I am new to Sinon mocking and trying to test a function

so my function is something like:
`save: function (data) {

var x = 10;
var y = [];

Deferred.when(
    app.ajax({
        type: 'GET',
        url: 'someurl/x/endpoint',
        success: function (response) {
            console.log("respsonse", response);
            if (!response.error) {
                y.push(response);
            }
        }
    })

).then(function (response1) {
    console.log("Printing 1");
    console.log("response1", response1);
});

}`

And test is:
`var x= {
“ID”:”10″
}

var data = {
“ID”:”10″
}

server.respondWith(‘GET’, “someurl/x/endpoint”, [
200,
{ ‘Content-Type’: ‘application/json’ },
JSON.stringify(x)
]);

save(data);`

The issue is when I am running the test I get

Printing 1

response1 undefined
ERROR: TypeError: undefined is not an object (evaluating ‘response1’)

I am using Sinon to mock and maybe going wrong somewhere.
I am trying to mock the GET AJAX call in order to take its response and proceed further.

Please can anyone guide me what’s the appropriate way to do so?