This one is super-useful. I'd rather have it standardized, in a slightly improved form that allows custom classes to easily be iterated.

A slightly different form, |for (val of obj)|, got standardized. To use it you define obj[@@iterator] as a function that returns an iterator object yielding values that are bound in sequence to val. @@iterator is -- very shortly -- going to be Symbol.iterator. Right now it's just "@@iterator", I think, as a temporary hack. So enabling symbol support in releases, and making Symbol.iterator work, are prerequisites to using for-of on objects other than those that we've already made iterable. Arrays are the most common already-iterable object right now.

Posted the site compatibility doc: https://www.fxsitecompat.com/en-US/docs/2015/for-each-in-loop-support-will-be-removed/

current telemetry result CONTENT | Nightly | Aurora | Beta | Release ---+---------+---------+---------+---------- 45 | 1 | 0 | 0 | 1 46 | 0 | 1 | 2 | 0 47 | 0 | 1 | 1 | 0 48 | 2 | 1 | 2 | 1 49 | 8 | 1 | 0 | 2 50 | 1 | 1 | 1 | - 51 | 3 | 7 | - | - 52 | 0 | - | - | - ADDON | Nightly | Aurora | Beta | Release ---+---------+---------+---------+---------- 45 | 133.27k | 645.85k | 230.41k | 649.69k 46 | 205.45k | 471.80k | 171.08k | 451.86k 47 | 192.48k | 279.69k | 157.39k | 658.98k 48 | 59.48k | 51.69k | 208.46k | 342.13k 49 | 2.88k | 82.57k | 118.06k | 422.39k 50 | 4.06k | 74.11k | 185.27k | - 51 | 4.21k | 71.60k | - | - 52 | 1.57k | - | - | - it's reducing on Nightly and aurora. it we also observe reduction on beta and release in next some cycles, we could remove it.

CONTENT | nightly | aurora | beta | release ----+------------+------------+------------+------------ 45 | 1 | 0 | 0 | 37 46 | 0 | 1 | 2 | 6 47 | 0 | 1 | 1 | 2 48 | 2 | 1 | 2 | 9 49 | 8 | 1 | 0 | 9 50 | 1 | 1 | 121 | 5 51 | 3 | 7 | 1 | 3 52 | 0 | 2 | 0 | 0 53 | 0 | 1 | - | 0 54 | 0 | - | - | - ADDONS | nightly | aurora | beta | release ----+------------+------------+------------+------------ 45 | 138460 | 650814 | 235841 | 650429 46 | 205489 | 471888 | 174660 | 453430 47 | 192617 | 283759 | 157472 | 680933 48 | 62149 | 51696 | 211752 | 408324 49 | 3341 | 83641 | 119269 | 433545 50 | 5024 | 74285 | 191778 | 514502 51 | 5825 | 73602 | 265392 | 131381 52 | 1657 | 120173 | 45066 | - 53 | 587 | 47502 | - | - 54 | 0 | - | - | - (note: latest versions don't have enough data to compare)

CONTENT | nightly | aurora | beta | release ----+------------+------------+------------+------------ 45 | 1 | 0 | 0 | 37 46 | 0 | 1 | 2 | 6 47 | 0 | 1 | 1 | 2 48 | 2 | 1 | 2 | 9 49 | 8 | 1 | 0 | 9 50 | 1 | 1 | 210 | 5 51 | 3 | 7 | 1 | 19 52 | 0 | 2 | 0 | 26 53 | 0 | 2 | 108 | 11 54 | 0 | 2 | 0 | 0 55 | 0 | 0 | 0 | - 56 | 0 | - | - | - ADDONS | nightly | aurora | beta | release ----+------------+------------+------------+------------ 45 | 138460 | 650827 | 235841 | 650431 46 | 205489 | 471902 | 174660 | 454734 47 | 192617 | 283759 | 157473 | 688471 48 | 62149 | 51732 | 212113 | 431219 49 | 3567 | 83641 | 119573 | 434457 50 | 5088 | 74294 | 191840 | 527190 51 | 5825 | 73668 | 267455 | 253480 52 | 1657 | 141589 | 91130 | 200836 53 | 881 | 96421 | 93702 | 262834 54 | 0 | 129100 | 89164 | 16238 55 | 0 | 80 | 6027 | - 56 | 0 | - | - | - (note: latest versions don't have enough data to compare)

We don't have to care about add-ons anymore post 57.

(In reply to Masatoshi Kimura [:emk] from comment #8) > We don't have to care about add-ons anymore post 57. do "XPCOM extensions" include "add-on SDK extensions" here?

(In reply to Tooru Fujisawa [:arai] from comment #9) > (In reply to Masatoshi Kimura [:emk] from comment #8) > > We don't have to care about add-ons anymore post 57. > > do "XPCOM extensions" include "add-on SDK extensions" here? Yes. All non-WebExtensions add-ons will not be supported since 57. And WebExtensions have banned versioned JavaScript since day one.

I believe content usage is enough low to kill this.

Comment on attachment 8894188 [details] Bug 1083470 - Disable E4X for-each in release. https://reviewboard.mozilla.org/r/165288/#review170750 Based on low count in normal content and discontinuation of old-style addons, I can't see why not.

Pushed by VYV03354@nifty.ne.jp: https://hg.mozilla.org/integration/autoland/rev/179deaedc512 Disable E4X for-each in release. r=luke

Posted the site compatibility note: https://www.fxsitecompat.com/en-CA/docs/2017/for-each-in-loop-support-has-been-removed/

https://hg.mozilla.org/projects/date/rev/179deaedc5123cd38c956f6f8fe3a319309b3135 Bug 1083470 - Disable E4X for-each in release. r=luke

Developer release notes: https://developer.mozilla.org/en-US/Firefox/Releases/57#JavaScript Reference page: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for_each...in Error page (the warning is removed now? page continues to serve migration help anyways): https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/For-each-in_loops_are_deprecated Compat data PR: https://github.com/mdn/browser-compat-data/pull/466