Convert json string to object

61.3% acceptance

Mar 31, 2026

Given a string str, return parsed JSON parsedStr. You may assume the str is a valid JSON string hence it only includes strings, numbers, arrays, objects, booleans, and null. str will not include invisible characters and escape characters. Please solve it without using the built-in JSON.parse method.

Solution

TypeScript

Time O(n³)

Space O(1)

LeetCode

solution.ts

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

function jsonParse(str: string): JSONValue {
  let i = 0;

  function parseValue(): JSONValue {
  if (str[i] === '"') return parseString();
  if (str[i] === "{") return parseObject();
  if (str[i] === "[") return parseArray();
  if (str[i] === "t") {
    i += 4;
    return true;
  }
  if (str[i] === "f") {
    i += 5;
    return false;
  }
  if (str[i] === "n") {
    i += 4;
    return null;
  }
  return parseNumber();
  }

  function parseString(): string {
  i++; // skip opening quote
  let result = "";
  while (str[i] !== '"') {
    result += str[i++];
  }
  i++; // skip closing quote
  return result;
  }

  function parseNumber(): number {
  let start = i;
  if (str[i] === "-") i++;
  while (i < str.length && str[i] >= "0" && str[i] <= "9") i++;
  if (str[i] === ".") {
    i++;
    while (i < str.length && str[i] >= "0" && str[i] <= "9") i++;
  }
  if (str[i] === "e" || str[i] === "E") {
    i++;
    if (str[i] === "+" || str[i] === "-") i++;
    while (i < str.length && str[i] >= "0" && str[i] <= "9") i++;
  }
  return Number(str.slice(start, i));
  }

  function parseArray(): JSONValue[] {
  i++; // skip [
  const arr: JSONValue[] = [];
  if (str[i] === "]") {
    i++;
    return arr;
  }
  arr.push(parseValue());
  while (str[i] === ",") {
    i++;
    arr.push(parseValue());
  }
  i++; // skip ]
  return arr;
  }

  function parseObject(): { [key: string]: JSONValue } {
  i++; // skip {
  const obj: { [key: string]: JSONValue } = {};
  if (str[i] === "}") {
    i++;
    return obj;
  }
  let key = parseString();
  i++; // skip :
  obj[key] = parseValue();
  while (str[i] === ",") {
    i++;
    key = parseString();
    i++; // skip :
    obj[key] = parseValue();
  }
  i++; // skip }
  return obj;
  }

  return parseValue();
}