Why does my JavaScript code receive a "No 'Access-Control-Allow-Origin' header is present on the requested resource" error, while Postman does not?

Understanding the two protocols: CORS and Same-Origin-Policy

The web's security infrastructure greatly depends upon the Same-Origin Policy (SOP). It stops web pages from making requests to a different domain from the one that served the web page. But CORS throws a lifeline, allowing controlled access across different origins by including specific HTTP headers from the server.

Remember that the cops of the web world, i.e., browsers, impose traffic rules (SOP) to protect user data and prevent malicious activities, while the cliff jumper, Postman, leaps off these restrictions.

Postman's superpowers and the browser's traffic rules

Postman, in the context of working with APIs, toes the line differently from browsers as it doesn't comply with CORS. It's an outsider to the browser, making it a rule-breaker, free to make unhampered requests. But remember, with great freedom comes great responsibility–it doesn't carry sensitive user information like a browser does, making it a perfect tool for experimenting with APIs but not emulating real-world client-side applications.

Getting your hands dirty with complex requests

Sometimes, a preflight request likes to gatecrash the party before the actual request. It uses the OPTIONS verb to place a long-distance call to the server to check its compatibility with specific sets of headers or HTTP methods. If the server responds well and doesn't drop the call, the browser dials the server again to place the real call.

Lock your doors: Security considerations

While Access-Control-Allow-Origin: * offers a convenient all-access pass, it's not recommended for production environments. It's like leaving all your doors and windows unlocked–any website can walk into your server, opening up the Pandora's box of security risks. Be a tough gatekeeper: Always accept requests from trusted domains only.

When you can't change servers: Alternatives and Workarounds

When modifying the server is as hard as locating a needle in a haystack, you need to be resourceful:

• CORS proxies such as cors-anywhere offer temporary relief, but remember they can cause delays and might not suit high traffic situations or when handling sensitive data.
• The server-side proxying: It's like hiring an assistant that acts as a middleman between your client and the external API.
• JSONP: This old-school hero, restricted to GET requests, though not recommended for modern applications, can still save the day occasionally.

Better be safe than sorry: Common pitfalls to avoid

Tread lightly and avoid these common traps that may lead to CORS errors:

• Failing to match Content-Type in AJAX calls: Always ensure that your request headers perfectly fit what the server expects, like a key into a lock.
• Missing OPTIONS method handling: For complex headers or HTTP methods, confirm the server responds correctly to preflight requests. It's like calling ahead to confirm the reservation at a restaurant.
• Misconfigured web server: Your servers, be it nginx or Apache, should be checked and double-checked for accurate CORS headers configuration.

Riding the CORS train for popular servers

Whether your weapon of choice is nginx, Apache, or Node.js, there's a way to handle CORS:

• In the world of nginx, slip add_header 'Access-Control-Allow-Origin' 'domain.com'; inside your location.
• For Apache enthusiasts, magic words are Header set Access-Control-Allow-Origin "domain.com" inside your .htaccess or conf file.
• In the Node.js universe, choose the cors package and configure to your heart's content.

Remember, testing with Postman offers convenience but always verify your code in the real-world browser environment. After all, even superheroes need to train under real-world conditions before saving the world!