Fetch API for POST requests in React Native

Introduction

We use POST requests to send data from our app to a server. The Fetch API helps us do this easily.

When a user submits a form and you want to save their data online.

When you need to send login details to check if a user is allowed in.

When uploading a photo or file from your app to a website.

When sending feedback or messages from the app to a server.

Syntax

React Native

fetch(url, {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json'
  },
  body: JSON.stringify(data)
})
.then(response => response.json())
.then(result => {
  // handle result
})
.catch(error => {
  // handle error
});

The method must be 'POST' to send data.

Use JSON.stringify to convert your data object into a string.

Examples

Sends a simple object with a name to the server.

React Native

fetch('https://example.com/api', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({ name: 'Anna' })
})

Sends login details securely to check user access.

React Native

fetch('https://example.com/login', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({ username: 'user1', password: 'pass123' })
})

Sample App

This React Native app has a button. When pressed, it sends a POST request to create a post on a test server. It then shows the new post ID or an error message.

React Native

import React, { useState } from 'react';
import { View, Button, Text } from 'react-native';

export default function App() {
  const [responseMessage, setResponseMessage] = useState('');

  const sendData = async () => {
    try {
      const response = await fetch('https://jsonplaceholder.typicode.com/posts', {
        method: 'POST',
        headers: { 'Content-Type': 'application/json' },
        body: JSON.stringify({ title: 'Hello', body: 'This is a test', userId: 1 })
      });
      const data = await response.json();
      setResponseMessage('Post created with ID: ' + data.id);
    } catch (error) {
      setResponseMessage('Error: ' + error.message);
    }
  };

  return (
    <View style={{ padding: 20, marginTop: 50 }}>
      <Button title="Send POST Request" onPress={sendData} />
      <Text style={{ marginTop: 20 }}>{responseMessage}</Text>
    </View>
  );
}

Important Notes

Always set the Content-Type header to 'application/json' when sending JSON data.

Use async/await for cleaner code and easier error handling.

Test your POST requests with free services like jsonplaceholder.typicode.com before using your real server.

Summary

POST requests send data from your app to a server.

Use Fetch API with method 'POST' and JSON.stringify your data.

Handle the server response to update your app UI or show messages.

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