React Native Academy · Aula

Armazenando e analisando objetos JSON

Serialize objetos JavaScript em JSON antes de gravá-los no AsyncStorage e desserialize-os durante a leitura, gerenciando valores nulos na primeira execução do aplicativo.

Armazenando e analisando objetos JSON é uma aula grátis de React Native Academy no CoddyKit. Esta é a aula 2 de 4. Você pode ler a aula completa abaixo gratuitamente — depois pratica ao vivo no navegador com um editor de código integrado e um tutor de IA 24/7. Faz parte do caminho de aprendizado de React Native Academy, e seu progresso é sincronizado entre a web e o app CoddyKit. O curso de React Native Academy inclui 4 aulas no total.

Partes desta aula ainda não foram traduzidas e aparecem em inglês.

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.

Perguntas Frequentes

A aula “Armazenando e analisando objetos JSON” é grátis?

Sim — o texto completo de “Armazenando e analisando objetos JSON” é grátis para ler aqui na web. Para praticá-la interativamente (um editor de código integrado e um tutor de IA 24/7) e desbloquear o restante do curso de React Native Academy, atualize para CoddyKit PRO. O curso de React Native Academy inclui 4 aulas no total.

O que vou aprender em “Armazenando e analisando objetos JSON”?

Serialize objetos JavaScript em JSON antes de gravá-los no AsyncStorage e desserialize-os durante a leitura, gerenciando valores nulos na primeira execução do aplicativo. Você pratica React Native Academy com código prático que executa diretamente no navegador, e um tutor de IA 24/7 responde suas dúvidas enquanto trabalha na aula.

Preciso ter experiência prévia para começar React Native Academy?

Nenhuma experiência prévia é necessária. React Native Academy no CoddyKit é estruturado para alunos iniciantes até avançados, então você pode começar aqui ou desde o início e aprender no seu ritmo. Esta é a aula 2 de 4.

Quanto tempo leva a aula “Armazenando e analisando objetos JSON”?

A maioria das aulas CoddyKit leva cerca de 5–10 minutos. Cada uma é compacta e interativa, então você faz progresso constante e retoma exatamente de onde parou entre web e app.

Posso escrever e executar código nesta aula de React Native Academy?

Sim. Cada aula de React Native Academy inclui um editor de código integrado, então você escreve e executa código real direto no navegador e recebe feedback de IA instantaneamente — nenhuma configuração local necessária.

Todas as aulas deste curso

Lendo e gravando com AsyncStorage
Armazenando e analisando objetos JSON
Criando uma tela de configurações persistente
Limpando o armazenamento e estratégias de migração

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