Allow Access-Control-Allow-Origin header using HTML5 fetch API

web-development

cors

fetch-api

access-control-allow-origin

byNikita Barsukov·Dec 21, 2024

Solving CORS issues with fetch requires the Access-Control-Allow-Origin header in server responses. Here's how to use fetch with cors mode:

fetch('https://example.com/data', { mode: 'cors' })
  .then(response => response.json())
  .then(console.log) // print the output, feeling like an archaeologist unearthing hidden treasure
  .catch(console.error); // catch the errors, like a baseball pro in the outfield

Implement appropriate CORS headers server-side (here's an Express on Node.js example):

app.use((req, res, next) => {
  res.header('Access-Control-Allow-Origin', '*'); // Adjust the '*' like you're dialing the magic radio frequency
  next();
});

Note: In a more secure environment, replace "*" with your domain to avoid any unwanted guests at your party.

The why: Origins of CORS and fetch API

The Same-Origin Policy is integral when dealing with network requests from your web application using the fetch API. It helps to ensure that scripts can only make requests to the same origin or domain from where the script loaded, unless CORS headers offer an open invitation.

On the client side, set your fetch mode to 'cors' for proper etiquette with the CORS protocol. In times when responses are opaque and you're not processing the returned data, { mode: 'no-cors' } could serve as a plan B, with some strings attached.

Securing those endpoints

Detailed domain list

Applying Access-Control-Allow-Origin: * can simplify development, but it's less safe in a production setting. Instead, spell out the domains you allow access like a parent leaving instructions for the babysitter.

To allow, or not to allow, that is the question

Apply Access-Control-Allow-Methods and Access-Control-Allow-Headers intentionally to provide a sophisticated level of control by specifying which HTTP methods and headers should be invited to the party.

Middleware for dynamic settings

A web application with diverse CORS requirements benefits from middleware automating the setting of CORS headers. This ensures your backend maintains consistent diplomacy across response types and routes.

Local development and opaque responses

Local-cors-proxy for local testing

When developing locally, local-cors-proxy is your savior, acting as a bridge that lifts CORS, making local development as smooth as your morning coffee.

Handling opaque responses

When CORS headers on the server side aren't modification-friendly, { mode: 'no-cors' } might be your only choice. But keep in mind, opaque responses have certain restrictions and buddy-system requirements.

Troubleshooting and best practices

Server-specific CORS guide

Every backend technology has a unique way of configuring CORS headers. Keep this in mind and refer to server-specific guidance and documentation to prevent your configuration from acting like a square peg in a round hole.

Considering IP-Address-based access

Although using an IP address in fetch requests can sometimes sidestep domain-based CORS restrictions, this practice should be used with care, considering the potential security implications in selecting this pathway.

Customizing settings to environment

CORS configurations should be tailored according to the environment. Keep a balance of loose settings for development and strict settings for production. This way, your code enjoys both functionality and security at their respective parties.

Understand ALL consequences of '*' wildcard

Using '*' in Access-Control-Allow-Origin can expose your application to potential threats. If your software handles sensitive data or operations, handle this setting like a delicate experiment with volatile compounds.