends string, Output extends Token[] = []> =
  Input extends `${infer First}${infer Rest}`
  ? First extends Space
    ? Tokenize<Rest, Output>
  : First extends Symbols
    ? Tokenize<Rest, [...Output, SymbolToken<First>]>
  : First extends '"'
    ? [TakeString<Rest>] extends [never]
      ? never
      : [...Output, ...TakeString<Rest>]
  : Input extends `true${infer Rest}`
    ? Tokenize<Rest, [...Output, BooleanToken<true>]>
  : Input extends `false${infer Rest}`
    ? Tokenize<Rest, [...Output, BooleanToken<false>]>
  : Input extends `null${infer Rest}`
    ? Tokenize<Rest, [...Output, NullToken]>
    : never
  : Input extends ''
    ? Output
    : never

type TakeString<Input extends string, Output extends string = ''> =
  Input extends `\\${infer Rest}`
  ? Rest extends `${infer First extends keyof Escape}${infer Rest}`
    ? TakeString<Rest, `${Output}${Escape[First]}`>
    : never
  : Input extends `${infer First}${infer Rest}`
    ? First extends '\n'
      ? never
      : First extends '"'
        ? [StringToken<Output>, ...Tokenize<Rest>]
        : TakeString<Rest, `${Output}${First}`>
    : never


type EvalPrimitive<Input extends Token[]> =
  Input extends TokenSplit<[1 | 2 | 3, infer Output], infer Rest>
  ? EvalResult<Output, Rest>
  : never

type EvalArrayElements<Input extends Token[], Output extends unknown[] = []> =
  Input extends TokenSplit<infer First, infer Rest>
  ? First extends SymbolToken<']'>
    ? EvalResult<Output, Rest>
    : Eval<Input> extends EvalResult<infer Element, infer Rest>
      ? Rest extends TokenSplit<infer First, infer Rest>
        ? First extends SymbolToken<']'>
          ? EvalResult<[...Output, Element], Rest>
          : First extends SymbolToken<','>
            ? EvalArrayElements<Rest, [...Output, Element]>
            : never
        : never
      : never
  : never

type EvalArray<Input extends Token[]> =
  Input extends TokenSplit<SymbolToken<'['>, infer Rest>
  ? EvalArrayElements<Rest>
  : never

type EvalObjectEntries<Input extends Token[], Output extends Record<string, unknown> = {}> =
  Input extends TokenSplit<infer First, infer Rest>
  ? First extends SymbolToken<'}'>
    ? EvalResult<Output, Rest>
    : Eval<Input> extends EvalResult<infer Key extends string, infer Rest>
      ? Rest extends TokenSplit<SymbolToken<':'>, infer Rest>
        ? Eval<Rest> extends EvalResult<infer Value, infer Rest>
          ? Output & Record<Key, Value> extends infer Output extends Record<string, unknown>
            ? Rest extends TokenSplit<infer First, infer Rest>
              ? First extends SymbolToken<'}'>
                ? EvalResult<Output, Rest>
                : First extends SymbolToken<','>
                  ? EvalObjectEntries<Rest, Output>
                  : never
              : never
            : never
          : never
        : never
      : never
  : never

type EvalObject<Input extends Token[]> =
  Input extends TokenSplit<SymbolToken<'{'>, infer Rest>
  ? EvalObjectEntries<Rest>
  : never

type Eval<Input extends Token[]> =
  | EvalPrimitive<Input>
  | EvalArray<Input>
  | EvalObject<Input>

type Merge<T> =
  T extends object
  ? { [P in keyof T]: Merge<T[P]> }
  : T

export type Parse<T extends string> =
  string extends T
  ? never
  : [Tokenize<T>] extends [never]
    ? never
    : Eval<Tokenize<T>> extends EvalResult<infer Output, []>
      ? Merge<Output>
      : never

Solution by hydrati #22390

My another answer

type Escape = {
  '"': '"',
  '\\': '\\',
  '/': '/',
  'b': '\b',
  'f': '\f',
  'n': '\n',
  'r': '\r',
  't': '\t',
}
type Space = ' ' | '\t' | '\0' | '\n'
type Symbol = '{' | '}' | '[' | ']' | ',' | ':'
type SymbolToken<T extends Symbol> = ['symbol', T]
type StringToken<T extends string> = ['string', T]
type BooleanToken<T extends boolean> = ['boolean', T]
type NullToken = ['null']
type Token =
  | SymbolToken<Symbol>
  | StringToken<string>
  | BooleanToken<boolean>
  | NullToken
type EvalResult<T, R extends Token[]> = [T, R]
type TokenFirst<T extends Token, R extends Token[]> = [T, ...R]
type SpreadObject<T> = T extends object ? {
  [P in keyof T] : SpreadObject<T[P]>
} : T
type MergeRecord<T extends Record<string, unknown>, K extends string, V> =
  SpreadObject<T & Record<K, V>>
type Tokenize<T extends string, O extends Token[] = []> =
  T extends `${infer P}${infer R}`
  ? P extends Space
    ? Tokenize<R, O>
  : P extends Symbol
    ? Tokenize<R, [...O, SymbolToken<P>]>
  : P extends '"'
    ? [...O, ...TakeString<R>]
  : T extends `true${infer R}`
    ? Tokenize<R, [...O, BooleanToken<true>]>
  : T extends `false${infer R}`
    ? Tokenize<R, [...O, BooleanToken<false>]>
  : T extends `null${infer R}`
    ? Tokenize<R, [...O, NullToken]>
    : never
  : O
type TakeString<T extends string, O extends string = ''> =
  T extends `\\${infer R}`
  ? R extends `${infer P extends keyof Escape}${infer R}`
    ? TakeString<R, `${O}${Escape[P]}`>
    : never
  : T extends `${infer P}${infer R}`
    ? P extends '\n'
      ? never
    : P extends '"'
      ? [StringToken<O>, ...Tokenize<R>]
      : TakeString<R, `${O}${P}`>
    : never
type EvalPrimitive<T extends Token[]> =
  T extends TokenFirst<NullToken, infer R>
  ? EvalResult<null, R>
  : T extends TokenFirst<BooleanToken<infer S>, infer R>
  ? EvalResult<S, R>
  : T extends TokenFirst<StringToken<infer S>, infer R>
  ? EvalResult<S, R>
  : never
type EvalArrayElements<T extends Token[], O extends unknown[] = []> =
  T extends TokenFirst<SymbolToken<']'>, infer R>
  ? EvalResult<O, R>
  : Eval<T> extends EvalResult<infer V, infer R>
    ? R extends TokenFirst<infer U, infer R>
      ? U extends SymbolToken<','>
        ? EvalArrayElements<R, [...O, V]>
        : U extends SymbolToken<']'>
          ? EvalResult<[...O, V], R>
          : never
      : never
    : never
type EvalArray<T extends Token[]> =
  T extends TokenFirst<SymbolToken<'['>, infer R>
  ? EvalArrayElements<R>
  : never
type EvalRecordEntries<T extends Token[], O extends Record<string, unknown> = {}> =
  T extends TokenFirst<SymbolToken<'}'>, infer R>
  ? EvalResult<O, R>
  : Eval<T> extends EvalResult<infer K extends string, infer R>
    ? R extends TokenFirst<SymbolToken<':'>, infer R>
      ? Eval<R> extends EvalResult<infer V, infer R>
        ? R extends TokenFirst<infer P, infer R>
          ? P extends SymbolToken<'}'>
            ? EvalResult<MergeRecord<O, K, V>, R>
            : P extends SymbolToken<','>
              ? EvalRecordEntries<R, MergeRecord<O, K, V>>
              : never
          : never
        : never
      : never
    : never
type EvalRecord<T extends Token[]> =
  T extends TokenFirst<SymbolToken<'{'>, infer R>
  ? EvalRecordEntries<R>
  : never
type Eval<T extends Token[]> =
  | EvalPrimitive<T>
  | EvalArray<T>
  | EvalRecord<T>
export type Parse<T extends string> =
  string extends T
  ? never
  : Eval<Tokenize<T>> extends EvalResult<infer P, []>
    ? unknown extends P ? Eval<Tokenize<T>> : P
    : never

Solution by hydrati #22162

// your answers
type P<T> = T extends string ? T : ''
type Nch = { n: '\n'; r: '\r'; f: '\f'; b: '\b'; }
type Igc = ' ' | Nch[keyof Nch]

type Counter = [0] | [Counter];
type Found<N, D extends Counter = [0], B extends string = ''> = N extends `${infer S}${infer L}` ? (
  S | D[0] extends ',' | 0 ? { B: Parse<B>; L: Parse<L, 0>; } :
  Found<L, S extends '{' | '[' ? [D] : S extends '}' | ']' ? D[0] extends Counter ? D[0] : D : D, `${B}${S}`>
) : { B: Parse<B>; L: ''; }

type PArr<N, R extends any[] = [], S extends { B: any; L: string; } = Found<N>> =
  N extends '' ? R : [S['B']] extends [never] ? never : PArr<S['L'], [...R, S['B']]>
type PObj<N, R = {}> = N extends `${infer K}:${infer D}` ? PStr<K> extends infer K ? unknown extends K ? never :
  Found<D> extends { B: infer B; L: infer L extends string; } ? PObj<L, unknown extends B ? R : R & Record<P<K>, B>> : {}
  : never : { [I in keyof R]: R[I]; }
type PStr<N, R = ''> = N extends `"${infer N}"` ? N extends `${any}${Igc}${any}` ? unknown : (
  N extends `${infer H}\\${infer K extends keyof Nch}${infer L}` ? PStr<`"${L}"`, `${P<R>}${H}${Nch[K]}`> : `${P<R>}${N}`
) : unknown
type POri<N> =
  N extends `${infer K extends boolean | null}` ? K :
  N extends `"${any}` ? PStr<N> :
  N extends `[${infer K}]` ? Parse<K, 2> :
  N extends `{${infer K}}` ? Parse<K, 3> :
  never
type Parse<T, O = 1> = T extends `${Igc}${infer T}` | `${Igc}${infer T}${Igc}` | `${infer T}${Igc}` ? Parse<T, O> :
  O extends 1 ? POri<T> : O extends 2 ? PArr<T> : O extends 3 ? PObj<T> : O extends 4 ? PStr<T> : T

Solution by E0SelmY4V #21755

This is a json parser implemented according to context-free grammar

json           → object | array ;
object         → "{" (member)? "}" ;
members        → pair ( "," pair )* ;
pair           → string ":" value ;
array          → "[" value ( "," value )* "]" | "[]" ;
value          → string | object | array | "true" | "false" | "null" ;
string         → """ ( character | escape )* """ ;
character      → any-Unicode-character-except-"-or-\-or-control-character ;
escape         → "\" ( "" | "\" | "/" | "b" | "f" | "n" | "r" | "t" ) ;

// your answers
type Pure<T> = {
  [P in keyof T]: T[P] extends object ? Pure<T[P]> : T[P];
};

type SetProperty<T, K extends PropertyKey, V> = {
  [P in keyof T | K]: P extends K ? V : P extends keyof T ? T[P] : never;
};

type Token = '{' | '}' | '[' | ']' | ':' | ',' | `"${string}"` | Primitive;

type Primitive = null | true | false;

type NumberType =
  | `0`
  | `1`
  | `2`
  | `3`
  | `4`
  | `5`
  | `6`
  | `7`
  | `8`
  | `9`
  | `-`;



type Parse<T extends string> = Pure<ParseLiteral<Tokenize<T>>[0]>;

type Tokenize<
  S,
  T extends Token[] = []
> = S extends `${infer First}${infer Rest}`
  ? First extends '{' | '}' | '[' | ']' | ':' | ','
    ? Tokenize<Rest, [...T, First]>
    : First extends `"`
    ? ParseStringResult<Rest> extends [
        infer Rest,
        infer Token extends `"${string}"`
      ]
      ? Tokenize<Rest, [...T, Token]>
      : never
    : First extends `t` | `f` | `n`
    ? ParsePrimitiveResult<S> extends [
        infer Rest,
        infer Token extends `"${string}"` | null | true | false
      ]
      ? Tokenize<Rest, [...T, Token]>
      : never
    : First extends NumberType
    ? never
    : First extends ` ` | `\t` | `\n`
    ? Tokenize<Rest, T>
    : never
  : T;


type ParseLiteral<T extends Token[]> = T extends [
  `"${string}"` | null | true | false
]
  ? [ParseLiteralResult<T[0]>]
  : ParseResult<T>;

type ParseResult<T extends Token[]> = T extends [
  infer FirstToken,
  ...infer RestTokens extends Token[]
]
  ? FirstToken extends '{'
    ? ParseObjectResult<RestTokens>
    : FirstToken extends '['
    ? ParseArrayResult<RestTokens>
    : never
  : never;

type ParseArrayResult<
  T extends Token[],
  Result extends unknown[] = []
> = T extends [infer First, ...infer RestToken extends Token[]]
  ? First extends ']'
    ? [Result, RestToken]
    : First extends ','
    ? ParseArrayResult<RestToken, Result>
    : Value<T> extends [infer value, infer vRestToken extends Token[]]
    ? ParseArrayResult<vRestToken, [...Result, value]>
    : never
  : never;

type ParseObjectResult<T extends Token[], Result = {}> = T extends [
  infer First,
  ...infer Rest extends Token[]
]
  ? First extends '}'
    ? [Result, Rest]
    : First extends ','
    ? ParseObjectResult<Rest, Result>
    : First extends `\"${infer lexeme}\"`
    ? Pair<Rest, Result, lexeme> extends [
        infer PResult,
        infer RestToken extends Token[]
      ]
      ? ParseObjectResult<RestToken, PResult>
      : never
    : never
  : never;

type Pair<
  Rest extends Token[],
  Result = {},
  Key extends string = ''
> = Rest extends [infer First, ...infer RestToken extends Token[]]
  ? First extends ':'
    ? Value<RestToken> extends [infer value, infer RestToken]
      ? [SetProperty<Result, Key, value>, RestToken]
      : never
    : never
  : [Result, Rest];

type Value<Rest extends Token[]> = Rest extends [
  infer First,
  ...infer RestToken extends Token[]
]
  ? First extends `\"${infer value}\"`
    ? [value, RestToken]
    : First extends '{'
    ? ParseObjectResult<RestToken> extends [infer VResult, infer VRest]
      ? [VResult, VRest]
      : never
    : First extends '['
    ? ParseArrayResult<RestToken> extends [infer VResult, infer VRest]
      ? [VResult, VRest]
      : never
    : First extends Primitive
    ? [First, RestToken]
    : never
  : never;

type ParseLiteralResult<T extends `"${string}"` | null | true | false> =
  T extends `"${infer StringContent}"` ? UnescapeString<StringContent> : T;

type UnescapeString<S extends string> =
  S extends `${infer First}${infer Second}${infer Rest}`
    ? `${First}${Second}` extends `\\n`
      ? `\n${UnescapeString<Rest>}`
      : `${First}${Second}` extends `\\r`
      ? `\r${UnescapeString<Rest>}`
      : `${First}${Second}` extends `\\f`
      ? `\f${UnescapeString<Rest>}`
      : `${First}${Second}` extends `\\b`
      ? `\b${UnescapeString<Rest>}`
      : `${First}${Second}` extends `\\t`
      ? `\t${UnescapeString<Rest>}`
      : `${First}${Second}${UnescapeString<Rest>}`
    : S;

type EscapeCharactor<S extends string> = S extends `n`
  ? `\n`
  : S extends `r`
  ? `\r`
  : S extends `f`
  ? `\f`
  : S extends `b`
  ? `\b`
  : S extends `t`
  ? `\t`
  : S;

type ParseStringResult<
  S extends string,
  Result extends string = ``
> = S extends `\\${infer First}${infer Rest}`
  ? ParseStringResult<Rest, `${Result}${EscapeCharactor<First>}`>
  : S extends `"${infer Rest}`
  ? [Rest, `"${Result}"`]
  : S extends `\n${string}`
  ? never
  : S extends `${infer First}${infer Rest}`
  ? ParseStringResult<Rest, `${Result}${First}`>
  : never;

type ParsePrimitiveResult<S extends string> = S extends `true${infer Rest}`
  ? [Rest, true]
  : S extends `false${infer Rest}`
  ? [Rest, false]
  : S extends `null${infer Rest}`
  ? [Rest, null]
  : never;

Solution by flyFatSeal #21318

// your answers
type Push<Arr extends readonly any[], Item > = [...Arr,Item] ;
type Pop<Arr extends readonly any[] > = Arr extends readonly [...infer R extends readonly any[],any] ? R : never ;
type Last<Arr extends readonly any[] > = Arr extends readonly [...readonly any[], infer R] ? R : never  ;
type StringToNumber<Str> = Str extends `${infer R extends number}` ? R : Str ;
type TrimLeft<Str extends string> = Str extends `${' '}${infer U extends string}` ? TrimLeft<U> : Str ;
type TrimRight<Str extends string> = Str extends `${infer U extends string}${' '}` ? TrimRight<U> : Str ;
type Trim<Str extends string> = TrimLeft<TrimRight<Str>>  ;

type ArrayKeys<
    A extends readonly any[]> =
    A extends [infer R,...infer U extends readonly any[]]
    ? U['length']|ArrayKeys<U>
    : never ;

type NestedValues<
    T extends object,
    Key = T extends readonly any[] ? ArrayKeys<T> : keyof T,
    Value = T[Key&keyof T]> =
    Value extends Value
    ? Value extends [never]
    ? [never]
    : Value extends object ? NestedValues<Value> : Value
    : never ;

type NestedKeysExceptArrayKeys<
    T extends object,
    Key = T extends readonly any[] ? ArrayKeys<T> : keyof T> =
    [Key] extends [never]
    ? never 
    :Key extends Key
    ? T[Key&keyof T] extends object 
    ? (T extends readonly any[] ? never : Key)|NestedKeysExceptArrayKeys<T[Key&keyof T]> 
    : (T extends readonly any[] ? never : Key)
    : never ;

type HasNumberKeysOrValues<
    T extends object,
    KeyOrValue = NestedValues<T>|NestedKeysExceptArrayKeys<T> > =
    KeyOrValue extends KeyOrValue
    ? KeyOrValue extends string
    ? `${KeyOrValue}` extends `${number}`
    ? true
    : false
    : KeyOrValue extends number
    ? true
    : false 
    : never ;

type NeverIfHasNumberKeysOrValues<
    Input> =
    Input extends string
    ? Input
    : Input extends object
    ? HasNumberKeysOrValues<Input> extends false
    ? Input
    : never
    : Input ;        

type UnicodeSymbols =
{
  'r':'\r',
  'b':'\b',
  'f':'\f',
  'n':'\n'
  't':'\t',
}   ;

type ParseUnicode<
    Str extends string> =
    Str extends `${infer R extends string}\\${infer S extends keyof UnicodeSymbols}${infer U extends string}`
    ? `${ParseUnicode<R>}${UnicodeSymbols[S]}${ParseUnicode<U>}`
    : `${Str}`  ;

type ExcludeUnicodeSymbols<
    Str extends string,
    Result extends string = '' > =
    Str extends `${infer R extends string}${infer U extends string}`
    ? R extends NestedValues<UnicodeSymbols>
    ? ExcludeUnicodeSymbols<U,Result>
    : ExcludeUnicodeSymbols<U,`${Result}${R}`>
    : Result  ;    

type Brackets = '{'|'}'|'['|']' ;

type BracketRelations =
{
  '}':'{',
  ']':'[',
}  ;

type SimpleTypes =
{
  'true':true,
  'false':false,
  'undefined':undefined,
  'null':null,
} ;

type HasBrackets<
    Str extends string> =
    Str extends `${infer R}${infer U}`
    ? R extends Brackets
    ? true
    : HasBrackets<U>
    : false ;

type AreBracketsOk<
    Str extends string,
    BracketsStack extends readonly Brackets[]=[],
    Steps extends readonly any[]=[] > =
    Str extends `${infer R extends string}${infer U extends string}`
    ? R extends '{'|'['
    ? AreBracketsOk<U,Push<BracketsStack,R>,Push<Steps,any>>    
    : R extends '}'|']'
    ? BracketRelations[R] extends Last<BracketsStack>
    ? AreBracketsOk<U,Pop<BracketsStack>,Push<Steps,any>>
    : false
    : AreBracketsOk<U,BracketsStack,Push<Steps,any>>    
    : BracketsStack extends [] 
    ? true
    : false ;

type Split<
    Str extends string,
    Separator extends string,
    Skip extends boolean = false,
    ArrMembers extends any[]=[],
    Member extends string='' > =
    Str extends ''
    ? [false,[never]]
    : Str extends `${infer R extends string}${Separator}${infer U extends string}`
    ? R extends ''
    ? [false,[never]]
    : AreBracketsOk<`${Member}${R}`> extends true
    ? Split<U,Separator,Skip,Push<ArrMembers,RoughParse<`${Member}${R}`,Skip>[1]>>    
    : Split<U,Separator,Skip,ArrMembers,`${Member}${R}${Separator}`>
    : AreBracketsOk<`${Member}${Str}`> extends true
    ? [true,Push<ArrMembers,RoughParse<`${Member}${Str}`,Skip>[1]>]
    : [false,[never]] ;    

type GetEntries<
    Parts extends readonly string[],
    Entries extends readonly [string,string][]=[] > =
    Parts extends readonly [infer R extends string,...infer U extends readonly string[]]
    ? Split<R,':',true> extends [false,[never]] 
    ? [false,[never]]
    : Split<R,':',true>[1] extends [string,string]
    ? GetEntries<U,[...Entries,Split<R,':',true>[1]]>
    : [false,[never]]
    : [true,Entries] ;    

type ObjectFromEntries<
    Entries extends readonly [string,string][],
    Result extends object = {}> =
    Entries extends readonly [infer R extends [string,string],...infer U extends [string,string][]]
    ? ObjectFromEntries<U,Result&{[J in R[0]] : R[1]}>
    : Result; 

  type ParseArr<
    Str extends string> =
    Str extends `[${infer R extends string}]`
    ? R extends ''
    ? [true,[]]
    : Split<R,','>
    : [false,[never]]   ;    

   type ParseObj<
    Str extends string,    
    SplitedObj extends readonly [boolean,any] = 
        Str extends `{${infer R extends string}}`
        ? R extends ''
        ? [true,[]] 
        : Split<R,',',true>
        : [false,[never]] ,
    Entries extends readonly [boolean,any] = SplitedObj[0] extends true
        ? GetEntries<SplitedObj[1]>
        : SplitedObj ,
    Result extends object = 
    Entries[0] extends false
    ? never
    : ObjectFromEntries<Entries[1]>      
      > =
      [Result] extends [never]
      ? [false,[never]] 
      :[true,{[J in keyof Result] : RoughParse<Result[J]&string>[1]}]  ;      

type RoughParse<
    Input extends string,
    Skip extends boolean =
    false,
    Trimmed extends string = 
    Trim<Input> > =
    Skip extends true
    ? [true,Input]
    : HasBrackets<Trimmed> extends false
    ? [true,Input]
    : Trimmed extends `${infer R extends string}${string}`
    ? R extends '{'
    ? ParseObj<Trimmed>
    : R extends '['
    ? ParseArr<Trimmed>
    : [false,never]
    : never;

type RoughParsedOrNever<
    T extends string,
    Result=RoughParse<T>[1],
    > =
    Result extends object
    ? Result extends [never]
    ? never
    : [never] extends NestedValues<Result>
    ? never
    : Result
    : Result  ;

type SetTypeToPrimitive<
    Str extends string,
    Trimmed = 
    Trim<Str> > =
    Trimmed extends `\"${infer R}\"`
    ? R
    : Trimmed extends keyof SimpleTypes 
    ? SimpleTypes [Trimmed]
    : StringToNumber<Trimmed> ;

type SetTypes<
    T> =
    T extends string
    ? SetTypeToPrimitive<T>
    : T extends readonly any[]
    ? T extends readonly [infer R,...infer U]
    ? [SetTypes<R>,...SetTypes<U>]
    : []
    : T extends object
    ? {[J in string&keyof T as SetTypes<J>] : SetTypes<T[J]>}
    : T ;

type Parse<
    Input extends string> =
    NeverIfHasNumberKeysOrValues<SetTypes<RoughParsedOrNever<ParseUnicode<ExcludeUnicodeSymbols<Input>>>>>  ;

Solution by justBadProgrammer #17145

type Parse<S extends string> = ParseValue<S> extends [infer L extends string, infer V] ? Peek<S, L> extends '' ? V : never : never

type ParseValue<S extends string, L extends string = ''>
  = S extends `${L}${infer C}${any}`
    ? C extends ' ' | '\t' | '\n' ? ParseValue<S, `${L}${C}`>
    : C extends '{' ? ParseObject<S, `${L}{`>
    : C extends '[' ? ParseArray<S, `${L}[`>
    : C extends 'n' ? S extends `${L}null${any}` ? [`${L}null`, null] : []
    : C extends 't' ? S extends `${L}true${any}` ? [`${L}true`, true] : []
    : C extends 'f' ? S extends `${L}false${any}` ? [`${L}false`, false] : []
    : C extends '"' ? ParseString<S, `${L}"`>
    : C extends '-'
      ? S extends `${L}${C}${infer C2}${any}`
        ? C2 extends Digit ? ParseNumber<S, `${L}${C}${C2}`, `${C}${C2}`>
        : []
      : []
    : C extends Digit ? ParseNumber<S, `${L}${C}`, C>
    : []
  : []

type ParseString<S extends string, L extends string = '"', Result extends string = ''>
  = S extends `${L}${infer C}${any}`
    ? C extends '"' ? [`${L}"`, Result]
    : C extends '\n' | '\r' | '\t' ?  []  // those chars must be escape
    : C extends `\\`
      ? S extends `${L}\\${infer C2}${any}`
        ? C2 extends '\\' ? ParseString<S, `${L}\\\\`, `${Result}\\`>
        : C2 extends '"' ? ParseString<S, `${L}\\"`, `${Result}\"`>
        : C2 extends 'b' ? ParseString<S, `${L}\\b`, `${Result}\b`>
        : C2 extends 'f' ? ParseString<S, `${L}\\f`, `${Result}\f`>
        : C2 extends 'n' ? ParseString<S, `${L}\\n`, `${Result}\n`>
        : C2 extends 'r' ? ParseString<S, `${L}\\r`, `${Result}\r`>
        : C2 extends 't' ? ParseString<S, `${L}\\t`, `${Result}\t`>
        : C2 extends 'u'
          ? S extends `${L}\\u${infer E}${infer F}${infer G}${infer H}${any}`
            ? E | F | G | H extends Hex
              ? ParseString<S, `${L}\\u${E}${F}${G}${H}`, `${Result}${GetUnicode<`${E}${F}${G}${H}`>}`>
            : [] // expecting 4 hex chars
          : []   // unexpected escape sequence
        : []     // unexpected escape sequence
      : []       // unexpected escape sequence
    : ParseString<S, `${L}${C}`, `${Result}${C}`>
  : []           // expecting token

type ParseObject<S extends string, L extends string = '{', Result extends object = {}>
  = S extends `${L}${infer C}${any}`
    ? C extends ' ' | '\t' | '\n' ? ParseObject<S, `${L}${C}`, Result>
    : C extends '}' ? Result extends {} ? [`${L}}`, {}] : []
    : C extends '"'
      ? ParseString<S, `${L}"`> extends [infer L extends string, infer P extends string]
        ? P extends keyof Result ? [] // duplicated property name
        : ParseObjectColon<S, L, P, Result>
      : []    // expecting Name
    : []      // expecting "
  : []        // expecting "

type ParseObjectColon<S extends string, L extends string, P extends string, Result extends object>
  = S extends `${L}${infer C}${any}`
    ? C extends ' ' | '\t' | '\n' ? ParseObjectColon<S, `${L}${C}`, P, Result>
    : C extends ':'
      ? ParseValue<S, `${L}:`> extends [infer L extends string, infer V]
        ? ParseObjectValue<S, L, P, V, Result>
      : []        // expecting Value
    : []          // expecting :
  : []            // expecting :

type ParseObjectValue<S extends string, L extends string, P extends string, V, Result extends object>
  = S extends `${L}${infer C}${any}`
    ? C extends ' ' | '\t' | '\n' ? ParseObjectValue<S, `${L}${C}`, P, V, Result>
    : C extends ',' ? ParseObject<S, `${L},`, {[K in keyof Result | P]: K extends keyof Result ? Result[K] : V}>
    : C extends '}' ? [`${L}}`, {[K in keyof Result | P]: K extends keyof Result ? Result[K] : V}]
    : []          // expecting , or }
  : []            // expecting , or }

type ParseArray<S extends string, L extends string = '[', Result extends unknown[] = []>
  = S extends `${L}${infer C}${any}`
    ? C extends ' ' | '\t' | '\n' ? ParseArray<S, `${L}${C}`, Result>
    : C extends ']' ? Result extends [] ? [`${L}]`, []] : []
    : ParseValue<S, L> extends [infer L extends string, infer V]
      ? ParseArrayValue<S, L, V, Result>
    : []          // expecting ]
  : []            // expecting ]

type ParseArrayValue<S extends string, L extends string, V, Result extends unknown[]>
  = S extends `${L}${infer C}${any}`
    ? C extends ' ' | '\t' | '\n' ? ParseArrayValue<S, `${L}${C}`, V, Result>
    : C extends ',' ? ParseArray<S, `${L},`, [...Result, V]>
    : C extends ']' ? [`${L}]`, [...Result, V]]
    : []        // expecting , or ]
  : []        // expecting , or ]

type ParseNumber<S extends string, L extends string = '', Result extends string = ''>
  = S extends `${L}${infer C}${any}`
    ? C extends Digit ? ParseNumber<S, `${L}${PeekDigits<S, L>}`, `${Result}${PeekDigits<S, L>}`>
    : C extends '.' ? Result extends `${any}.${any}` ? [] : ParseNumber<S, `${L}.`, `${Result}.`>
    : C extends 'e' | 'E'
      ? S extends `${L}${C}-${Digit}${any}` ? [`${L}${C}-${PeekDigits<S, `${L}${C}-`>}`, ParseExponent<Result, `-${PeekDigits<S, `${L}${C}-`>}`>]
      : S extends `${L}${C}+${Digit}${any}` ? [`${L}${C}+${PeekDigits<S, `${L}${C}+`>}`, ParseExponent<Result, PeekDigits<S, `${L}${C}+`>>]
      : S extends `${L}${C}${Digit}${any}` ? [`${L}${C}${PeekDigits<S, `${L}${C}`>}`, ParseExponent<Result, PeekDigits<S, `${L}${C}`>>]
      : [L, ParseExponent<Result, '0'>]
    : [L, ParseExponent<Result, '0'>]
  : [L, ParseExponent<Result, '0'>]  // expecting token

type NormalizeExponent<Mantissa extends string, Exponent extends string>
  = Exponent extends `0` | '-0'
    ? Mantissa extends `${infer I}.${infer F}0` ? NormalizeExponent<`${I}.${F}`, Exponent>
    : Mantissa
  : Exponent extends `-${infer E extends bigint}`
    ? NormalizeExponent<
        Mantissa extends `${infer I}.${infer F}`
          ? I extends Digit ? `0.${I}${F}`
          : I extends `${infer I1}${Digit}` ? `${I1}.${TrimStart<I, I1>}${F}`
          : Mantissa
        : Mantissa extends `${infer I}0` ? I
        : Mantissa extends `${infer I}${Digit}` ? `${I}.${TrimStart<Mantissa, I>}`
        : Mantissa
        , `-${MinusOne<`${E}`>}`>
  : NormalizeExponent<
      Mantissa extends `${infer I}.${infer F}`
        ? F extends Digit ? `${I}${F}`
        : F extends `${infer F1}${infer F2}` ? `${I}${F1}.${F2}`
        : Mantissa
      : `${Mantissa}0`
      , MinusOne<Exponent>>

type NegSciForm<Mantissa extends string, Exponent extends string = '0'>
  = [Mantissa & `0.${any}`] extends [never]
    ? Exponent extends '0' ? Mantissa : `${Mantissa}e-${Exponent}`
  : Mantissa extends `0.${Digit}` ? NegSciForm<TrimStart<Mantissa, '0.'>, PlusOne<Exponent>>
  : Mantissa extends `0.${infer F1}${infer F2}` ? NegSciForm<`${F1}.${F2}`, PlusOne<Exponent>>
  : Mantissa

type PosSciForm<Mantissa extends string, Exponent extends string = '0'>
  = Mantissa extends Digit | `${Digit}.${any}`
    ? Exponent extends '0' ? Mantissa : `${Mantissa}e+${Exponent}`
  : Mantissa extends `${infer I}.${infer F}`
    ? PosSciForm<I extends `${infer I1}${Digit}` ? `${I1}.${TrimStart<I, I1>}${F}` : never, PlusOne<Exponent>>
  : Mantissa extends `${infer I}0` ? PosSciForm<I, PlusOne<Exponent>>
  : Mantissa extends `${infer I}${Digit}` ? PosSciForm<`${I}.${TrimStart<Mantissa, I>}`, PlusOne<Exponent>>
  : Mantissa

type ParseExponent<Mantissa extends string, Exponent extends string, Normalized extends string = NormalizeExponent<Mantissa, Exponent>>
  = Normalized extends `0.000000${any}`
    ? NegSciForm<Normalized> extends infer N extends string ? Num<N> : never
  : Normalized extends `${any}.${any}`
    ? Normalized extends `${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}.${any}`
      ? PosSciForm<Normalized> extends infer N extends string ? Num<N> : never
    : Num<Normalized>
  : Normalized extends `${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}`
    ? PosSciForm<Normalized> extends infer N extends string ? Num<N> : never
  : Num<Normalized>

type PlusOne<N extends string, Result extends string = ''>
  = N extends `${infer L}9` ? L extends '' ? `10${Result}` : PlusOne<L, `0${Result}`>
  : N extends `${infer L}8` ? `${L}9${Result}`
  : N extends `${infer L}7` ? `${L}8${Result}`
  : N extends `${infer L}6` ? `${L}7${Result}`
  : N extends `${infer L}5` ? `${L}6${Result}`
  : N extends `${infer L}4` ? `${L}5${Result}`
  : N extends `${infer L}3` ? `${L}4${Result}`
  : N extends `${infer L}2` ? `${L}3${Result}`
  : N extends `${infer L}1` ? `${L}2${Result}`
  : N extends `${infer L}0` ? `${L}1${Result}`
  : never

type MinusOne<N extends string, Result extends string = ''>
  = N extends `${infer L}0` ? L extends '1' ? `9${Result}` : MinusOne<L, `9${Result}`>
  : N extends `${infer L}1` ? `${L}0${Result}`
  : N extends `${infer L}2` ? `${L}1${Result}`
  : N extends `${infer L}3` ? `${L}2${Result}`
  : N extends `${infer L}4` ? `${L}3${Result}`
  : N extends `${infer L}5` ? `${L}4${Result}`
  : N extends `${infer L}6` ? `${L}5${Result}`
  : N extends `${infer L}7` ? `${L}6${Result}`
  : N extends `${infer L}8` ? `${L}7${Result}`
  : N extends `${infer L}9` ? `${L}8${Result}`
  : never

type Peek<S extends string, L extends string = ''>
  = S extends `${L}${infer C}${any}` ? C extends ' ' | '\t' | '\n' ? Peek<S, `${L}${C}`> : `${C}${L}` : ''

type PeekDigits<S extends string, L extends string = '', Result extends string = ''>
  = S extends `${L}${infer C}${any}` ? C extends Digit ? PeekDigits<S, `${L}${C}`, `${Result}${C}`> : Result : Result

type TrimStart<S extends string, Start extends string> = S extends `${Start}${infer End}` ? End : ''

type Num<S extends string> = S extends `${infer N extends number}` ? number extends N ? never : N : never

type Digit = '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'
type Hex = Digit | 'a' | 'b' | 'c' | 'd' | 'e' | 'f'

type GetUnicode<H extends string> = Unicode extends GetPadStart<H, `${infer C}${any}`> ? C : never
type GetPadStart<H extends string, End extends string, Result extends string = ''>
  = H extends `${infer F extends keyof PadStart}${infer R}` ? GetPadStart<R, End, PadStart<Result>[F]>
  : `${Result}${End}`
type PadStart<H extends string = '', H10 extends string = `${H}${H}${H}${H}${H}${H}${H}${H}${H}${H}${H}${H}${H}${H}${H}${H}`> = {
  '0': `${H10}`,
  '1': `${H10}${any}`,
  '2': `${H10}${any}${any}`,
  '3': `${H10}${any}${any}${any}`,
  '4': `${H10}${any}${any}${any}${any}`,
  '5': `${H10}${any}${any}${any}${any}${any}`,
  '6': `${H10}${any}${any}${any}${any}${any}${any}`,
  '7': `${H10}${any}${any}${any}${any}${any}${any}${any}`,
  '8': `${H10}${any}${any}${any}${any}${any}${any}${any}${any}`,
  '9': `${H10}${any}${any}${any}${any}${any}${any}${any}${any}${any}`,
  'a': `${H10}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}`,
  'b': `${H10}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}`,
  'c': `${H10}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}`,
  'd': `${H10}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}`,
  'e': `${H10}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}`,
  'f': `${H10}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}${any}`,
}
type Unicode = '\u0000\u0001\u0002\u0003\u0004\u0005\u0006\u0007\u0008\u0009\u000a\u000b\u000c\u000d....'
// full code in playground

Playground

Solution by teamchong #11695

type Parse<JSON extends string>
  = ParseValue<TrimLeft<JSON>> extends [infer Parsed, infer Tail extends string]
    ? [Parsed] extends [never]
      ? never
      : TrimLeft<Tail> extends ''
        ? Parsed
        : never
      : never

type TrimLeft<JSON extends string> = JSON extends `${' ' | '\n' | '\t'}${infer R}`
  ? TrimLeft<R>
  : JSON

type ParseValue<JSON extends string>
  = JSON extends `undefined${infer R}` ? [undefined, TrimLeft<R>]
  : JSON extends `null${infer R}` ? [null, TrimLeft<R>]
  : JSON extends `true${infer R}` ? [true, TrimLeft<R>]
  : JSON extends `false${infer R}` ? [false, TrimLeft<R>]
  : JSON extends `"${infer Str}"${infer R}` ? [ParseString<Str>, TrimLeft<R>]
  : JSON extends `[${infer R}` ? TrimLeft<R> extends `]${infer R}`
    ? [[], TrimLeft<R>]
    : ParseArray<`,${TrimLeft<R>}`>
  : JSON extends `{${infer R}` ? TrimLeft<R> extends `}${infer R}`
    ? [{}, TrimLeft<R>]
    : ParseObject<`,${TrimLeft<R>}`>
  : never

type ParseString<S extends string>
  = S extends `${infer L}\\${infer C extends keyof EscChars}${infer R}`
    ? ParseString<`${L}${EscChars[C]}${R}`>
    : S
type EscChars = { n: '\n', t: '\t', r: '\r', b: '\b', f: '\f' }

type ParseArray<JSON extends string, Arr extends any[] = []>
  = JSON extends `]${infer R}`
    ? [Arr, TrimLeft<R>]
    : JSON extends `,${infer R}`
      ? ParseValue<TrimLeft<R>> extends [infer Value, infer R extends string]
        ? [Value] extends [never]
          ? never
          : ParseArray<TrimLeft<R>, [...Arr, Value]>
        : never
      : never

type ParseObject<JSON extends string, Obj extends object = {}>
  = JSON extends `}${infer R}`
    ? [Merge<Obj>, TrimLeft<R>]
    : JSON extends `,${infer R}`
      ? ParseKey<TrimLeft<R>> extends [infer Key extends string, infer R]
        ? R extends `:${infer R}`
          ? ParseValue<TrimLeft<R>> extends [infer Value, infer R extends string]
            ? ParseObject<TrimLeft<R>, Obj & { [K in Key]: Value }>
            : never
          : never
        : never
      : never

type ParseKey<JSON extends string>
  = JSON extends `"${infer Key}"${infer R}`
    ? [ParseString<Key>, TrimLeft<R>]
    : never

type Merge<O> = { [K in keyof O]: O[K] }

The solution is designed to pass more complicated test cases than there are provided. Also, it's really easy to extend it to parse Numbers as well - not sure why they have been excluded from the problem in the first place

Solution by Alexsey #11692

// your answers

type TrimBase<T extends string, R extends string = ' '> = T extends `${R}${infer S}`
  ? TrimBase<S, R>
  : T extends `${infer S}${R}`
  ? TrimBase<S, R>
  : T
type Trim<T extends string> = TrimBase<TrimBase<T>, '"'>

type ParserObject<
  T extends string,
  R extends Record<string, any> = {},
  K extends string = '',
  V extends string = '',
  F extends boolean = false,
  L extends unknown[] = [],
> = T extends `${infer S}${infer E}`
  ? F extends false
    ? S extends ':'
      ? ParserObject<E, R, Trim<K>, V, true, L>
      : ParserObject<E, R, `${K}${S}`, V, F, L>
    : S extends ','
    ? L extends []
      ? ParserObject<E, { [P in keyof R]: R[P] } & { [P in K]: Parser<V> }, '', '', false, L>
      : ParserObject<E, R, K, `${V}${S}`, F, L>
    : S extends '[' | '{'
    ? ParserObject<E, R, K, `${V}${S}`, F, [...L, unknown]>
    : S extends ']' | '}'
    ? L extends [unknown, ...infer B]
      ? ParserObject<E, R, K, `${V}${S}`, F, B>
      : ParserObject<E, R, K, `${V}${S}`, F, L>
    : ParserObject<E, R, K, `${V}${S}`, F, L>
  : K extends ''
  ? { [P in keyof R]: R[P] }
  : ParserObject<'', { [P in keyof R]: R[P] } & { [P in K]: Parser<V> }, '', '', false, L>

type ParserArray<
  T extends string,
  R extends string[] = [],
  V extends string = '',
  L extends unknown[] = [],
> = T extends `${infer S}${infer E}`
  ? S extends ','
    ? L extends []
      ? ParserArray<E, [...R, Parser<V>], '', L>
      : ParserArray<E, R, `${V}${S}`, L>
    : S extends '[' | '{'
    ? ParserArray<E, R, `${V}${S}`, [...L, unknown]>
    : S extends '}' | ']'
    ? L extends [unknown, ...infer B]
      ? ParserArray<E, R, `${V}${S}`, B>
      : ParserArray<E, R, `${V}${S}`, L>
    : ParserArray<E, R, `${V}${S}`, L>
  : [...R, Parser<V>]

type ParserHelper<T extends string> = T extends `{${infer S}}`
  ? ParserObject<Trim<S>>
  : T extends `[${infer S}]`
  ? ParserArray<S>
  : T extends 'true'
  ? true
  : T extends 'false'
  ? false
  : T extends 'null'
  ? null
  : // Numbers and Unicode escape (\uxxxx) in JSON can be ignored. You needn't to parse them.
    T

type Parser<T extends string> = ParserHelper<Trim<T>>

type A =
  Parser<'  { "asdasd ":{} , "   a123  "  :  true,"b123":{"b1acads": 1," b2":{"aasd ":false},"b3asda":2},"c":3,"d":{"d":null}}'>
// type A = {
//   'asdasd ': {}
//   '   a123  ': true
//   b123: {
//     b1acads: '1'
//     ' b2': {
//       'aasd ': false
//     }
//     b3asda: '2'
//   }
//   c: '3'
//   d: {
//     d: null
//   }
// }

type B = Parser<'[  1  ,true, false,null ,{  },{ " a ":1,"b":{"b":"2"}},2,3]'>
// type B = [
//   '1',
//   true,
//   false,
//   null,
//   {},
//   {
//     'a ': '1'
//     b: {
//       b: '2'
//     }
//   },
//   '2',
//   '3',
// ]

Solution by ZangYuSong #6959

// your answers
type Pure<T> = {
  [P in keyof T]: T[P] extends object ? Pure<T[P]> : T[P]
}

type SetProperty<T, K extends PropertyKey, V> = {
  [P in (keyof T) | K]: P extends K ? V : P extends keyof T ? T[P] : never
}

type NumberToken = '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' | '-';
type BeginObjectToken = "{";
type EndObjectToken = "}";
type BeginArrayToken = "[";
type EndArrayToken = "]";
type SepColonToken = ":";
type SepCommaToken = ",";
type DoubleQuoteToken = "\"";

type StringToken = `\"${string}\"`;

type PrimitiveToken = StringToken | null | true | false;

type SyntaxToken = BeginObjectToken | EndObjectToken | BeginArrayToken | EndArrayToken | SepColonToken | SepCommaToken;

type Token = SyntaxToken | PrimitiveToken;

type ToVal<F extends PrimitiveToken> = F extends `\"${infer S}\"` ? S : F;

type ToEscape<S extends string> = S extends `'` ? '\'' :
    S extends `"` ? '\"' :
    S extends `n` ? '\n' :
    S extends `r` ? '\r' :
    S extends `f` ? '\f' :
    S extends `b` ? '\b' :
    S extends `t` ? '\t' : S;

type GetString<S extends string, R extends string = ""> = S extends `\\${infer F}${infer T}` ? GetString<T, `${R}${ToEscape<F>}`>
  : S extends `${infer F}${infer T}` ? (F extends DoubleQuoteToken ? [T, `${R}${F}`] : F extends "\n" ? never : GetString<T, `${R}${F}`>)
  : never;

type ToPrimitive<T extends string> = T extends `true` ? true : T extends 'false' ? false : T extends 'null' ? null : never;

type GetPrimitive<S extends string, R extends string = ""> = S extends `${infer F}${infer T}` ? (`${R}${F}` extends 'true' | 'false' | 'null' ? [T, ToPrimitive<`${R}${F}`>] : GetPrimitive<T, `${R}${F}`>) : never;

type ExpectToken = SepCommaToken | EndArrayToken;

// 提取数组
type ParseArrayResult<T extends any, R extends any[] = [], E extends any = PrimitiveToken | EndArrayToken | BeginArrayToken | BeginObjectToken> = T extends [infer F, ...infer S] ?
    (F extends E ?
        (   // 数组结束
            F extends EndArrayToken ? [R, S] :
            // 子数组
            F extends BeginArrayToken ? (ParseArrayResult<S> extends [infer F1, infer F2] ? ParseArrayResult<F2, [...R, F1], ExpectToken> : never) :
            // 子对象
            F extends BeginObjectToken ? (ParseObjectResult<S> extends [infer F1, infer F2] ? ParseArrayResult<F2, [...R, F1], ExpectToken> : never) :
            // 逗号标识
            F extends SepCommaToken ? ParseArrayResult<S, R, PrimitiveToken | BeginArrayToken | BeginObjectToken> :
            // 值
            F extends PrimitiveToken ? ParseArrayResult<S, [...R, ToVal<F>], ExpectToken> : never
        ) : never
    ) : never;

// 提取对象
type ParseObjectResult<T extends any, R extends any = {}, E extends any = EndObjectToken | StringToken, K extends string = ''> = T extends [infer F, ...infer S] ?
  (
      // 是否符合预期标识
      F extends E ? (
          // 提取值
          K extends StringToken ? (
              // : 标识符
              F extends SepColonToken ? ParseObjectResult<S, R, PrimitiveToken | BeginArrayToken | BeginObjectToken, K> :
              // 普通值
              F extends PrimitiveToken ? ParseObjectResult<S, SetProperty<R, ToVal<K>, ToVal<F>>, SepCommaToken | EndObjectToken> :
              // 数组
              F extends BeginArrayToken ? (ParseArrayResult<S> extends [infer F1, infer F2] ? ParseObjectResult<F2, SetProperty<R, ToVal<K>, F1>, SepCommaToken | EndObjectToken> : never) :
              // 子对象
              F extends BeginObjectToken ? (ParseObjectResult<S> extends [infer F1, infer F2] ? ParseObjectResult<F2, SetProperty<R, ToVal<K>, F1>, SepCommaToken | EndObjectToken> : never) :
              never
          ) : (
              // , token
              F extends SepCommaToken ? ParseObjectResult<S, R, StringToken, ''> :
              // key token
              F extends StringToken ? (ParseObjectResult<S, R, SepColonToken, F>) :
              // } end object
              F extends EndObjectToken ? [R, S] : never
          )
      ) : never
  ) : never;

type ParseResult<K extends Token[]> = K extends [infer F, ...infer S] ? F extends BeginObjectToken ? ParseObjectResult<S> : F extends BeginArrayToken ? ParseArrayResult<S> : never : never;

type Tokenize<S, T extends any[] = []> = S extends `${infer F}${infer N}` ?
  F extends SyntaxToken ? Tokenize<N, [...T, F]> :
  F extends DoubleQuoteToken ? (GetString<N, F> extends [infer A, infer B] ? Tokenize<A, [...T, B]> : never) :
  F extends 't' | 'f' | 'n' ? (GetPrimitive<S> extends [infer A, infer B] ? Tokenize<A, [...T, B]> : never) :
  F extends NumberToken ? never : Tokenize<N, T> : T;

type ParseLiteral<T extends Token[]> = T extends Token[] ? T extends [PrimitiveToken] ? [ToVal<T[0]>] : ParseResult<T> : never;

type Parse<T extends string> = Pure<ParseLiteral<Tokenize<T>>[0]>

Solution by venusliang #6457

//My answer is only for the test cases
type Parse<T extends string> = Eval<T> extends [infer V, infer U] ? V : never

type Eval<T>
  = T extends `${' '|'\n'}${infer U}` ? Eval<U>
  : T extends `true${infer U}` ? [true, U]
  : T extends `false${infer U}` ? [false, U]
  : T extends `null${infer U}` ? [null, U]
  : T extends `"${infer U}` ? EvalString<U>
  : T extends `${'['}${infer U}` ? EvalArray<U>
  : T extends `${'{'}${infer U}` ? EvalObject<U>
  : false

type Escapes = {r:'\r', n:'\n', b:'\b', f:'\f'}

type EvalString<T, S extends string = ''>
  = T extends `"${infer U}` ? [S, U]
  : (T extends `\\${infer C}${infer U}` ? C extends keyof Escapes ? [C, U] : false : false) extends [infer C, infer U] 
    ? EvalString<U, `${S}${C extends keyof Escapes ? Escapes[C] : never}`>
  : T extends `${infer C}${infer U}` ? EvalString<U, `${S}${C}`>
  : false

type EvalArray<T, A extends any[] = []> 
  = T extends `${' '|'\n'}${infer U}` ? EvalArray<U, A>
  : T extends `]${infer U}` ? [A, U]
  : T extends `,${infer U}` ? EvalArray<U, A>
  : Eval<T> extends [infer V, infer U] ? EvalArray<U, [...A, V]> 
  : false

type EvalObject<T, K extends string = '', O = {}> 
  = T extends `${' '|'\n'}${infer U}` ? EvalObject<U, K, O>
  : T extends `}${infer U}` ? [O, U]
  : T extends `,${infer U}` ? EvalObject<U, K, O>
  : T extends `"${infer U}` ? Eval<`"${U}`> extends [`${infer KK}`, infer UU] ? EvalObject<UU, KK, O> : false  
  : T extends `:${infer U}` ? Eval<U> extends [infer V, infer UU] ? EvalObject<UU, '', Merge<{[P in K]: V} & O>> : false
  : false

type Merge<T> = {[P in keyof T]: T[P]}

Solution by okayu29 #6329

New version.

/**
 * JSON Subset Parser, in Types
 * By Hydrogen (https://github.com/hyroge)
 */

interface Token {
  typ: "LCurly" | "RCurly" | "LBracket" | "RBracket" | "String" | "Comma" | "Colon" | "Keyword" | "Number"
  inner?: string
}

type Pure<T> = T extends object ? {
  [P in keyof T] : Pure<T[P]>
} : T

type Alpha = 'a' | 'b' | 'c' | 'd' | 'e' | 'f' | 'g' | 'h' | 'i' | 'j' | 'k' | 'l' | 'm' | 'n' | 'o' | 'p' | 'q' | 'r' | 's' | 't' | 'u' | 'v' | 'w' | 'x' | 'y' | 'z'

type TakeString<T extends string, S extends string = ''> = T extends `${infer P}${infer E}` ? (
  P extends '\\' ? (
    E extends `${infer R}${infer F}` ? (
      R extends '"' ? (
        TakeString<F, `${S}\"`>
      ) : (
        R extends '\\' ? (
          TakeString<F, `${S}\\`>
        ) : (
          R extends '/' ? (
            TakeString<F, `${S}/`>
          ) : (
            R extends 'b' ? (
              TakeString<F, `${S}\b`>
            ) : (
              R extends 'f' ? (
                TakeString<F, `${S}\f`>
              ) : (
                R extends 'n' ? (
                  TakeString<F, `${S}\n`>
                ) : (
                  R extends 'r' ? (
                    TakeString<F, `${S}\r`>
                  ) : (
                    R extends 't' ? (
                      TakeString<F, `${S}\t`>
                    ) : 0
                  )
                )
              )
            ) 
          )
        )
      )
    ) : never
  ) : (
    P extends '"' ? (
      [{typ: "String", inner: S}, E]
    ) : (
      P extends '\n' ? (
        never
      ) : (
        TakeString<E, `${S}${P}`>
      )
    )
  )
) : never

type TakeKeyword<T, S extends string = ''> = T extends `${infer P}${infer E}` ? (
  P extends Alpha ? (
    TakeKeyword<E, `${S}${P}`>
  ) : (
    S extends '' ? (
      never
    ) : (
      [{typ: "Keyword", inner: S}, T]
    )
  )
) : (
  S extends '' ? (
    never
  ) : (
    [{typ: "Keyword", inner: S}, T]
  )
)

type Tokenize<T extends string, S extends Token[] = []> = T extends `${infer P}${infer E}` ? (
  P extends '\r' | '\t' | ' ' | '\n' ? (
    Tokenize<E, S>
  ) : (
    (
      P extends '{' ? (
        Tokenize<E, [...S, { typ: "LCurly" }]>
      ) : (
        P extends '}' ? (
          Tokenize<E, [...S, { typ: "RCurly" }]>
        ) : (
          P extends '[' ? (
            Tokenize<E, [...S, { typ: "LBracket" }]>
          ) : (
            P extends ']' ? (
              Tokenize<E, [...S, { typ: "RBracket" }]>
            ) : (
              P extends ',' ? (
                Tokenize<E, [...S, { typ: "Comma" }]>
              ) : (
                P extends ':' ? (
                  Tokenize<E, [...S, { typ: "Colon" }]>
                ) : (
                  P extends '"' ? (
                    TakeString<E> extends [Token, string] ? (
                      Tokenize<TakeString<E>[1], [...S, TakeString<E>[0]]>
                    ) : never
                  ) : (
                    P extends Alpha ? (
                      TakeKeyword<T> extends [Token, string] ? (
                        Tokenize<TakeKeyword<T>[1], [...S, TakeKeyword<T>[0]]>
                      ) : never
                    ) : (
                      never
                    )
                  )
                )
              )
            )
          )
        )
      )
    )
  )
) : S

type SetProperty<T , K extends PropertyKey, V> = {
  [P in (keyof T) | K]: P extends K ? V : P extends keyof T ? T[P] : never
}

type ParseRecordImpl<T extends Token[], S = {}> = (
  ParsePair<T> extends never ? (
    never
  ) : (
    TakeToken<ParsePair<T>[1]> extends never ? (
      never
    ) : (
      TakeToken<ParsePair<T>[1]>[0]['typ'] extends 'RCurly' ? (
        [SetProperty<S, ParsePair<T>[0][0], ParsePair<T>[0][1]>, TakeToken<ParsePair<T>[1]>[1]]
      ) : (
        TakeToken<ParsePair<T>[1]>[0]['typ'] extends 'Comma' ? (
          ParseRecordImpl<TakeToken<ParsePair<T>[1]>[1], SetProperty<S, ParsePair<T>[0][0], ParsePair<T>[0][1]>>
        ) : never
      )
    )
  )
)

type ParseRecord<T extends Token[]> = T extends [infer P, ...(infer E)] ? (
  P extends Token ? (
    E extends Token[] ? (
      P['typ'] extends 'RCurly' ? (
        [{}, E]
      ) : ParseRecordImpl<T, {}>
    ) : never
  ) : never
) : never



type ParsePair<T extends Token[]> = ParseString<T> extends never ? (
  ParseString<T>
) : (
  TakeToken<ParseString<T>[1]> extends never ? (
    never
  ) : (
    TakeToken<ParseString<T>[1]>[0]['typ'] extends 'Colon' ? (
      ParseLiteral<TakeToken<ParseString<T>[1]>[1]> extends never ? (
        never
      ) : (
        [
          [ParseString<T>[0], ParseLiteral<TakeToken<ParseString<T>[1]>[1]>[0]],
          ParseLiteral<TakeToken<ParseString<T>[1]>[1]>[1]
        ]
      )
    ) : never
  )
)

type TakeToken<T extends Token[]> = T extends [infer P, ...(infer S)] ? (
  [P, S] extends [Token, Token[]] ? ( 
    [P, S]
  ) : never
) : never

type ParseArrayImpl<T extends Token[], S extends unknown[] = []> = (
  ParseLiteral<T> extends never ? (
    never
  ) : (
    TakeToken<ParseLiteral<T>[1]> extends never ? (
      never
    ) : (
      TakeToken<ParseLiteral<T>[1]>[0]['typ'] extends 'Comma' ? (
        ParseArrayImpl<TakeToken<ParseLiteral<T>[1]>[1], [...S, ParseLiteral<T>[0]]>
      ) : (
        TakeToken<ParseLiteral<T>[1]>[0]['typ'] extends 'RBracket' ? (
          [[...S, ParseLiteral<T>[0]], TakeToken<ParseLiteral<T>[1]>[1]]
        ) : never
      )
    )
  )
)

type ParseArray<T extends Token[]> = (
  T extends [infer P, ...(infer E)] ? (
    [P, E] extends [Token, Token[]] ? ( 
        [P, E][0]['typ'] extends 'RBracket' ? (
          [[], E]
        ) : ParseArrayImpl<T, []>
    ) : never
  ) : never
)

type ParseKeyword<T extends Token[]> = (
  TakeToken<T> extends never ? never : (
    TakeToken<T>[0]['typ'] extends 'Keyword' ? (
      Exclude<TakeToken<T>[0]['inner'], undefined> extends 'true' ? (
        [true, TakeToken<T>[1]]
      ) : (
        Exclude<TakeToken<T>[0]['inner'], undefined> extends 'false' ? (
          [false, TakeToken<T>[1]]
        ) : (
          Exclude<TakeToken<T>[0]['inner'], undefined> extends 'null' ? (
            [null, TakeToken<T>[1]]
          ) : never
        )
      )
    ) : never
  )
)

type ParseString<T extends Token[]> = (
  TakeToken<T> extends never ? never : (
    TakeToken<T>[0]['typ'] extends 'String' ? (
      [Exclude<TakeToken<T>[0]['inner'], undefined>, TakeToken<T>[1]]
    ) : never
  )
)


type ParseNumber<T extends Token[]> = (
  TakeToken<T> extends never ? never : (
    TakeToken<T>[0]['typ'] extends 'Number' ? (
      TakeToken<T>[0]
    ) : never
  )
)

type ParseRoot<T extends Token[]> = (
  TakeToken<T> extends never ? never : (
    TakeToken<T>[0]['typ'] extends 'LCurly' ? (
      ParseRecord<TakeToken<T>[1]>
    ) : (
      TakeToken<T>[0]['typ'] extends 'LBracket' ? (
        ParseArray<TakeToken<T>[1]>
      ) : never
    )
  )
)


type ParseLiteral<T extends Token[]> = (
  | ParseRoot<T>
  | ParseString<T>
  | ParseKeyword<T>
  | ParseNumber<T>
)

//
// support data types:
//  - `boolean` (`true` `false`)
//  - `string` (`"\\r\\n"`)
//  - `object` & `array`
//  - `null` (`null`)
//
type Parse<T extends string> = Pure<ParseLiteral<Tokenize<T>>[0]>

Solution by hydrati #6310