Definition: What is "Failed to Fetch" Error?
"Failed to fetch" is a JavaScript error that occurs when the fetch() API cannot complete an HTTP request. It's a generic error message that indicates the fetch operation failed, but doesn't specify the exact cause. The error can occur due to network issues, CORS problems, HTTPS/SSL errors, invalid URLs, server unavailability, or browser security restrictions.
Unlike specific HTTP status codes (404, 500), "Failed to fetch" is thrown before the request completes, meaning the browser couldn't even establish a connection or the request was blocked. This makes it different from HTTP errors where the server responds but with an error status code.
This error is common in web development when making API requests, especially cross-origin requests, HTTPS connections, or when network connectivity is poor. Understanding and fixing "Failed to fetch" errors is essential for robust API integration and error handling in JavaScript applications.
Key Point: "Failed to fetch" occurs when fetch() requests fail before completion due to CORS, network issues, HTTPS problems, or browser security. The solution depends on the specific cause: configure CORS, fix network connectivity, resolve HTTPS issues, or handle errors properly.
What: Understanding the Error
"Failed to fetch" can be caused by several issues:
CORS (Cross-Origin Resource Sharing)
CORS blocks cross-origin requests when servers don't send proper CORS headers. Browsers block requests to different domains/protocols/ports unless servers explicitly allow them. CORS errors cause "Failed to fetch" before requests reach servers.
Example: Request from localhost:3000 to api.example.com blocked without CORS headers
HTTPS/SSL Issues
HTTPS errors occur when SSL certificates are invalid, expired, self-signed, or misconfigured. Browsers block insecure connections, causing fetch to fail. Mixed content (HTTP from HTTPS) also causes failures. HTTPS issues prevent secure connections.
Example: Invalid SSL certificate or HTTP request from HTTPS page causes fetch failure
Network Connectivity
Network issues (no internet, server down, firewall blocking, DNS failures, timeouts) prevent fetch requests from completing. When networks are unavailable or servers don't respond, fetch fails with "Failed to fetch" error.
Example: No internet connection, server offline, or request timeout causes fetch failure
Invalid URLs or Server Errors
Invalid URLs (typos, malformed, wrong protocol), unreachable servers, or server errors before response cause fetch to fail. When URLs are incorrect or servers don't respond, fetch cannot establish connections, resulting in "Failed to fetch".
Example: Wrong URL, server crash, or DNS resolution failure causes fetch error
Important: Understanding CORS, HTTPS/SSL, network connectivity, and URL/server issues is key to fixing "Failed to fetch" errors. The main causes are browser security (CORS), connection security (HTTPS), network availability, and server accessibility.
When: When This Error Occurs
"Failed to fetch" occurs in these situations:
Cross-Origin API Requests
When making requests from web pages to different domains, protocols, or ports, CORS policies block requests unless servers send proper CORS headers. CORS blocks cause "Failed to fetch" before requests complete, especially in local development.
HTTPS/SSL Certificate Issues
When APIs use HTTPS with invalid, expired, or self-signed certificates, browsers block connections for security. HTTPS errors cause fetch to fail immediately. Mixed content (HTTP from HTTPS) also triggers "Failed to fetch" errors.
Network Connectivity Problems
When internet connections are unavailable, servers are down, firewalls block requests, or requests timeout, fetch cannot complete. Network issues cause "Failed to fetch" errors when connections cannot be established.
Invalid URLs or Server Errors
When API URLs are incorrect, malformed, or point to unreachable servers, fetch fails immediately. Server crashes, DNS failures, or unreachable endpoints cause "Failed to fetch" before any response is received.
Common Scenario: "Failed to fetch" is most common in cross-origin requests (CORS), HTTPS connections with certificate issues, network connectivity problems, and invalid URLs. CORS is the primary cause in local development, while network and HTTPS issues are common in production.
How To: Fix "Failed to Fetch" Errors
Follow these methods to fix "Failed to fetch" errors:
Method 1: Fix CORS Errors
Configure CORS headers on your API server:
Express.js CORS Configuration
const express = require('express');
const cors = require('cors');
const app = express();
// Enable CORS for all routes
app.use(cors({
origin: 'https://yourdomain.com', // or ['http://localhost:3000'] for dev
methods: ['GET', 'POST', 'PUT', 'DELETE', 'OPTIONS'],
allowedHeaders: ['Content-Type', 'Authorization'],
credentials: true
}));
// Handle preflight requests
app.options('*', cors());Method 2: Handle Network Errors
Implement proper error handling for fetch requests:
Robust Fetch with Error Handling
async function fetchWithErrorHandling(url, options = {}) {
try {
const response = await fetch(url, {
...options,
signal: AbortSignal.timeout(10000) // 10 second timeout
});
if (!response.ok) {
throw new Error(`HTTP error! status: ${response.status}`);
}
return await response.json();
} catch (error) {
if (error.name === 'AbortError') {
throw new Error('Request timeout - server took too long to respond');
} else if (error.message === 'Failed to fetch') {
// Check specific causes
if (error.cause) {
console.error('Fetch error cause:', error.cause);
}
throw new Error('Network error - check internet connection and API availability');
} else {
throw error;
}
}
}
// Usage
try {
const data = await fetchWithErrorHandling('https://api.example.com/data');
console.log(data);
} catch (error) {
console.error('Fetch failed:', error.message);
// Show user-friendly error message
}Method 3: Fix HTTPS/SSL Issues
Resolve HTTPS and SSL certificate problems:
Handle HTTPS Errors
// For development with self-signed certificates
// Use HTTP instead of HTTPS, or configure valid SSL certificates
// Check if page is HTTPS
if (window.location.protocol === 'https:') {
// Ensure API also uses HTTPS (no mixed content)
const apiUrl = 'https://api.example.com/data';
} else {
// Development: can use HTTP
const apiUrl = 'http://localhost:8000/api/data';
}
// Verify SSL certificate is valid
// Use tools like SSL Labs to check certificate validity
// Ensure certificates are not expired
// Use Let's Encrypt for free valid certificatesMethod 4: Debug and Verify Requests
Use browser DevTools to debug fetch errors:
Debug Fetch Requests
// 1. Open Browser DevTools (F12)
// 2. Go to Network tab
// 3. Make fetch request
// 4. Check request details:
// - Request URL (is it correct?)
// - Request Method (GET, POST, etc.)
// - Request Headers (CORS, authentication)
// - Response Status (if any)
// - Response Headers (CORS headers)
// - Error message in console
// Check in console
fetch('https://api.example.com/data')
.then(response => {
console.log('Status:', response.status);
console.log('Headers:', response.headers);
return response.json();
})
.then(data => console.log('Data:', data))
.catch(error => {
console.error('Error type:', error.name);
console.error('Error message:', error.message);
console.error('Error stack:', error.stack);
});Best Practice: Always configure CORS properly, handle network errors with try-catch, verify HTTPS/SSL certificates, check URLs are correct, implement timeouts, provide user-friendly error messages, and use browser DevTools to debug fetch errors. Test API endpoints with curl or Postman to verify they work.
Why: Why "Failed to Fetch" Happens
"Failed to fetch" happens for these reasons:
Browser Security
Browsers enforce CORS policies and security restrictions to prevent malicious websites from making unauthorized requests. When servers don't send proper CORS headers, browsers block requests before they complete, causing "Failed to fetch" errors.
HTTPS Security
Browsers block insecure HTTPS connections when SSL certificates are invalid, expired, or self-signed. Mixed content (HTTP from HTTPS) is also blocked. HTTPS security prevents fetch from establishing connections, causing "Failed to fetch" errors.
Network Availability
When networks are unavailable, servers are down, or connections timeout, fetch cannot complete requests. Network issues prevent fetch from establishing connections or receiving responses, causing "Failed to fetch" errors before requests complete.
Request Validity
When URLs are invalid, malformed, or point to unreachable servers, fetch fails immediately. Invalid requests cannot be processed, causing "Failed to fetch" errors before any network activity occurs. URL validation prevents invalid requests.
Important: "Failed to fetch" happens due to browser security (CORS), HTTPS security (SSL), network availability, and request validity. The solution is to configure CORS properly, use valid SSL certificates, ensure network connectivity, and verify URLs are correct.
Frequently Asked Questions
What causes "Failed to fetch" error in JavaScript?
"Failed to fetch" occurs when fetch() API requests fail due to: CORS (Cross-Origin Resource Sharing) blocking requests, network connectivity issues, HTTPS/SSL certificate problems, invalid URLs, server not responding, request timeouts, or browser security restrictions. It's a generic error indicating the fetch request couldn't complete.
How do I fix CORS "Failed to fetch" error?
Configure your API server to send CORS headers: Access-Control-Allow-Origin, Access-Control-Allow-Methods, Access-Control-Allow-Headers. For development, use a proxy or CORS browser extension. For production, configure backend (Express.js, Flask) to allow cross-origin requests. Ensure preflight OPTIONS requests are handled correctly.
Why does fetch fail with HTTPS errors?
Fetch fails with HTTPS when: SSL certificates are invalid or expired, mixed content (HTTP from HTTPS page), self-signed certificates, certificate authority issues, or HTTPS misconfiguration. Browsers block insecure connections. Fix by using valid SSL certificates, avoiding mixed content, and configuring HTTPS properly.
How do I handle network errors in fetch?
Use try-catch blocks, check response.ok, handle different error types (network, CORS, timeout), implement retry logic, set appropriate timeouts, verify network connectivity, and provide user-friendly error messages. Use fetch with proper error handling: try { const response = await fetch(url); if (!response.ok) throw error; } catch (error) { handle error }.
How do I debug "Failed to fetch" errors?
Check browser DevTools Network tab for request details, verify URL is correct and accessible, check CORS headers in response, inspect console for specific error messages, test API with curl or Postman, verify network connectivity, check SSL certificates, and review server logs. Use response.status and error.message for debugging.