Javascript: check if file exists

In JavaScript programming, you’ll often have to determine the presence of a file within the file system. This guide covers how to check if a file exists in different JavaScript environments such as the browser, Node.js, and using third-party libraries.

Understanding the environment

JavaScript runs in multiple environments; in-browser, it's limited due to security concerns, while in server-side environments like Node.js, you have more freedom to access the file system.

Check file existence in the browser

In a browser context, direct file system access is restricted. You can only check if a file exists if the user has provided it, for example, through an <input> element.

document.getElementById('fileInput').addEventListener('change', function(event) {
  const file = event.target.files[0];
  if (file) {
    console.log('File exists:', file.name);
  } else {
    console.log('No file selected.');
  }
});

Check file existence in Node

Node.js provides the fs module, which you can use to check if a file exists.

Using fs.existsSync

The fs.existsSync method is the simplest synchronous way to check for file existence without reading the file.

const fs = require('fs');

const fileExists = fs.existsSync('path/to/file.txt');
console.log(fileExists); // true or false

Using fs.stat or fs.access

For asynchronous checks, fs.stat or fs.access are the preferred methods. fs.access is specifically designed to check permissions and, by default, will check for file existence.

const fs = require('fs');

fs.access('path/to/file.txt', fs.constants.F_OK, (err) => {
  console.log(err ? 'File does not exist' : 'File exists');
});

Using fs.promises

With the fs.promises API, you can use async/await for cleaner asynchronous code.

const fs = require('fs').promises;

async function checkFileExists(file) {
  try {
    await fs.access(file, fs.constants.F_OK);
    console.log('File exists');
  } catch {
    console.log('File does not exist');
  }
}

checkFileExists('path/to/file.txt');

Using third-party libraries

Third-party libraries like axios for HTTP requests or the jQuery library for AJAX can be used to check the existence of a file over a network.

Using axios

const axios = require('axios');

async function checkFileExists(url) {
  try {
    const response = await axios.head(url);
    console.log(response.status === 200 ? 'File exists' : 'File does not exist');
  } catch (error) {
    console.log('File does not exist');
  }
}

checkFileExists('<http://example.com/file.txt>');

Using jQuery

$.ajax({
  type: 'HEAD',
  url: '<http://example.com/file.txt>',
  success: function(){
    console.log('File exists');
  },
  error: function() {
    console.log('File does not exist');
  }
});

Handling errors and exceptions

When checking file existence, especially in Node.js, it’s important to handle potential errors such as permissions issues, broken links, or network errors when checking remote files.

Watching for file presence

In scenarios where you need to watch for a file to appear or disappear, you can use fs.watch in Node.js:

const fs = require('fs');
const filename = 'path/to/file.txt';

fs.watch(filename, (eventType, filename) => {
  if (eventType === 'rename') {
    console.log('File has been created or deleted.');
  }
});

Permissions considerations

Permissions can affect your ability to check for a file. Always ensure that the process has the appropriate permissions to avoid false negatives.

Cross-environment checks

Writing code that might run in different JavaScript environments requires checks that work in both the browser and server-side:

function checkFileExists(file) {
  if (typeof window !== 'undefined') {
    // Browser environment
    // ... browser-specific code
  } else if (typeof process !== 'undefined') {
    // Node.js environment
    const fs = require('fs');
    return fs.existsSync(file); // or async variant with fs.access
  }
}

Network error handling

When checking file existence over a network, implement retries and timeouts to handle latency and errors:

const axios = require('axios');
const axiosRetry = require('axios-retry');

axiosRetry(axios, { retries: 3, retryDelay: axiosRetry.exponentialDelay});

async function checkFileExists(url) {
  try {
    const response = await axios.head(url, { timeout: 5000 });
    console.log('File exists');
  } catch (error) {
    console.log('File does not exist or network error:', error.message);
  }
}