JSON response parsing in React Native - Mini App: Build & Ship

Build: User List Screen

This screen fetches a list of users from a JSON API and displays their names in a scrollable list.

Target UI

-------------------------
|      User List        |
|-----------------------|
| - Loading...          |
|                       |
|                       |
|                       |
|                       |
|                       |
-------------------------

Fetch JSON data from https://jsonplaceholder.typicode.com/users

Parse the JSON response to extract user names

Display the user names in a vertical scrollable list

Show a loading indicator while fetching data

Handle errors by showing an error message

Starter Code

React Native

import React, { useEffect, useState } from 'react';
import { View, Text, FlatList, ActivityIndicator, StyleSheet } from 'react-native';

export default function UserListScreen() {
  const [users, setUsers] = useState([]);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);

  useEffect(() => {
    // TODO: Fetch JSON data and update state
  }, []);

  return (
    <View style={styles.container}>
      {/* TODO: Show loading, error or user list here */}
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    padding: 16,
    backgroundColor: '#fff'
  },
  itemText: {
    fontSize: 18,
    paddingVertical: 8
  },
  errorText: {
    color: 'red',
    fontSize: 16
  }
});

Task 1

Task 2

Task 3

Task 4

Task 5

Task 6

Task 7

Solution

React Native

import React, { useEffect, useState } from 'react';
import { View, Text, FlatList, ActivityIndicator, StyleSheet } from 'react-native';

export default function UserListScreen() {
  const [users, setUsers] = useState([]);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);

  useEffect(() => {
    fetch('https://jsonplaceholder.typicode.com/users')
      .then(response => response.json())
      .then(data => {
        setUsers(data);
        setLoading(false);
      })
      .catch(err => {
        setError('Failed to load users');
        setLoading(false);
      });
  }, []);

  if (loading) {
    return (
      <View style={styles.container}>
        <ActivityIndicator size="large" color="#0000ff" />
      </View>
    );
  }

  if (error) {
    return (
      <View style={styles.container}>
        <Text style={styles.errorText}>{error}</Text>
      </View>
    );
  }

  return (
    <View style={styles.container}>
      <FlatList
        data={users}
        keyExtractor={item => item.id.toString()}
        renderItem={({ item }) => <Text style={styles.itemText}>{item.name}</Text>}
      />
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    padding: 16,
    backgroundColor: '#fff'
  },
  itemText: {
    fontSize: 18,
    paddingVertical: 8
  },
  errorText: {
    color: 'red',
    fontSize: 16
  }
});

This solution uses React Native hooks to manage state and side effects.

We start with three states: users for the list, loading to show a spinner, and error to show any problems.

Inside useEffect, we fetch the JSON from the API. When the data arrives, we save it in users and stop loading. If there is an error, we save an error message and stop loading.

The UI shows a spinner while loading, an error message if there is an error, or the list of user names when data is ready.

Using FlatList makes the list scrollable and efficient.

Final Result

Completed Screen

-------------------------
|      User List        |
|-----------------------|
| Leanne Graham          |
| Ervin Howell           |
| Clementine Bauch       |
| Patricia Lebsack       |
| Chelsey Dietrich       |
| Mrs. Dennis Schulist   |
| Kurtis Weissnat        |
| Nicholas Runolfsdottir |
| Glenna Reichert        |
| Clementina DuBuque     |
-------------------------

When the screen loads, a spinner appears while fetching data.

After data loads, the spinner disappears and the list of user names appears.

User can scroll vertically through the list.

If fetching fails, an error message is shown instead.

Stretch Goal

Add a pull-to-refresh feature to reload the user list when the user swipes down.

💡 Hint

Use FlatList's refreshing and onRefresh props to implement pull-to-refresh.

Practice

1. What does response.json() do in React Native when fetching data?

easy

A. It displays the JSON data on the screen.

B. It sends data back to the server.

C. It converts the server response into a JavaScript object.

D. It stops the fetch request.

Solution

1. Step 1: Understand the fetch response

   The fetch API returns a response object that contains the raw data from the server.

2. Step 2: Use response.json() to parse data

   The response.json() method reads the raw response and converts it into a usable JavaScript object.

3. Final Answer:

   It converts the server response into a JavaScript object. -> Option C

4. Quick Check:

   response.json() = parse JSON [OK]

Hint: Remember: response.json() parses JSON to object [OK]

Common Mistakes:

• Thinking response.json() sends data to server
• Assuming response.json() displays data directly
• Confusing response.json() with stopping fetch

2. Which of the following is the correct syntax to fetch and parse JSON data in React Native?

easy

A. fetch(url).then(response => response.json())

B. fetch(url).then(response => response.text())

C. fetch(url).json()

D. fetch(url).parseJSON()

Solution

1. Step 1: Recall fetch syntax

   The fetch function returns a promise with a response object.

2. Step 2: Use response.json() to parse JSON

   The correct method to parse JSON from the response is response.json(), which returns a promise with the parsed object.

3. Final Answer:

   fetch(url).then(response => response.json()) -> Option A

4. Quick Check:

   Correct fetch JSON syntax = fetch(url).then(response => response.json()) [OK]

Hint: Use response.json() after fetch to parse JSON [OK]

Common Mistakes:

• Using response.text() instead of response.json()
• Calling json() directly on fetch without then
• Using non-existent parseJSON() method

3. Given the code snippet:

fetch('https://api.example.com/data')
  .then(response => response.json())
  .then(data => console.log(data.name))
  .catch(error => console.error(error));

What will be logged if the JSON response is {"name": "Alice", "age": 25}?

medium

A. "Alice"

B. undefined

C. 25

D. Error

Solution

1. Step 1: Parse JSON response

   The response JSON is parsed into an object: {name: "Alice", age: 25}.

2. Step 2: Access the 'name' property

   The code logs data.name, which is "Alice".

3. Final Answer:

   "Alice" -> Option A

4. Quick Check:

   data.name = "Alice" [OK]

Hint: Access parsed JSON properties directly by name [OK]

Common Mistakes:

• Logging data instead of data.name
• Expecting age instead of name
• Not parsing JSON before accessing properties

4. What is wrong with this React Native fetch code?

fetch('https://api.example.com/data')
  .then(response => response.json)
  .then(data => console.log(data))
  .catch(error => console.error(error));

medium

A. No catch block for errors

B. Missing parentheses after response.json

C. console.log should be console.error

D. fetch URL is incorrect

Solution

1. Step 1: Check response.json usage

   The code uses response.json without parentheses, so it references the function but does not call it.

2. Step 2: Correct function call syntax

   It should be response.json() to actually parse the JSON.

3. Final Answer:

   Missing parentheses after response.json -> Option B

4. Quick Check:

   Call response.json() with () [OK]

Hint: Always call response.json() with parentheses [OK]

Common Mistakes:

• Forgetting parentheses after response.json
• Assuming fetch URL is wrong without checking
• Confusing console.log and console.error usage

5. You receive this JSON response from a server:

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

How do you extract an array of user names in React Native after parsing?

hard

A. data.names

B. data.map(user => user.name)

C. data.users.name

D. data.users.map(user => user.name)

Solution

1. Step 1: Understand JSON structure

   The JSON has a key "users" which is an array of user objects.

2. Step 2: Use map to extract names

   To get all names, use data.users.map(user => user.name) which creates a new array of names.

3. Final Answer:

   data.users.map(user => user.name) -> Option D

4. Quick Check:

   Map over data.users to get names [OK]

Hint: Map over array property to extract values [OK]

Common Mistakes:

• Mapping directly on data instead of data.users
• Trying to access data.users.name which is undefined
• Assuming data.names exists