Cross-Origin Resource Sharing (CORS) with JavaScript and jQuery

Cross-Origin Resource Sharing (CORS) is a security feature implemented in web browsers to prevent web pages from making requests to a different origin (domain, protocol, or port) than the one the web page was loaded from. This tutorial will explain how CORS works, how to implement it with plain JavaScript and jQuery, and common pitfalls to avoid.

Introduction to CORS

CORS is a security feature that prevents malicious scripts from making unauthorized requests on behalf of the user. It does this by requiring the server to include specific headers in its response, indicating which origins are allowed to make requests.

When a web page makes a request to a different origin, the browser sends an OPTIONS request (also known as a preflight request) to the server before sending the actual request. The server responds with a set of headers that indicate what methods and headers are allowed.

Implementing CORS with Plain JavaScript

To make a CORS request with plain JavaScript, you can use the XMLHttpRequest object or the fetch API. Here is an example using the fetch API:

fetch('https://example.com/api/data', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({ name: 'John Doe' })
})
.then(response => response.json())
.then(data => console.log(data))
.catch(error => console.error(error));

Implementing CORS with jQuery

To make a CORS request with jQuery, you can use the ajax method. Here is an example:

$.ajax({
  type: 'POST',
  url: 'https://example.com/api/data',
  data: JSON.stringify({ name: 'John Doe' }),
  contentType: 'application/json',
  success: function(data) {
    console.log(data);
  },
  error: function(xhr, status) {
    console.error(xhr, status);
  }
});

Common Pitfalls

One common pitfall when making CORS requests with jQuery is that the Access-Control-Request-Headers header is not properly set. This can cause the request to fail even if the server includes the correct headers in its response.

To fix this issue, you need to ensure that the server includes the required headers in its response. Specifically, the Access-Control-Allow-Headers header should include the x-requested-with header.

Here is an example of how to set the Access-Control-Allow-Headers header on a server:

response = HttpResponse(json.dumps({'status': 'success'}))
response['Content-type'] = 'application/json'
response['Access-Control-Allow-Origin'] = '*'
response['Access-Control-Allow-Headers'] = 'x-requested-with, Content-Type'

return response

Another common pitfall is that the withCredentials flag is not set to true. This can cause the request to fail if the server requires credentials to be sent with the request.

To fix this issue, you need to add the xhrFields property to the ajax options and set withCredentials to true:

$.ajax({
  type: 'POST',
  url: 'https://example.com/api/data',
  data: JSON.stringify({ name: 'John Doe' }),
  contentType: 'application/json',
  xhrFields: {
    withCredentials: true
  },
  success: function(data) {
    console.log(data);
  },
  error: function(xhr, status) {
    console.error(xhr, status);
  }
});

Conclusion

In conclusion, CORS is an important security feature that prevents malicious scripts from making unauthorized requests on behalf of the user. To implement CORS with plain JavaScript and jQuery, you need to ensure that the server includes the correct headers in its response and that the client sets the Access-Control-Request-Headers header properly.

By following the examples and guidelines outlined in this tutorial, you should be able to make successful CORS requests with both plain JavaScript and jQuery.