breaking change: dropping support for EcmaScript5

[sigmundch]

Two years ago we announced a plan to deprecate support for ES5. Changes in priorities made us delay our plans until now.

In an upcoming Dart release (likely 2.14), dart2js will start generating code using ES6 features by default.

What can break?
Apps will stop running in old browsers like IE11 that don't support these ES6 features.

Modern browsers will not be affected, dart2js continues to support the last two major releases of Edge, Safari, Firefox, and Chrome.

Mitigation
Users that continue needing support on legacy browsers have two options:

• Translate the output from dart2js using a tool like BabelJs.
• Delay migrating to Dart releases that lack ES5 support until they find a viable a migration path. The caveat here is that we only provide bug fixes to the most recent SDK version.

For a short period we will also provide a --legacy-javascript flag that removes the modern ES6 features. The intend of this flag is to make it easy to opt-out in case bugs are discovered after the default changes, but this flag will not be kept around long term and will likely be removed one or two stable releases after this first breaking change.

@Markzipan @franklinyow @vsmenon @mit-mit @kevmoo

@grouma @Hixie - for approvals

[vsmenon]

lgtm

[sigmundch]

@grouma @Hixie - friendly ping for approvals

[Hixie]

What percentage of active web users will remain supported after this change?

[sigmundch]

We are not aware of any apps that currently rely on IE11 support anymore. @kevmoo @mariamhas - do you have any data available to verify this or data to address @Hixie's question?

FWIW, the change would align our compiler support with the messaging about supported browsers in Dart, flutter web, and internal Dart web apps.

[kevmoo]

According to Google's properties, MSIE is now 5x less popular than Opera. It's usage is dropping by >50% m/m

…

On Tue, Jul 13, 2021 at 11:33 AM sigmundch ***@***.***> wrote: We are not aware of any apps that currently rely on IE11 support anymore. @kevmoo <https://github.com/kevmoo> @mariamhas <https://github.com/mariamhas> - do you have any data available to verify this or data to address @Hixie <https://github.com/Hixie>'s question? FWIW, the change would align our compiler support with the messaging about supported browsers in Dart <https://dart.dev/faq#q-what-browsers-do-you-support-as-javascript-compilation-targets>, flutter web <https://flutter.dev/docs/development/platform-integration/web#which-web-browsers-are-supported-by-flutter>, and internal Dart web apps. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#46545 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAAEFCXESLS6KICR6IXC2ALTXSBGVANCNFSM47VV24KQ> .

[Hixie]

That's not quite what I asked. To evaluate this proposal I am interested in comparing the fraction of users we current support, and the fraction of users we would support after this change.

[vsmenon]

I'm not aware of any data to that affect. We've announced and not gotten any push back.

Do we have the means to collect hard data? E.g., number of app users running with IE?

[Hixie]

Sorry, I meant web users in general. As in, what percentage of web users use browsers that we would still support, vs what percentage use browsers we used to support but would no longer support plus what percentage of users use browsers we already don't support.

[vsmenon]

I don't believe this changes from a Flutter perspective. Flutter doesn't support IE, and apps I've just tested (e.g., gallery.flutter.dev) choke in IE on the JS in index.html (not dart2js output) from template:

https://github.com/flutter/flutter/blob/76d5e62f42dffe3aba42bba9364c4469af1ff332/packages/flutter_tools/templates/app_shared/web/index.html.tmpl#L58

The number of non-Flutter web users is small, and we've checked with specific teams.

[robrbecker]

Workiva is probably one of the largest dart for web users (non flutter). This change is ok for us.

…

On Tue, Jul 13, 2021, 1:14 PM vsmenon ***@***.***> wrote: I don't believe this changes from a Flutter perspective. Flutter doesn't support IE, and apps I've just tested (e.g., gallery.flutter.dev) choke in IE on the JS in index.html (not dart2js output) from template: https://github.com/flutter/flutter/blob/76d5e62f42dffe3aba42bba9364c4469af1ff332/packages/flutter_tools/templates/app_shared/web/index.html.tmpl#L58 The number of non-Flutter web users is small. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#46545 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AACM32WD7UTWERETDFHE2XLTXSGBXANCNFSM47VV24KQ> .

[grouma]

LGTM.

[gmpassos]

FYI:

Browser Support for ES6 (2015)

Browser	Version	Date
Chrome	51	May 2016
Firefox	52	Mar 2017
Edge	14	Aug 2016
Safari	10	Sep 2016
Opera	38	Jun 2016

• Internet Explorer does not support ECMAScript 2015.

Source: https://www.w3schools.com/js/js_versions.asp

[sigmundch]

@Hixie - let us know if @vsmenon comments addressed your concerns.

To provide some additional context, we have customers that don't expect their users to be on the latest versions of our supported browsers. For example, we know of a decent volume of users in browsers as old as Chrome 70.

We are evaluating each JS feature individually and advancing cautiously. At this time, we are starting with Symbol and the ES6 method syntax.

[franklinyow]

@Hixie Ping

[msoham123]

This is looking promising, what are the performance implications of this? Faster loading times? Smaller payload sizes?

[SteveAlexander]

@Hixie

My estimate based on the data sources below: around 8% of desktop browser usage is currently unable to use ES6.

Desktop vs mobile is split about 50/50. To estimate mobile will need some data about mobile browser versions or mobile OS versions (seeing as iOS makes you use webkit).

This estimate is based on data from the following data sources:

1. Worldwide browser version market share may-july 2021 (see CSV file)

https://gs.statcounter.com/browser-version-market-share#monthly-202105-202107-bar

I have no idea how reliable this data source is.

2. Based on that file, excluding mobile browsers, and for the remainder, figuring out which support ES6. The following resources were helpful:

https://github.com/browserslist/map-to-chrome
https://caniuse.com/?search=es6

[vsmenon]

@Hixie @ferhatb - we're still waiting on a green light on the flutter side. Any further concerns?

We're seeing a modest code size decrease (~3%) on one of our large internal web apps. Per @sigmundch 's comment, this is with fairly conservative use of ES6.

[Hixie]

Cutting out 8% of the potential audience seems a bit drastic, and is more than we cut off for Android and iOS. (For context, that's about 320,000,000 people and would like cutting out the entire population of the US.)

[vsmenon]

@Hixie - Flutter already doesn't support these browsers on the Web (independent of dart2js):

https://flutter.dev/docs/development/platform-integration/web#which-web-browsers-are-supported-by-flutter

E.g., I wasn't able to run Gallery on IE11 independent of this change.

Are you suggesting that Flutter support these older browsers?

[msoham123]

@vsmenon Right, so what you are saying is that the only audience that is (potentially) being cut off is non-flutter dart users on older platforms. With that in mind, I feel like this is a reasonable trade-off (3% size decrease for dropping support for a browser replaced by the first generation of edge in 2015, 8 years ago).

[Hixie]

Certainly on the long term we'd want to support more browsers than we do now, but I don't know when that would happen. We have our hands full dealing with the browsers we officially support as it is.

That said, given https://dart.dev/faq#web-general this seems like something that shouldn't need any kind of approval. We should just judge this mechanically based on our stated policy.

[kevmoo]

To pile on here: https://aws.amazon.com/blogs/aws/heads-up-aws-support-for-internet-explorer-11-is-ending/

(Timely)

[vsmenon]

Thanks, all! Marking this as approved.

[gmpassos]

FYI:

End of IE support by Microsoft 365 online services:

https://techcommunity.microsoft.com/t5/microsoft-365-blog/microsoft-365-apps-say-farewell-to-internet-explorer-11-and/ba-p/1591666

"the Internet Explorer 11 desktop application will be retired and go out of support on June 15, 2022":

https://blogs.windows.com/windowsexperience/2021/05/19/the-future-of-internet-explorer-on-windows-10-is-in-microsoft-edge/

[bernaferrari]

Should this be closed now that it is in CHANGELOG?

[sigmundch]

Yes thank you!

The change has now landed and will be part of the Dart 2.14 stable release. The --legacy-javascript flag is still available to revert to the old behavior, but we expect to remove that flag in 2.15.

[imcss4html]

I have practiced it, but the effect is not good. FlutterWeb main.dart.js just reduced from 1.7M to 1.6M. any plan for continuous improvement, or am I using the wrong method?