JSONP (JSON with Padding) is a technique that leverages the fact that <script> tags are not subject to the same-origin policy to enable cross-origin requests.
Since the <script> tag was originally designed for loading static JavaScript files, it is limited to the GET method for requesting resources and does not support POST requests. This is the fundamental reason why JSONP does not support POST requests.
When using JSONP for communication, you include the request parameters in the URL and send the request by dynamically creating a <script> tag.
Upon receiving a GET request, the server wraps the data in a function call and returns it as the response.
After the client defines the callback function, the JavaScript containing the JSON data is executed, and the callback function is invoked to process the returned data.
For example, suppose your page needs to retrieve some user data from http://example.com; you might send the following JSONP request:
<script type="text/javascript"> // Define callback function function handleResponse(data) { console.log('Received data:', data); } </script> <script type="text/javascript" src="http://example.com/data?callback=handleResponse"></script>
The server needs to receive the callback parameter and wrap the data in the function call:
// Server-side response handleResponse({ "user": "Alice", "age": 25 });
As mentioned above, a JSONP request is essentially a GET request initiated through the src attribute of the <script> tag. Therefore, it cannot utilize the POST method, which is typically used for transmitting large amounts of data or sending data that requires secure transmission. If you need to perform cross-origin POST requests, consider using more modern technologies such as CORS (Cross-Origin Resource Sharing), which allows cross-origin requests for various HTTP methods while providing better security.