Fetch API for GET requests in React Native - UI Render Trace

Component - Fetch API for GET requests

This React Native component fetches data from a web address using the Fetch API with a GET request. It shows the data on the screen or a loading message while waiting.

Widget Tree

View
├─ Text (title)
├─ Button (Fetch Data)
└─ Text (data or loading message)

The main container is a View. Inside it, a Text shows a title. Below is a Button that triggers the data fetch. Finally, another Text shows the fetched data or a loading message.

Render Trace - 4 Steps

Step 1: View

Step 2: Text (title)

Step 3: Button

Step 4: Text (data or loading)

State Change - Re-render

Trigger:User presses the 'Fetch Data' button

Before

data is empty string, loading is false

→

After

loading is true, data is empty string; then loading is false, data is set to fetched text

Re-renders:The entire component re-renders to update the loading message and then show fetched data

UI Quiz - 3 Questions

What happens on the screen immediately after pressing the 'Fetch Data' button?

A The text changes to 'Loading...'

B The button disappears

C The title text changes

D Nothing changes

Key Insight

Using the Fetch API in React Native involves triggering a network request on user action, managing loading state, and updating the UI to show feedback. The component re-renders when state changes to reflect loading and data results, providing a smooth user experience.

Practice

1.

What does the Fetch API do in React Native?

Choose the best description.

easy

A. It retrieves data from a web server using HTTP requests.

B. It styles components with CSS.

C. It manages app navigation between screens.

D. It stores data locally on the device.

Solution

1. Step 1: Understand Fetch API purpose

   The Fetch API is designed to make network requests to get or send data.

2. Step 2: Match description to Fetch API

   Only It retrieves data from a web server using HTTP requests. describes retrieving data from a web server, which is what Fetch does.

3. Final Answer:

   It retrieves data from a web server using HTTP requests. -> Option A

4. Quick Check:

   Fetch API = Data retrieval [OK]

Hint: Fetch is for network data requests, not styling or storage [OK]

Common Mistakes:

• Confusing Fetch with styling or navigation libraries
• Thinking Fetch stores data locally
• Assuming Fetch only works with JSON

2.

Which of the following is the correct syntax to fetch JSON data from https://api.example.com/data?

fetch(_____).then(response => response.json())

easy

A. "https://api.example.com/data"

B. url

C. https://api.example.com/data

D. 'https://api.example.com/data'

Solution

1. Step 1: Identify correct string syntax in JavaScript

   URLs must be passed as strings, enclosed in quotes.

2. Step 2: Choose proper quotes for URL string

   'https://api.example.com/data' uses single quotes correctly.

3. Final Answer:

   'https://api.example.com/data' -> Option D

4. Quick Check:

   URL as string = 'https://api.example.com/data' [OK]

Hint: Always wrap URLs in quotes for fetch [OK]

Common Mistakes:

• Not using quotes around the URL
• Using undefined variable url without declaration
• Using double quotes inside double quotes without escaping

3.

What will be logged to the console after running this code?

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

Assume the server returns { "name": "Alice" }.

medium

A. SyntaxError

B. undefined

C. Alice

D. null

Solution

1. Step 1: Understand fetch and JSON parsing

   The fetch call gets the response, then response.json() parses it into an object.

2. Step 2: Access the 'name' property from parsed data

   The data object has a 'name' key with value "Alice", so console.log(data.name) prints "Alice".

3. Final Answer:

   Alice -> Option C

4. Quick Check:

   Parsed JSON name = Alice [OK]

Hint: Parsed JSON data properties print their values [OK]

Common Mistakes:

• Logging before JSON parsing
• Trying to access data.name before fetch resolves
• Ignoring asynchronous nature of fetch

4.

Identify the error in this fetch code snippet:

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

medium

A. Missing parentheses after response.json() call

B. URL string is not quoted properly

C. No catch block to handle errors

D. Using console.log instead of alert

Solution

1. Step 1: Check response.json usage

   response.json is a function and must be called with parentheses to parse JSON.

2. Step 2: Identify missing parentheses error

   The code uses response.json without () which returns the function itself, not the parsed data.

3. Final Answer:

   Missing parentheses after response.json() call -> Option A

4. Quick Check:

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

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

Common Mistakes:

• Forgetting parentheses on response.json
• Misplacing catch block
• Using wrong quotes for URL

5.

You want to fetch user data and show a loading message while waiting. Which code snippet correctly handles loading state and errors?

const [loading, setLoading] = React.useState(true);
const [error, setError] = React.useState(null);

React.useEffect(() => {
  fetch('https://api.example.com/user')
    .then(res => res.json())
    .then(data => {
      // handle data
      setLoading(false);
    })
    .catch(err => {
      setError(err.message);
      setLoading(false);
    });
}, []);

What is missing to properly show loading and error states?

hard

A. Remove catch block to avoid errors

B. Render UI that shows loading and error messages based on state

C. Call setLoading(true) inside fetch then block

D. Use async/await instead of then/catch

Solution

1. Step 1: Understand state variables usage

   loading and error states are set correctly during fetch lifecycle.

2. Step 2: Identify missing UI rendering logic

   Without UI code that checks loading and error states, user won't see messages.

3. Final Answer:

   Render UI that shows loading and error messages based on state -> Option B

4. Quick Check:

   Show loading/error UI based on state [OK]

Hint: Set state is not enough; render UI accordingly [OK]

Common Mistakes:

• Not rendering loading or error messages
• Setting loading true too late
• Ignoring error handling in UI