How do I send a cross-domain POST request via JavaScript?

javascript

cross-domain-requests

cors

javascript-apis

byAlex Kataev·Dec 12, 2024

// Fetch API: a modern, efficient solution for HTTP requests, like the avocado toast of JavaScript
fetch('http://example.com/api/resource', {
    method: 'POST', // It's a post man, so POST it!
    headers: { 'Content-Type': 'application/json' }, // Just tell them, we're speaking JSON
    body: JSON.stringify({ key: 'value' }), // Here's the stuff you want to send, neatly packaged
    credentials: 'include' // Send your cookies along, just don't eat them all!
})
.then(response => response.json()) // We got a response! Let's see what's in the box
.then(data => console.log(data)) // All the treasure from the open box
.catch(error => console.error('Error:', error)); // Oh Oh! Let's see what went wrong

Essentials: Fetch API gives an elegant, promise-based alternative to run cross-domain POST requests. The credentials: 'include' bit is critical if you're managing cookies and session data. Do make sure your server is CORS-compliant and can setup apt response headers. This is a standalone, ready-to-use example!

Preparing CORS on the server side

To have a smooth cross-domain POST request, ensure your server is configured the right way.

Set the Access-Control-Allow-Origin header. This dictates who gets access to the resources, like a bouncer at a club.
Station ‘HTTP method cops’ with the Access-Control-Allow-Methods header. They'll handle the permissible HTTP methods.
Gear up for pre-flight requests and process the OPTIONS method correctly.
Use Access-Control-Allow-Origin with specific and trusted domains for better security. No free passes for everyone!

Dealing with Old Internet Explorer versions

For older versions of IE, use XDomainRequest. Here are some additional pointers:

Make use of window.postMessage for secure cross-domain communication. It's the secret handshake among websites.
To send form data, use an iframe. Set the action attribute with the target URL for the POST request.
Embed hidden input elements in the iframe to securely transfer data, like passing a secret note under the desk.

Aspects for a Secure Cross-domain POST Request

Security is paramount when you're letting requests travel across domains.

Be careful to parse and verify server responses asynchronously. It's like checking the contents of a suitcase at customs.
Avoid Access-Control-Allow-Origin: * as it's wide open like an unlocked door.
Not all mobile browsers treat cross-domain POST requests in the same way, so test thoroughly.
In certain cases, you might need to use libraries like EasyXDM or postMessage API.
When CORS isn't available and you're making read-only requests, consider JSONP as a last resort alternative.

Troubleshooting your cross-domain POST request

Facing issues while handling your cross-domain POST requests? Here are some troubleshooting tips:

Pre-flight requests

The browser sends a trial balloon (OPTIONS method) to test the waters before making the actual request. It checks if the server allows the specific types of requests. Prepare your server to process these.

Processing server responses

After the POST request is made, processing the response correctly is key. Write your code to handle errors and to asynchronously process the data.

Handling variant content types

Our examples have application/json content type, but you might work with other types such as multipart/form-data or text/plain. Alter your headers and data processing logic according to the content type.

Final Note

This guide will help you make cross-domain POST requests in Javascript using best practices and with security in mind. Make sure to understand and follow all steps for optimal results.