In my blog Cross domain request in ABAP and Java with two workaround I introduce the step how to deal with Cross Domain issue using Cross-origin resource sharing ( CORS ) supported by almost all modern browsers.

And there is another alternative for cross domain issue, that is JSONP which can work on legacy browsers which predate CORS support.
In this blog, I will first explain how to use JSONP and then introduce the secret behind it.

JSONP in nodeJS server

Suppose I have two employee ID lookup service hosted by the port 3000 and 3001 in my local server. The service will simply return employee name by ID.
The client web page is hosted in port 3000. According to same origin policy, the web page hosted in port 3000 is allowed to access the service hosted in localhost:3000, but forbidden for localhost:3001.
Let’s now do a verification.
This is my server listening to port 3000:
const express = require('express');  
const app = express();  
const port = 3000;

var path = require('path');
var repo = {
	"I042416": "Jerry",
	"I042417": "Tom",
	"I042418": "Jim"
}
app.use(express.static(path.join(__dirname, 'public')));

app.get('/request', (request, response) => {  
 console.log("The employee ID sent from Client:" + request.query.id);
  response.json({
    UserName: repo[request.query.id] + " ( handled in port 3000 )"
  });
});
app.listen(port, (err) => {  
  if (err) {
    return console.log('something bad happened', err)
  }
  console.log(`server is listening on ${port}`)
});
And this is my client page which allows end user to type the employee ID and send the query request:
<html>
<body>
<form action="">
  ID: <input type="text" id = "inumber" name="ID" value="I042416"><br>
  <input type="submit" value="Submit">
</form> 
</body>
<script src="jquery1.7.1.js"></script>
<script>
$(document).ready(function(){	
     $("form").click(function(e){
     	e.preventDefault();
        var data = {
            id: $("#inumber").val()
        };
        $.ajax({
            type: 'GET',
            data: data,
            url: 'http://localhost:3000/request',
            dataType: 'json',
            success: function(data) {
                alert(data.UserName);
            },
            error: function(jqXHR, textStatus, errorThrown) {
                console.log('error ' + textStatus + ' ' + errorThrown);
            }
        }); 
    });
});
</script>
</html>
When I click Submit button, I get query response returned from service in port 3000 as expected:
And this is the log output in the console of service in port 3000:
Now I make small modification on the web page in port 3000, forcing it to send request to port 3001 instead:
And resend the id query, this time I saw the expected cross domain error message:

How to resolve cross domain issue using JSONP

Both minor changes in client and server side are necessary.
In service working in port 3001, I add a new service end point “request_jsonp”:
app.get('/request_jsonp', (request, response) => {  
  console.log("This service supports JSONP now: " + request.query.id);
  var data = "{" + "UserName:'" + repo[request.query.id] + " ( handled in port 3001 )'"
  + "}";
  var callback = request.query.callback;
  var jsonp = callback + '(' + data + ');';
  response.send(jsonp);
  response.end();
});
In client web page, I change the send AJAX data type from json to jsonp, and inform server that “please parse the callback function name from literal “callback” in request header.
$.ajax({
            type: 'GET',
            data: data,
            url: 'http://localhost:3001/request_jsonp',
            dataType: 'jsonp',
            jsonp: 'callback',
            jsonpCallback: 'jsonpCallback',
            success: function(data) {
                alert(data.UserName);
            },
            error: function(jqXHR, textStatus, errorThrown) {
                console.log('error ' + textStatus + ' ' + errorThrown);
            }
        }); 
Now send the query again from localhost:3000 page, and the request could successfully reach service in localhost:3001, handled there and return to localhost:3000 again:

Magic behind JSONP

In fact, no magic at all. The mechanism of JSONP just utilize the “benefit” that the HTML <script> element is allowed to execute content retrieved from foreign origins.Let’s debug the working example to understand how it works.
When I send the AJAX call with data type jsonp, a new script element is created on the fly. The employee ID specified by end user is also appended as a query field in request header.
In server side, the response to this JSONP request is NOT json data, but a fragment of executable JavaScript code. I add a print statement to make it more clear:
Once this response returns to client side, it will get executed immediately there as a reaction to jsonp request sent from client.
For more detail explanation you can refer to Wikipedia How JSONP works.

JSONP in ABAP Server

Suppose I have a web page in system AG3/001 which would like to access service in AG3/815.
Create a ICF service in AG3/815:
AG3/001 has port 44354 and AG3/815 port: 44356
The web page is put in AG3/001 which has almost exactly the same code in previous nodeJS chapter.
This request fails as expected.
Now enhance the ICF handler class with JSONP support: return a string which contains executable JavaScript code instead:
 METHOD if_http_extension~handle_request.
    DATA(lv_userid) = server->request->get_form_field( 'id' ).
    DATA(lv_callback) = server->request->get_form_field( 'callback' ).
    DATA(lv_name) = COND #( WHEN lv_userid = 'I042416' THEN 'Jerry' ELSE 'Unknown').

    DATA(lv_response) = |\{ UserName:'{ lv_name }'\}|.
    DATA(lv_jsonp) = |{ lv_callback }({ lv_response });|.
    server->response->append_cdata(
                         data   = lv_jsonp
                         length = strlen( lv_jsonp ) ).
  ENDMETHOD.
And click on submit button of web page in AG3/001, this time it works:

Further reading

I have written a series of blogs which compare the language feature among ABAP, JavaScript and Java. You can find a list of them below:
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