Recognizing CORS Errors in Postman, but Not in jQuery

Why Do CORS Errors Occur in Browsers?

When using JavaScript to connect to a RESTful API, you may receive the error "No 'Access-Control-Allow-Origin' header is present on the requested resource". This error is usually caused by the browser's Same Origin Policy, which prevents web pages from making requests to domains other than the one that supplied the page.

Interestingly, when the same request is made using a tool like Postman, no such error happens. This distinction can be confusing for developers. Understanding why CORS blocks XMLHttpRequest or retrieve calls in the browser but not Postman is critical for debugging and designing safe web apps.

Solving CORS Issues with JavaScript Using Flask Backend

JavaScript Frontend Code with jQuery

// JavaScript (jQuery) frontend script
$(document).ready(function() {
  $("#loginButton").click(function() {
    $.ajax({
      type: 'POST',
      dataType: 'json',
      url: 'http://localhost:5000/login',
      data: JSON.stringify({
        username: 'user',
        password: 'pass'
      }),
      contentType: 'application/json',
      crossDomain: true,
      xhrFields: {
        withCredentials: true
      }
    }).done(function(data) {
      console.log('Login successful');
    }).fail(function(xhr, textStatus, errorThrown) {
      alert('Error: ' + xhr.responseText);
    });
  });
});

Setting up CORS in Flask.

Python Backend Code With Flask

# Python (Flask) backend script
from flask import Flask, request, jsonify
from flask_cors import CORS

app = Flask(__name__)
CORS(app, supports_credentials=True)

@app.route('/login', methods=['POST'])
def login():
    data = request.get_json()
    username = data.get('username')
    password = data.get('password')

    if username == 'user' and password == 'pass':
        return jsonify({'message': 'Login successful'}), 200
    else:
        return jsonify({'message': 'Invalid credentials'}), 401

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000)

Setting up CORS in Flask.

Python Backend Code With Flask

# Python (Flask) backend script
from flask import Flask, request, jsonify
from flask_cors import CORS

app = Flask(__name__)
CORS(app, supports_credentials=True)

@app.route('/login', methods=['POST'])
def login():
    data = request.get_json()
    username = data.get('username')
    password = data.get('password')

    if username == 'user' and password == 'pass':
        return jsonify({'message': 'Login successful'}), 200
    else:
        return jsonify({'message': 'Invalid credentials'}), 401

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000)

Why do CORS issues occur in browsers but not in Postman?

One important feature to grasp about Cross-Origin Resource Sharing (CORS) is the security method that browsers utilize to protect users. Browsers enforce the Same-Origin Policy, which stops web pages from making requests to domains other than those that served them. This is a security feature that prevents rogue websites from accessing sensitive information on other websites using JavaScript. When making an XMLHttpRequest or fetch request from a browser, it checks for the header in the server's response. If this header is missing or does not allow the requesting origin, the browser will deny the request and return a CORS error.

In contrast, Postman is an API testing tool rather than a browser. It does not enforce the Same-Origin Policy because it does not run in the browser. As a result, it does not perform the same security checks and permits unrestricted requests to any domain. This is why making the same call through Postman does not result in CORS difficulties. Understanding the difference is critical for developers who want to successfully troubleshoot and resolve CORS-related issues. By configuring the server to contain the necessary CORS headers, you can ensure that your web application communicates with external APIs safely and without mistake.

1. What is CORS?
2. CORS stands for Cross-Origin Resource Sharing, which is a method that permits restricted resources on a web page to be requested from a domain other than the one from which the resource originated.
3. Why do browsers enforce the Same-origin Policy?
4. The Same-Origin Policy is implemented to protect users' data and prevent fraudulent websites from accessing sensitive information from other domains using JavaScript.
5. Why does Postman not enforce CORS?
6. Postman does not enforce CORS because it is not a browser and hence does not need to follow the Same-Origin Policy.
7. How can I fix a CORS problem in my online application?
8. To fix a CORS error, set the server to add the necessary header in the response, which allows the requesting origin.
9. What does the header accomplish?
10. The header determines which sources can access the resource, allowing cross-origin requests.
11. What is the use of in XMLHttpRequest?
12. The attribute determines whether cross-site Access-Control requests should be made using credentials like cookies or authorization headers.
13. Why do I receive a CORS error when my server contains the header?
14. You may still get a CORS error if additional essential CORS headers, such as or , are missing or wrongly configured.
15. Can I turn off CORS in my browser?
16. Disabling CORS in a browser is not recommended as it jeopardizes security. Instead, set up your server to handle CORS appropriately.
17. What exactly is a pre-flight request in CORS?
18. A preflight request is an initial request sent by the browser via the OPTIONS method to determine whether the actual request is safe to submit. It looks for needed CORS headers on the server.

Understanding the subtleties of CORS and Same Origin Policy is critical for web developers. While browsers utilize strict security protections to protect users, programs like Postman circumvent these restrictions, making it easier to test APIs. By properly configuring the backend with the necessary CORS headers, developers can ensure that the frontend and backend communicate smoothly and securely. Addressing CORS concerns is critical for developing functional and usable online apps.