使用jQuery.when（）来组合jquery Deferred的promise和jqXHR：为什么jqXHR的数据包含在数组中？

Question

请参阅jsFiddle：https://jsfiddle.net/dtds1n2t/2/¸

这是来自jsFiddle的代码：

var resultData = "result-data-string";

// Create a jQuery deferred
defA = $.Deferred();

// Make an ajax call, which creates a jqXHR object
defB = $.ajax({
  type: "POST",
  url: "/echo/json/",
  data: {
    json: JSON.stringify(resultData),
    delay: 1.5
  }
});

defA.done(function(data) {
  console.log("defA resolved");
  console.log(data);
  // data is resultData, as expected
});

defB.done(function(data) {
  console.log("defB resolved");
  console.log(data);
  // data is resultData, as expected
});


defAB = $.when(defA, defB);

defAB.done(function(aData, bData) {
  console.log("defAB resolved");
  console.log(aData);
  // aData is resultData, as expected
  console.log(bData);
  // bData is an array for some reason:
  // [ resultData, "success", jqXHR object? 
});

defA.resolve(resultData);

这是控制台输出：

defA resolved
result-data-string
defB resolved
result-data-string
defAB resolved
result-data-string
["result-data-string", "success", Object]

为什么bData在数组中的$ .when（）。done回调部分？

我的defA和defB可以是来自$ .ajax（）调用的jqXHR对象，也可以是$ .Deferred（）。promise（）对象。我希望$ .when（）。done回调以相同的格式接收数据，无论底层的promise / Deferred对象是如何创建的！