What does the url /echo/json/ in this jsfiddle do?

Question

I have no confusion about the code below, I just do not understand what kind of url this is: /echo/json/.

See this jsfiddle.

You can see that data getting posted to this url /echo/json/ but this url probably does not exist. So tell me how this url /echo/json/ is working.

$.ajax({
  type: 'POST',
  url: '/echo/json/',
  data: {
    json: ko.toJSON(entries),
    delay: .1
  },
  success: function(entries) {
    ko.utils.arrayForEach(entries, function(entry) {
      viewModel.items.push(entry);
    });
    viewModel.pendingRequest(false);
  },
  error: function() {
    viewModel.pendingRequest(false);
  },
  dataType: 'json'
});

The only issue I have is about this url /echo/json/, I'd like to know how it is working.

Answer 1

As @nemesv pointed out in the comments, this is a feature of JSFiddle.

The JSFiddle docs provide these URLs (/echo/html, /echo/json, /echo/jsonp, /echo/xml, and /echo/js) as stubs which simply echo back the given data. You can use this to emulate a response with an optionally-specified delay, which would be useful for testing a nonexistent REST action or AJAX call handler of some sort, for example.

The format for using the JSON URL from raw Javascript is as follows (ripped from their example page):

new Request.JSON({
    url: '/echo/json/',
    data: {
        json: JSON.encode({
            text: 'some text',
            array: [1, 2, 'three'],
            object: {
                par1: 'another text',
                par2: [3, 2, 'one'],
                par3: {}
            }
        }),
        delay: 3
    },
    onSuccess: function(response) {
        show_response(response, $('post'));
    }
}).send();