/** * @param {string} str * @return {object | Array | string | number | boolean | null} */function parse(str) {  let i = 0;  function skipWhitespace() {    while (i < str.length && /\s/.test(str[i])) i++;  }  function parseValue() {    skipWhitespace();    const value =      parseString() ??      parseNumber() ??      parseObject() ??      parseArray() ??      parseKeyword('true', true) ??      parseKeyword('false', false) ??      parseKeyword('null', null);    if (value === undefined) {      throw new Error(`Unexpected token at position ${i}: "${str[i]}"`);    }    return value;  }  function parseString() {    if (str[i] !== '"') return undefined;    i++; // 跳过开头的 "    let result = '';    while (i < str.length) {      const ch = str[i];      if (ch === '"') {        i++; // 跳过结尾的 "        return result;      }      if (ch === '\\') {        i++;        const escaped = str[i];        const escapeMap = {          '"': '"', '\\': '\\', '/': '/',          b: '\b', f: '\f', n: '\n', r: '\r', t: '\t',        };        if (escaped in escapeMap) {          result += escapeMap[escaped];          i++;        } else if (escaped === 'u') {          const hex = str.slice(i + 1, i + 5);          if (!/^[0-9a-fA-F]{4}$/.test(hex)) {            throw new Error('Invalid Unicode escape');          }          result += String.fromCharCode(parseInt(hex, 16));          i += 5;        } else {          throw new Error(`Invalid escape character: \\${escaped}`);        }      } else {        result += ch;        i++;      }    }    throw new Error('Unterminated string');  }  function parseNumber() {    const start = i;    if (str[i] === '-') i++;    // 整数部分    if (str[i] === '0') {      i++;    } else if (str[i] >= '1' && str[i] <= '9') {      while (str[i] >= '0' && str[i] <= '9') i++;    } else {      return undefined;    }    // 小数部分    if (str[i] === '.') {      i++;      if (!(str[i] >= '0' && str[i] <= '9')) {        throw new Error('Invalid number: expected digit after decimal point');      }      while (str[i] >= '0' && str[i] <= '9') i++;    }    // 指数部分    if (str[i] === 'e' || str[i] === 'E') {      i++;      if (str[i] === '+' || str[i] === '-') i++;      if (!(str[i] >= '0' && str[i] <= '9')) {        throw new Error('Invalid number: expected digit in exponent');      }      while (str[i] >= '0' && str[i] <= '9') i++;    }    return Number(str.slice(start, i));  }  function parseObject() {    if (str[i] !== '{') return undefined;    i++; // 跳过 {    const result = {};    skipWhitespace();    if (str[i] === '}') {      i++;      return result;    }    while (true) {      skipWhitespace();      const key = parseString();      if (key === undefined) throw new Error('Expected string key');      skipWhitespace();      if (str[i] !== ':') throw new Error('Expected ":"');      i++;      const value = parseValue();      result[key] = value;      skipWhitespace();      if (str[i] === ',') {        i++;        continue;      }      if (str[i] === '}') {        i++;        return result;      }      throw new Error('Expected "," or "}"');    }  }  function parseArray() {    if (str[i] !== '[') return undefined;    i++; // 跳过 [    const result = [];    skipWhitespace();    if (str[i] === ']') {      i++;      return result;    }    while (true) {      result.push(parseValue());      skipWhitespace();      if (str[i] === ',') {        i++;        continue;      }      if (str[i] === ']') {        i++;        return result;      }      throw new Error('Expected "," or "]"');    }  }  function parseKeyword(name, value) {    if (str.slice(i, i + name.length) !== name) return undefined;    i += name.length;    return value;  }  const result = parseValue();  skipWhitespace();  if (i < str.length) {    throw new Error(`Unexpected non-whitespace character after JSON at position ${i}`);  }  return result;}