Question

Please, consider the code sample below, and focus on variable assignments. Since I have never seen such form in C++, what does the following mean: “upload” in new XMLHttpRequest`.

I would need a good explanation of what does the following statement mean: progress: "upload" in new XMLHttpRequest. Especially, the in is not in C++ present. What is that in supposed to do?

tests = {
  filereader: typeof FileReader != 'undefined',
  dnd: 'draggable' in document.createElement('span'),
  formdata: !!window.FormData,
  progress: "upload" in new XMLHttpRequest
};

thank you.

Answer 1

Chapter 11.8.7 – The in operator says

Return the result of calling the [[HasProperty]] internal method of rval with argument ToString(lval).

which means that

(lval in rval)

is true when rval is an object and it has a property named String(lval).

in is also used in for (... in ...) loops but that is just similar syntax, not a use of this operator.

---

"upload" in new XMLHttpRequest

This is asking “does an XMLHttpRequest instance have a property named ‘upload’?” It’s effectively checking whether this browser has a particular feature which might not be present on all browsers.

upload in particular is specified in XMLHttpRequest Level 2 as an object that supports certain event handler to let you monitor the progress of an upload:

interface XMLHttpRequestEventTarget : EventTarget {
  // event handlers
  [TreatNonCallableAsNull] attribute Function? onloadstart;
  [TreatNonCallableAsNull] attribute Function? onprogress;
  [TreatNonCallableAsNull] attribute Function? onabort;
  [TreatNonCallableAsNull] attribute Function? onerror;
  [TreatNonCallableAsNull] attribute Function? onload;
  [TreatNonCallableAsNull] attribute Function? ontimeout;
  [TreatNonCallableAsNull] attribute Function? onloadend;
};