I’m connecting to a Flask-based RESTful API using JavaScript, and I keep hitting the ‘No Access-Control-Allow-Origin’ header is present on the requested resource error. However, the same request works just fine in Postman. I understand this is related to CORS, but why does the browser enforce this while Postman doesn’t? What’s the technical reason behind this behavior difference?
I’ve been working with front-end integrations for several years, and I’ve run into this “no ‘access-control-allow-origin’ header is present on the requested resource” error more times than I can count. Basically, browsers enforce CORS (Cross-Origin Resource Sharing) strictly for security reasons mainly to stop malicious scripts from one origin messing with another origin’s resources. Postman, though, isn’t a browser and doesn’t live inside the same sandbox, so it doesn’t have those restrictions. That’s why the same request that fails in JavaScript in your browser works perfectly fine in Postman.
Totally relate, @emma-crepeau I’ve been building APIs and web apps for a few years too, and that “no ‘access-control-allow-origin’ header is present on the requested resource” error tripped me up early on. Just to add to what you said: when you use fetch or XMLHttpRequest in the browser, the browser automatically checks whether the server explicitly allows the origin by looking for the Access-Control-Allow-Origin header. If it’s missing, the browser stops your JavaScript code from even seeing the response. Postman skips this check entirely because it’s not confined by the browser’s security sandbox.
Great points, both of you! I’ve been explaining this for years while helping folks debug CORS headaches. I like to put it this way: Postman acts like a trusted client talking straight to your server, so it’s not blocked by CORS at all. Browsers, on the other hand, are super cautious, they don’t want JavaScript from site-a.com poking around on site-b.com unless site-b explicitly says, “yeah, it’s cool.” So unless your Flask API sets the proper headers (like with Flask-CORS or manually), you’ll keep seeing that “no ‘access-control-allow-origin’ header is present on the requested resource” error in the browser. But tools like Postman will still breeze through the same request because they’re outside the browser’s security model.