JavaScript API request fails with CORS error, but succeeds in Postman. Why?

Other Questions JavaScript
1

I’m having trouble with a JavaScript API request. When I try to connect to my Flask-based RESTful API for authorization, I get an error about ‘Access-Control-Allow-Origin’ header missing. But when I use Postman, it works fine. What’s going on?

Here’s a simplified version of my code:

fetch('https://myapi.example.com/auth', {
  method: 'POST',
  credentials: 'include',
  headers: {
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    username: 'testuser',
    password: 'secretpass'
  })
})
.then(response => response.json())
.then(data => console.log('Success:', data))
.catch(error => console.error('Error:', error));

I know the API needs to set some headers, but I’m confused why Postman works and my code doesn’t. Can someone explain the difference? Is there a way to make my JavaScript behave more like Postman?

2

I’ve experienced similar CORS issues with Flask APIs before, and it really can be frustrating. The main difference with Postman is that it bypasses the browser’s security measures, like the Same-Origin Policy. When you run your JavaScript in a browser, CORS restrictions come into play. To resolve this, you can use the flask-cors extension. After installing it, you just need to initialize it (for example, by calling CORS(app)) so that the proper headers are set. Just remember to configure it securely for production use. Hope this helps!

3

sounds like a CORS issue mate. postman bypasses browser security, so it works fine. ur API needs to set proper CORS headers. try adding flask-cors to ur flask app and use @cross_origin decorator. that should fix it. good luck!

4

The issue arises because browsers enforce the Same-Origin Policy, which restricts JavaScript from making requests to a domain different from that which served the webpage. Postman does not enforce this policy, so it successfully makes the API call. To resolve this in your Flask application, you need to enable Cross-Origin Resource Sharing (CORS). One way to do this is by using the flask-cors extension. By installing flask-cors, importing it, and initializing it within your Flask app, you can ensure the necessary headers are set for cross-domain requests. Adjusting the CORS configuration further can improve the security for your production environment.