Oh, and from C++, there's also now a range iterator wrapper with similar semantics:

 for (auto& docShell : SimpleEnumerator<nsIDocShell>(docShellEnum)) {

On Thu, Aug 23, 2018 at 03:19:55PM -0700, Kris Maglione wrote:
As of bug 1484496, any C++-implemented nsISimpleEnumertor instance can be used as a JS iterator. And, when used this way, the iterators now have intrinsic type information, and therefore do not require QIing their elements.

Which is to say, now you can simply do:

for (let window of Services.wm.getEnumerator("navigator:browser")) {

for (let docShell of docShellEnum) {

rather than:

let winEnum = Services.wm.getEnumerator("navigator:browser");
while (winEnum.hasMoreElements()) {
  let window = winEnum.getNext();

while (docShellEnum.hasMoreElements()) {
  let docShell = winEnum.getNext().QueryInterface(Ci.nsIDocShell);

If you happen to be using an nsIArray from directly from JavaScript, you unfortunately still need to specify the expected types, since nsIArray has no idea what types it can contain:

for (let thing of array.enumerate(Ci.nsIThing)) {

Aside from being easier to maintain, these forms should also be somewhat faster than the old protocol, since they only require one XPConnect call per iteration rather than 3(+).


Kris Maglione
Senior Firefox Add-ons Engineer
Mozilla Corporation

There is no greater mistake than the hasty conclusion that opinions
are worthless because they are badly argued.
        --Thomas Huxley

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