File system access in React Native

Introduction

File system access lets your app read and write files on the device. This helps save user data or load files anytime.

Saving user settings or preferences locally

Downloading and storing images or documents

Reading a saved text file to show content

Caching data to improve app speed

Logging app events for debugging

Syntax

React Native

import RNFS from 'react-native-fs';

// Write a file
RNFS.writeFile(path, contents, 'utf8')
  .then(() => console.log('File written'))
  .catch(err => console.log(err));

// Read a file
RNFS.readFile(path, 'utf8')
  .then(contents => console.log(contents))
  .catch(err => console.log(err));

Use react-native-fs library for file system operations in React Native.

Always handle errors to avoid app crashes.

Examples

This example saves a text file named test.txt with a simple message.

React Native

const path = RNFS.DocumentDirectoryPath + '/test.txt';
const content = 'Hello from React Native!';

RNFS.writeFile(path, content, 'utf8')
  .then(() => console.log('File saved'))
  .catch(err => console.log(err));

This reads the previously saved file and logs its content.

React Native

RNFS.readFile(RNFS.DocumentDirectoryPath + '/test.txt', 'utf8')
  .then(contents => console.log('File content:', contents))
  .catch(err => console.log(err));

Sample App

This React Native component has two buttons: one to save a file with a message, and one to read that file and show its content below.

React Native

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

export default function FileExample() {
  const [fileContent, setFileContent] = useState('');
  const path = RNFS.DocumentDirectoryPath + '/hello.txt';

  const saveFile = () => {
    RNFS.writeFile(path, 'Hello React Native!', 'utf8')
      .then(() => Alert.alert('File saved'))
      .catch(err => Alert.alert('Error: ' + err.message));
  };

  const readFile = () => {
    RNFS.readFile(path, 'utf8')
      .then(contents => setFileContent(contents))
      .catch(err => Alert.alert('Error: ' + err.message));
  };

  return (
    <View style={{ padding: 20 }}>
      <Button title="Save File" onPress={saveFile} />
      <Button title="Read File" onPress={readFile} />
      <Text style={{ marginTop: 20, fontSize: 16 }}>Content: {fileContent}</Text>
    </View>
  );
}

Important Notes

File paths differ by platform; use RNFS.DocumentDirectoryPath for safe storage.

Always ask for permissions if your app needs to access external storage on Android.

Test file operations on real devices as simulators may behave differently.

Summary

File system access lets apps save and load files on the device.

Use react-native-fs for easy file reading and writing.

Handle errors and permissions carefully for smooth user experience.

Practice

1. What is the main purpose of using react-native-fs in a React Native app?

easy

A. To read and write files on the device's file system

B. To create user interface components

C. To manage app navigation between screens

D. To handle network requests and API calls

Solution

1. Step 1: Understand the role of react-native-fs

   This library is designed to provide file system access in React Native apps.

2. Step 2: Identify the correct functionality

   It allows reading and writing files on the device, unlike UI or network libraries.

3. Final Answer:

   To read and write files on the device's file system -> Option A

4. Quick Check:

   File system access = read/write files [OK]

Hint: Remember: react-native-fs is for file operations only [OK]

Common Mistakes:

• Confusing file system access with UI components
• Thinking it manages navigation or network calls
• Assuming it handles permissions automatically

2. Which of the following is the recommended ES6 import syntax for react-native-fs in a React Native file?

easy

A. import * as RNFS from 'react-native-fs';

B. import RNFS from 'react-native-fs';

C. import { RNFS } from 'react-native-fs';

D. const RNFS = require('react-native-fs');

Solution

1. Step 1: Recall ES6 import syntax for default exports

   react-native-fs exports a default object, so use import RNFS from 'react-native-fs';.

2. Step 2: Check other options

   const RNFS = require('react-native-fs'); uses CommonJS require (valid but less common in modern React Native). import { RNFS } from 'react-native-fs'; incorrectly destructures. import * as RNFS from 'react-native-fs'; imports all as namespace but is not the recommended style here.

3. Final Answer:

   import RNFS from 'react-native-fs'; -> Option B

4. Quick Check:

   Default import syntax = import RNFS from 'react-native-fs'; [OK]

Hint: Use default import for react-native-fs: import RNFS from '...' [OK]

Common Mistakes:

• Using destructuring import for default export
• Mixing require and import styles
• Using namespace import unnecessarily

3. What will the following code output if the file test.txt contains the text "Hello World"?

import RNFS from 'react-native-fs';

RNFS.readFile(RNFS.DocumentDirectoryPath + '/test.txt', 'utf8')
  .then(contents => console.log(contents))
  .catch(err => console.log('Error:', err.message));

medium

A. Error: File not found

B. An empty string

C. undefined

D. "Hello World"

Solution

1. Step 1: Understand readFile behavior

   The readFile method reads the file content as a string when encoding is 'utf8'.

2. Step 2: Check file content and output

   The file contains "Hello World", so the promise resolves and logs this string.

3. Final Answer:

   "Hello World" -> Option D

4. Quick Check:

   readFile with utf8 returns file text [OK]

Hint: readFile with 'utf8' returns file text content [OK]

Common Mistakes:

• Assuming readFile returns a buffer or object
• Expecting error if file exists
• Ignoring promise and expecting synchronous output

4. Identify the error in this code snippet that tries to write text to a file:

import RNFS from 'react-native-fs';

RNFS.writeFile('/path/to/file.txt', 'Sample text', 'utf8')
  .then(() => console.log('File written'))
  .catch(error => console.log('Write error:', error));

medium

A. The file path should use RNFS.DocumentDirectoryPath or similar, not an absolute string

B. The encoding 'utf8' is not supported by writeFile

C. writeFile does not return a promise

D. The text argument must be an object, not a string

Solution

1. Step 1: Check file path usage

   In React Native, absolute paths like '/path/to/file.txt' are invalid; use RNFS.DocumentDirectoryPath or similar directory constants.

2. Step 2: Validate other parameters

   Encoding 'utf8' is valid, writeFile returns a promise, and text can be a string.

3. Final Answer:

   The file path should use RNFS.DocumentDirectoryPath or similar, not an absolute string -> Option A

4. Quick Check:

   Use RNFS paths, not absolute strings [OK]

Hint: Always use RNFS directory constants for file paths [OK]

Common Mistakes:

• Using absolute file paths directly
• Misunderstanding writeFile return type
• Passing wrong data types for text

5. You want to save user notes in a file and then read them back later. Which approach correctly handles permissions, writes the file, and reads it back safely in React Native using react-native-fs?

hard

A. Use fetch API to write and read files on device storage

B. Write file directly to '/storage/emulated/0/notes.txt' without permission, then read it

C. Request storage permission, write file using RNFS.DocumentDirectoryPath, then read file with error handling

D. Write file with no encoding specified, then read file with 'ascii' encoding

Solution

1. Step 1: Handle permissions properly

   On mobile devices, storage permission must be requested before file operations.

2. Step 2: Use safe file paths and encoding

   Use RNFS.DocumentDirectoryPath for file location and specify encoding like 'utf8' for writing and reading.

3. Step 3: Implement error handling

   Use promises with then and catch to handle success and errors safely.

4. Final Answer:

   Request storage permission, write file using RNFS.DocumentDirectoryPath, then read file with error handling -> Option C

5. Quick Check:

   Permissions + RNFS paths + error handling = Request storage permission, write file using RNFS.DocumentDirectoryPath, then read file with error handling [OK]

Hint: Always request permission and use RNFS paths with error handling [OK]

Common Mistakes:

• Skipping permission requests
• Using hardcoded absolute paths
• Ignoring promise errors
• Using wrong encoding for text files