JSON response parsing in React Native - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

Displaying parsed JSON data in a list

You receive this JSON response from a server:

{"users": [{"id": 1, "name": "Alice"}, {"id": 2, "name": "Bob"}]}

Which React Native code snippet correctly parses this JSON and displays the user names in a FlatList?

React Native

const jsonResponse = '{"users": [{"id": 1, "name": "Alice"}, {"id": 2, "name": "Bob"}]}';

// Which snippet correctly parses and displays the names?

A

const data = JSON.stringify(jsonResponse);
<FlatList data={data.users} keyExtractor={item => item.id.toString()} renderItem={({item}) => <Text>{item.name}</Text>} />

B

const data = JSON.parse(jsonResponse.users);
<FlatList data={data} keyExtractor={item => item.id.toString()} renderItem={({item}) => <Text>{item.name}</Text>} />

C

const data = JSON.parse(jsonResponse);
<FlatList data={data.users} keyExtractor={item => item.id.toString()} renderItem={({item}) => <Text>{item.name}</Text>} />

D

const data = JSON.parse(jsonResponse);
<FlatList data={data} keyExtractor={item => item.id.toString()} renderItem={({item}) => <Text>{item.name}</Text>} />

💡 Hint

Remember to use JSON.parse to convert the JSON string into an object before accessing its properties.

Option C correctly parses the JSON string into an object, then accesses the 'users' array to pass as data to FlatList. Option C wrongly uses JSON.stringify which converts an object to a string. Option C passes the whole parsed object instead of the users array. Option C tries to parse jsonResponse.users which is undefined because jsonResponse is a string.

📝 Syntax

intermediate

Correct JSON parsing syntax

Which option correctly parses a JSON string stored in variable response and accesses the title property of the first item in the posts array?

React Native

const response = '{"posts": [{"id": 1, "title": "Hello"}, {"id": 2, "title": "World"}]}';

A

const data = JSON.parse(response.posts);
const title = data[0].title;

B

const data = JSON.stringify(response);
const title = data.posts[0].title;

C

const data = JSON.parse(response);
const title = data[0].posts.title;

D

const data = JSON.parse(response);
const title = data.posts[0].title;

💡 Hint

Use JSON.parse on the whole JSON string before accessing nested properties.

Option D correctly parses the JSON string and accesses the first post's title. Option D uses JSON.stringify which converts an object to a string, so data.posts is undefined. Option D tries to parse response.posts which is undefined because response is a string. Option D tries to access data[0] which is undefined because data is an object with a posts array.

🔧 Debug

advanced

Fixing runtime error when parsing JSON

You have this code snippet:

const response = '{"items": [{"id": 1, "value": 10}, {"id": 2, "value": 20}]}';
const data = JSON.parse(response);
console.log(data.items.length);

But it throws an error: TypeError: Cannot read property 'length' of undefined. What is the most likely cause?

A The variable <code>response</code> is not a string but already an object, so parsing it again causes error.

B The code is correct and should not throw an error; the error must be from elsewhere.

C The JSON string is correct but the property <code>items</code> does not exist in the parsed object.

D The JSON string is malformed and JSON.parse fails silently returning undefined.

💡 Hint

Check the type of the variable before parsing it again.

If response is already an object, calling JSON.parse on it throws an error or returns undefined. This causes data.items to be undefined, so accessing length fails. The JSON string shown is correct, so A and C are unlikely. B is incorrect because the error message points to this code.

🧠 Conceptual

advanced

Understanding asynchronous JSON fetching

In React Native, you fetch JSON data from a server using fetch. Which code snippet correctly parses the JSON response and updates state with the data?

React Native

fetch('https://example.com/data')
  .then(response => ???)
  .then(data => setData(data))
  .catch(error => console.error(error));

A response.json()

B JSON.parse(response)

C response.text()

D JSON.stringify(response)

💡 Hint

The fetch response object has a method to parse JSON directly.

The fetch API response object has a .json() method that returns a promise resolving to the parsed JSON. JSON.parse expects a string, but response is a Response object, so B is wrong. A returns raw text, not parsed JSON. D converts an object to a string, not parsing.

❓ navigation

expert

Passing parsed JSON data between screens

You parsed JSON data in Screen A and want to pass the array items to Screen B using React Navigation. Which code snippet correctly passes and accesses the data?

React Native

// In Screen A:
navigation.navigate('ScreenB', { items: parsedData.items });

// In Screen B:
const items = ???;

A const items = navigation.getParam('items');

B const items = route.params.items;

C const items = props.navigation.state.params.items;

D const items = navigation.params.items;

💡 Hint

In React Navigation v5 and later, route.params contains passed parameters.

Option B uses route.params which is the correct way to access passed parameters in functional components with React Navigation v5+. Option B is deprecated from older versions. Option B uses props.navigation.state.params which is outdated and may not work. Option B is invalid syntax.