React Native Academy · 강의

JSON 객체 저장 및 파싱

AsyncStorage에 쓰기 전에 JavaScript 객체를 JSON으로 직렬화하고 읽을 때 역직렬화하며, 앱을 처음 실행할 때의 null 값을 처리합니다.

JSON 객체 저장 및 파싱은(는) CoddyKit의 무료 React Native Academy 강의입니다. 이것은 4개 중 2번째 강의입니다. 아래에서 전체 강의를 무료로 읽을 수 있으며, 내장 코드 에디터와 24/7 AI 튜터와 함께 브라우저에서 직접 실습할 수 있습니다. 이 강의는 React Native Academy 학습 경로의 일부이며, 진행 상황이 웹과 CoddyKit 앱에 동기화됩니다. React Native Academy 강의에는 총 4개의 강의가 포함되어 있습니다.

이 강의의 일부는 아직 번역되지 않았으며 영어로 표시됩니다.

Why AsyncStorage Only Stores Strings

AsyncStorage is a string-only key-value store — you cannot store JavaScript objects, arrays, numbers, or booleans directly. To save any non-string value you must convert it to a string first, and convert it back when reading. The standard approach is JSON.stringify before writing and JSON.parse after reading.

Saving an Object with JSON.stringify

Call JSON.stringify on your JavaScript object before passing it to setItem. This converts the object to a JSON string that AsyncStorage can store. If the object contains functions, undefined, or circular references, JSON.stringify will omit or error on those values — keep stored objects plain and serializable.

const userProfile = {
  name: 'Alice',
  email: 'alice@example.com',
  preferences: { theme: 'dark', language: 'en' },
};

async function saveProfile(profile: object) {
  try {
    await AsyncStorage.setItem('userProfile', JSON.stringify(profile));
  } catch (err) {
    console.error('Failed to save profile:', err);
  }
}

Reading and Parsing with JSON.parse

When reading back a stored JSON string, call JSON.parse on the returned value to convert it back into a JavaScript object. Always wrap this in a try-catch because JSON.parse throws a SyntaxError if the stored value is not valid JSON — for example, if it was corrupted or written by a different code path.

async function loadProfile() {
  try {
    const json = await AsyncStorage.getItem('userProfile');
    if (json === null) return null; // never saved before
    return JSON.parse(json);
  } catch (err) {
    console.error('Failed to load or parse profile:', err);
    return null;
  }
}

Handling null on First Launch

A user who has never opened the app will have nothing in AsyncStorage. getItem returns null before any setItem call has been made. Always check for null before parsing and return an appropriate default value — typically the initial state used when the app launches for the very first time.

const DEFAULT_SETTINGS = { theme: 'light', language: 'en', notifications: true };

async function loadSettings() {
  const json = await AsyncStorage.getItem('settings');
  if (json === null) return DEFAULT_SETTINGS; // first launch
  try {
    return JSON.parse(json);
  } catch {
    return DEFAULT_SETTINGS; // corrupted data — fall back to defaults
  }
}

Storing Arrays

Arrays are also not directly supported by AsyncStorage but serialize perfectly with JSON.stringify. Use this pattern to persist lists such as recent searches, bookmarked items, or favorite IDs. After parsing the JSON string back, the result is a proper JavaScript array that you can spread, map, and filter normally.

// Save an array
const favorites = ['post_1', 'post_3', 'post_7'];
await AsyncStorage.setItem('favorites', JSON.stringify(favorites));

// Load the array
const json = await AsyncStorage.getItem('favorites');
const favorites = json ? JSON.parse(json) as string[] : [];
console.log(favorites); // ['post_1', 'post_3', 'post_7']

Merging New Data into an Existing Object

When updating only one field of a stored object, read the full object first, merge in the new value using spread, then write it back. This avoids accidentally overwriting other fields. For performance in write-heavy scenarios, consider batching updates and debouncing writes so you are not writing to disk on every keystroke.

async function updateTheme(newTheme: string) {
  const json = await AsyncStorage.getItem('settings');
  const current = json ? JSON.parse(json) : DEFAULT_SETTINGS;
  const updated = { ...current, theme: newTheme };
  await AsyncStorage.setItem('settings', JSON.stringify(updated));
}

Type-Safe Storage Helpers

Create typed wrapper functions for each stored object to avoid scattering JSON.stringify and JSON.parse throughout the codebase. Each helper encapsulates the key name, the serialization, and the default value. TypeScript generics make it easy to create a single reusable helper for any type.

async function saveObject<T>(key: string, value: T): Promise<void> {
  await AsyncStorage.setItem(key, JSON.stringify(value));
}

async function loadObject<T>(key: string, defaultValue: T): Promise<T> {
  const json = await AsyncStorage.getItem(key);
  if (json === null) return defaultValue;
  try {
    return JSON.parse(json) as T;
  } catch {
    return defaultValue;
  }
}

// Usage:
const profile = await loadObject<UserProfile>('profile', defaultProfile);

Storing Primitive Numbers and Booleans

Numbers and booleans must also be converted to strings when stored in AsyncStorage. JSON.stringify(42) gives '42' and JSON.parse('42') gives 42 back. For simple values you can also use String(value) to write and then Number(value) or compare to 'true' to read, but JSON.stringify/parse is more consistent across all types.

// Store a boolean
await AsyncStorage.setItem('onboardingComplete', JSON.stringify(true));

// Read it back
const raw = await AsyncStorage.getItem('onboardingComplete');
const complete = raw !== null ? JSON.parse(raw) as boolean : false;

// Store a number
await AsyncStorage.setItem('highScore', JSON.stringify(2048));
const score = JSON.parse(await AsyncStorage.getItem('highScore') ?? '0') as number;

Dates and Timestamps

JavaScript Date objects are not directly JSON-serializable — JSON.stringify(new Date()) produces an ISO string, but JSON.parse gives you back a string, not a Date. Store dates as ISO strings or Unix timestamps (numbers) and explicitly convert them back to Date objects after parsing.

// Store date as ISO string
await AsyncStorage.setItem('lastLogin', new Date().toISOString());

// Read and convert back to Date
const raw = await AsyncStorage.getItem('lastLogin');
const lastLogin = raw ? new Date(raw) : null;
console.log(lastLogin?.toLocaleDateString());

Validating Stored Data Shape

Stored data can become outdated as your app evolves. After parsing JSON from AsyncStorage, validate the returned object shape before using it. Check for required fields and fall back to defaults if they are missing. This prevents crashes when an old app version stored data with fewer fields than the current version expects.

function isValidProfile(obj: any): obj is UserProfile {
  return (
    typeof obj === 'object' &&
    obj !== null &&
    typeof obj.name === 'string' &&
    typeof obj.email === 'string'
  );
}

const parsed = json ? JSON.parse(json) : null;
const profile = isValidProfile(parsed) ? parsed : DEFAULT_PROFILE;

Performance: Avoid Frequent Reads

Reading from AsyncStorage on every render or in a tight loop is slow because it involves asynchronous I/O. Load data once on app startup (or screen mount), store it in component state or a global store (like Zustand), and read from state for the rest of the session. Write to AsyncStorage only when values actually change, not on every re-render.

Quick Check

Test your understanding of React Native Mobile Development concepts from this lesson.

Lesson Recap

In this lesson you learned: JSON.stringify before writing and JSON.parse after reading are the standard pattern for storing objects in AsyncStorage, checking for null prevents crashes on first launch before any data has been saved, and type-safe wrapper functions encapsulate serialization to keep your codebase clean. Next up we explore building a persistent settings screen.

자주 묻는 질문

“JSON 객체 저장 및 파싱” 강의는 무료인가요?

네 — “JSON 객체 저장 및 파싱” 전체 내용을 이 웹사이트에서 무료로 읽을 수 있습니다. 인터랙티브하게 실습하려면(내장 코드 에디터와 24/7 AI 튜터), CoddyKit PRO로 업그레이드하면 React Native Academy 강의 전체를 잠금 해제할 수 있습니다. React Native Academy 강의에는 총 4개의 강의가 포함되어 있습니다.

“JSON 객체 저장 및 파싱”에서 뭘 배우나요?

AsyncStorage에 쓰기 전에 JavaScript 객체를 JSON으로 직렬화하고 읽을 때 역직렬화하며, 앱을 처음 실행할 때의 null 값을 처리합니다. 브라우저에서 직접 실행하는 실습 코드로 React Native Academy을(를) 배우며, 24/7 AI 튜터가 강의를 진행하면서 질문에 답변해줍니다.

React Native Academy을(를) 시작하는 데 경험이 필요한가요?

사전 경험은 필요하지 않습니다. CoddyKit의 React Native Academy은(는) 초급자부터 고급 학습자까지를 위해 구성되어 있으므로, 여기서 시작하거나 처음부터 시작할 수 있으며 자신의 속도대로 진행할 수 있습니다. 이것은 4개 중 2번째 강의입니다.

“JSON 객체 저장 및 파싱” 강의는 얼마나 걸리나요?

대부분의 CoddyKit 강의는 약 5~10분이 소요됩니다. 각 강의는 간결하고 인터랙티브하여 꾸준한 진행이 가능하며, 웹과 앱에서 중단한 부분부터 바로 시작할 수 있습니다.

이 React Native Academy 강의에서 코드를 작성하고 실행할 수 있나요?

네. 모든 React Native Academy 강의에는 내장 코드 에디터가 포함되어 있으므로, 브라우저에서 바로 실제 코드를 작성하고 실행한 후 즉시 AI 피드백을 받을 수 있습니다 — 로컬 설정이 필요 없습니다.

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