You can do something like

for(var k in result) {
   console.log(k, result[k]);
}

which loops over all the keys in the returned json and prints the values. However, if you have a nested structure, you will need to use

typeof result[k] === "object"

to determine if you have to loop over the nested objects. Most APIs I have used, the developers know the structure of what is being returned, so this is unnecessary. However, I suppose it's possible that this expectation is not good for all cases.

var arr = [ {"id":"10", "class": "child-of-9"}, {"id":"11", "class": "child-of-10"}];
    
for (var i = 0; i < arr.length; i++){
  document.write("<br><br>array index: " + i);
  var obj = arr[i];
  for (var key in obj){
    var value = obj[key];
    document.write("<br> - " + key + ": " + value);
  }
}

note: the for-in method is cool for simple objects. Not very smart to use with DOM object.

obj.users.forEach(function(user) {
  console.log(user.username);
});

With a basic for loop:

for (var i = 0; i < obj.users.length; i++) {
    var user = obj.users[i];
    console.log(user.username);
}

Your data should already be a javascript array because you've specified the JSON type for the jQuery Ajax call so it should have already parsed the JSON into javascript. As such, you can just directly iterate it as the array:

success: function (data) {
    for (var i = 0; i < data.length; i++) {
        var checkBox = "<input type='checkbox' data-price='" + data[i].Price + "' name='" + data[i].Name + "' value='" + data[i].ID + "'/>" + data[i].Name + "<br/>";
        $(checkBox).appendTo('#modifiersDiv');
    }
    $('#addModifiers').modal('show');
}

Or, if you want to use jQuery's .each() iterator instead of a for loop, you can do this:

success: function (data) {
    $.each(data, function(key, item) {
        var checkBox = "<input type='checkbox' data-price='" + item.Price + "' name='" + item.Name + "' value='" + item.ID + "'/>" + item.Name + "<br/>";
        $(checkBox).appendTo('#modifiersDiv');
    });
    $('#addModifiers').modal('show');
}