Recent array method discussion reminded me of this addition in ES5. Trivial patch, in-browser users want it, out-of-browser users want it, about as no-risk a feature as they get -- nice driveby pickup for 3.6. We should skim the spec for other ten-minute features... Test fodder: assertEq(Array.isArray.length, 1); var empty = []; var fastSmall = [1]; var slow = []; slow.slow = 0; var slowSmall = []; slowSmall[999999] = 0; slowSmall.length = 0; var all = [empty, fastSmall, slow, slowSmall]; all.forEach(function(a) { assertEq(Array.isArray(a), true); }); assertEq(Array.isArray(), false); assertEq(Array.isArray({}), false); assertEq(Array.isArray(null), false); assertEq(Array.isArray(undefined), false); assertEq(Array.isArray(17), false); assertEq(Array.isArray("Array"), false); assertEq(Array.isArray(Math.PI), false); assertEq(Array.isArray(true), false); assertEq(Array.isArray(false), false);

Comment on attachment 394528 [details] [diff] [review] Patch Looks good.

Any more like this missing from the es5 buglist? Why not a traceable native? /be

I don't know if anything else is missing -- I've been too busy diving into Object.defineProperties and all those object meta-hooks at the moment to really look lately (and those are much orders of magnitude less trivial than this). I probably should remedy that, maybe I'll do a spec skim over the weekend at some point. As for why not a traceable native, 1) It's a driveby patch, didn't want to spend any more time on it than necessary. 2) We can do better than a traceable native; this really cries out for a bug 497173-based solution. 3) I think we get better ROI if I spend time on something like Object.defineProperties than if I spend time wringing out a couple dozen instructions' execution time for one method that nobody uses (yet). I'm working on a web-tech post on this at the moment, although to be honest I'm less up on the reasons why someone would want to know if a value is an array than I'd like to be. I'm probably going to lean a little on this URL for rationales and use cases for doing so, maybe ask a few people around Mozilla as well. No rush, especially since this wants to be on branch before anything would actually be published. http://thinkweb2.com/projects/prototype/instanceof-considered-harmful-or-how-to-write-a-robust-isarray/

Just askin', no sweat. Indeed bug 497173 approach could do a lot better than a traceable native! You might ask jresig, bgalbraith, and dalmaer about isArray use cases. /be

Draft post: http://web.mit.edu/jwalden/www/isArray.html (Mother Superior) jumping the gun a bit as this hasn't made 192 yet, but that'll happen eventually enough to make it worth posting this now...

Would appreciate it if someone would let me know when this officially lands on 1.9.2.

Is this going to land on 1.9.2?

I wonder why searching for "Array.isArray" in bugzilla (before I created bug 537548) returned 0 results. Sigh.

dev-doc under https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Global_Objects/Array/isArray

The eventual blog post following on comment 6 was made here: https://developer.mozilla.org/web-tech/2010/07/26/determining-with-absolute-accuracy-whether-or-not-a-javascript-object-is-an-array/