How to pass parameters in $ajax POST?

I would recommend you to make use of the $.post or $.get syntax of jQuery for simple cases:

$.post('superman', { field1: "hello", field2 : "hello2"}, 
    function(returnedData){
         console.log(returnedData);
});

If you need to catch the fail cases, just do this:

$.post('superman', { field1: "hello", field2 : "hello2"}, 
    function(returnedData){
         console.log(returnedData);
}).fail(function(){
      console.log("error");
});

Additionally, if you always send a JSON string, you can use $.getJSON or $.post with one more parameter at the very end.

$.post('superman', { field1: "hello", field2 : "hello2"}, 
     function(returnedData){
        console.log(returnedData);
}, 'json');

Try using GET method,

var request = $.ajax({
    url: 'url',
    type: 'GET',
    data: { field1: "hello", field2 : "hello2"} ,
    contentType: 'application/json; charset=utf-8'
});

request.done(function(data) {
      // your success code here
});

request.fail(function(jqXHR, textStatus) {
      // your failure code here
});

You cannot see parameters in URL with POST method.

Edit:

Deprecation Notice: The jqXHR.success(), jqXHR.error(), and jqXHR.complete() callbacks are removed as of jQuery 3.0. You can use jqXHR.done(), jqXHR.fail(), and jqXHR.always() instead.

Jquery.ajax does not encode POST data for you automatically the way that it does for GET data. Jquery expects your data to be pre-formated to append to the request body to be sent directly across the wire.

A solution is to use the jQuery.param function to build a query string that most scripts that process POST requests expect.

$.ajax({
    url: 'superman',
    type: 'POST',
    data: jQuery.param({ field1: "hello", field2 : "hello2"}) ,
    contentType: 'application/x-www-form-urlencoded; charset=UTF-8',
    success: function (response) {
        alert(response.status);
    },
    error: function () {
        alert("error");
    }
}); 

In this case the param method formats the data to:

field1=hello&field2=hello2

---

The Jquery.ajax documentation says that there is a flag called processData that controls whether this encoding is done automatically or not. The documentation says that it defaults to true, but that is not the behavior I observe when POST is used.