Fetch API for POST requests in React Native - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens when this React Native fetch POST request runs?

Consider this React Native code snippet that sends a POST request. What will be the value of response.status if the server accepts the request successfully?

React Native

fetch('https://example.com/api/data', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({ name: 'Alice' })
})
.then(response => console.log(response.status))
.catch(error => console.error(error));

A 200

B 404

C 500

D 201

💡 Hint

A successful POST request usually returns a status code in the 200 range.

The fetch POST request sends data to the server. If the server accepts it successfully, it usually responds with status 200 (OK) or 201 (Created). Since this example does not specify server behavior, 201 is the most appropriate success code for a POST that creates a resource.

📝 Syntax

intermediate

Which option correctly formats the fetch POST request body?

You want to send JSON data in a POST request using fetch in React Native. Which option correctly sets the body property?

A body: JSON.stringify({ username: 'bob' })

B body: { username: 'bob' }

C body: JSON.parse('{"username":"bob"}')

D body: 'username=bob'

💡 Hint

The body must be a string when sending JSON data.

The fetch API requires the body to be a string for JSON data. JSON.stringify converts the object to a JSON string. Option A passes an object directly, which causes an error. Option A parses a string to an object, which is wrong. Option A sends a plain string not JSON.

❓ lifecycle

advanced

What is the correct way to handle fetch POST request errors in React Native?

You want to catch network or server errors when sending a POST request with fetch. Which code snippet correctly handles errors?

React Native

fetch('https://api.example.com/login', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({ user: 'test' })
})
.then(response => {
  if (!response.ok) throw new Error('Server error');
  return response.json();
})
.then(data => console.log(data))
.catch(error => console.error('Error:', error.message));

A It catches both network errors and server response errors correctly.

B It only catches network errors, not server errors.

C It does not catch any errors because fetch never throws.

D It catches errors but does not parse JSON correctly.

💡 Hint

Check how response.ok is used to detect server errors.

fetch only rejects on network errors. To catch server errors (like 404 or 500), you must check response.ok and throw an error manually. This code does both, so it handles all errors properly.

❓ navigation

advanced

After a successful POST request, how to navigate to another screen in React Native?

You send a POST request to login a user. After success, you want to navigate to the Home screen. Which code snippet correctly does this using React Navigation?

React Native

fetch('https://api.example.com/login', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({ username: 'user', password: 'pass' })
})
.then(response => response.json())
.then(data => {
  if (data.success) {
    // Navigate to Home
  }
})
.catch(error => console.error(error));

A navigate('Home');

B navigation.navigate('Home');

C Navigation.push('Home');

D this.props.navigation.navigate('Home');

💡 Hint

In functional components, navigation is usually passed as a prop.

In React Navigation with functional components, you use the navigation prop directly: navigation.navigate('Home'). Option B is for class components. Options A and C are invalid functions.

🧠 Conceptual

expert

Why must you include 'Content-Type': 'application/json' header in a fetch POST request?

You send JSON data in a POST request but forget to set the 'Content-Type' header. What is the likely effect?

A The JSON data will be automatically converted to form data.

B The fetch request will fail with a syntax error.

C The server may not parse the JSON body correctly and reject the request.

D The server will accept the request but ignore the body.

💡 Hint

The Content-Type header tells the server how to read the request body.

Without the 'Content-Type': 'application/json' header, the server may treat the body as plain text or another format and fail to parse the JSON, causing errors or rejection.

Practice

1. What is the main purpose of using a POST request with the Fetch API in React Native?

easy

A. To delete data from the app

B. To retrieve data from a server

C. To send data from the app to a server

D. To update the app's local state only

Solution

1. Step 1: Understand POST request purpose

   POST requests are used to send data to a server, unlike GET which retrieves data.

2. Step 2: Match Fetch API usage

   Using Fetch with method 'POST' means sending data from the app to the server.

3. Final Answer:

   To send data from the app to a server -> Option C

4. Quick Check:

   POST request = send data [OK]

Hint: POST means sending data to server, not just getting it [OK]

Common Mistakes:

• Confusing POST with GET method
• Thinking POST only updates local state
• Assuming POST deletes data

2. Which of the following is the correct way to specify a POST request using Fetch API in React Native?

easy

A. fetch(url, { method: 'POST' })

B. fetch(url, { method: 'GET' })

C. fetch(url, { method: 'PUT' })

D. fetch(url, { method: 'DELETE' })

Solution

1. Step 1: Identify HTTP method for POST

   The method property in Fetch options must be 'POST' to send data.

2. Step 2: Check options object

   Only fetch(url, { method: 'POST' }) uses method: 'POST' correctly.

3. Final Answer:

   fetch(url, { method: 'POST' }) -> Option A

4. Quick Check:

   Fetch POST syntax = method: 'POST' [OK]

Hint: Use method: 'POST' exactly to send data [OK]

Common Mistakes:

• Using method: 'GET' for sending data
• Confusing PUT or DELETE with POST
• Omitting the method property

3. What will be the output of this code snippet in React Native?

fetch('https://example.com/api', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({ name: 'John' })
})
.then(response => response.json())
.then(data => console.log(data.message))
.catch(error => console.error(error));

Assuming the server responds with {"message": "Success"}.

medium

A. Throws a syntax error

B. Logs undefined to the console

C. Logs the entire response object

D. Logs 'Success' to the console

Solution

1. Step 1: Understand fetch and JSON parsing

   The fetch call sends a POST request with JSON body and parses the JSON response.

2. Step 2: Check console.log output

   Since server returns {"message": "Success"}, data.message is 'Success', so it logs 'Success'.

3. Final Answer:

   Logs 'Success' to the console -> Option D

4. Quick Check:

   response.json() = parsed JSON, data.message = 'Success' [OK]

Hint: response.json() parses JSON, then access properties [OK]

Common Mistakes:

• Not parsing response as JSON
• Logging response instead of data.message
• Syntax errors in JSON.stringify

4. Identify the error in this React Native POST request code:

fetch('https://api.example.com/data', {
  method: 'POST',
  body: { name: 'Alice' }
})

medium

A. Missing headers for content type

B. body should be a JSON string, not an object

C. method should be 'GET' instead of 'POST'

D. URL is invalid

Solution

1. Step 1: Check body format in fetch POST

   The body must be a string, usually JSON stringified, not a plain object.

2. Step 2: Identify missing JSON.stringify

   Here, body is an object { name: 'Alice' } which causes an error; it should be JSON.stringify({ name: 'Alice' }).

3. Final Answer:

   body should be a JSON string, not an object -> Option B

4. Quick Check:

   body = JSON.stringify(object) [OK]

Hint: Always JSON.stringify body for POST requests [OK]

Common Mistakes:

• Passing object directly as body
• Forgetting Content-Type header
• Using wrong HTTP method

5. You want to send user data with a POST request and handle errors gracefully in React Native. Which code snippet correctly implements this with async/await and error handling? A)

async function sendData() {
  try {
    const response = await fetch('https://api.example.com/user', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({ username: 'user1' })
    });
    const data = await response.json();
    console.log(data);
  } catch (error) {
    console.error('Error:', error);
  }
}

B)

async function sendData() {
  const response = fetch('https://api.example.com/user', {
    method: 'POST',
    body: { username: 'user1' }
  });
  const data = response.json();
  console.log(data);
}

C)

function sendData() {
  fetch('https://api.example.com/user', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ username: 'user1' })
  })
  .then(response => response.text())
  .then(text => console.log(text))
  .catch(error => console.error(error));
}

D)

async function sendData() {
  try {
    const response = await fetch('https://api.example.com/user');
    const data = await response.json();
    console.log(data);
  } catch (error) {
    console.error(error);
  }
}

hard

A. Correct async/await POST with JSON body and error handling

B. Missing await and body is not stringified

C. Uses then but parses response as text, not JSON

D. No POST method or body specified

Solution

1. Step 1: Check async/await usage and POST setup

   Correct async/await POST with JSON body and error handling uses async/await correctly, sets method to 'POST', includes headers, and stringifies body.

2. Step 2: Verify error handling and response parsing

   Correct async/await POST with JSON body and error handling has try/catch for errors and parses response as JSON, then logs data.

3. Final Answer:

   Correct async/await POST with JSON body and error handling -> Option A

4. Quick Check:

   Async POST with try/catch = Correct async/await POST with JSON body and error handling [OK]

Hint: Use async/await with try/catch and JSON.stringify body [OK]

Common Mistakes:

• Not awaiting fetch or response.json()
• Not stringifying body object
• Missing headers for JSON content
• Not handling errors