Swap JavaScript minifier from UglifyJS to terser

[colinrotherham]

Enable minification of ES6+ features

Swaps UglifyJS with terser (forked version) as explained on the terser README.md

uglify-es is no longer maintained and uglify-js does not support ES6+.

terser is a fork of uglify-es that mostly retains API and CLI compatibility with uglify-es and uglify-js@3.

Related:

• Fix stray let in I18n preventing uglify (into main) #2870
• Add gulp-plumber to JavaScript compile task to log UglifyJS issues #2872
• Run browser tests on minified code #3050

[romaricpascal]

Besides the sourcemap being missing, looks like Uglify was quite "aggressive" in its minification with things like:

• inlining function body that were used once, like normaliseString
• reusing variables, like in Tabs.prototype.getHref

Not really a concern, more of an observation and something we may want to look into in the future to see how far we can minify things again tweaking terser's options.

I think I'd me more confident for these tweaks to happen after we run our review app against the minified files, as you proposed. Which actually would give us safety to merge this PR as well, so maybe we could look into making the review app use a minified file and then merge this one?

[36degrees]

What are the things that we're worried about with this change?

As this only affects dist, I can't immediately see any reason this needs to wait until v5.0 as long as browser support doesn't change and the file size isn't massively increased.

[colinrotherham]

@36degrees Added a CHANGELOG entry under Fixes

Fixes

We've fixed errors in IE8 caused by updates to our JavaScript minifier UglifyJS. The issue was limited to the release-v4.4.1.zip and release-v4.4.0.zip assets on GitHub releases and prevented some polyfills from running:

• #3136: Enable UglifyJS compatibility workarounds
• #3013: Swap JavaScript minifier from UglifyJS to terser

Too much info?

[36degrees]

LGTM, but worth checking with @claireashworth.

Might be worth using the term 'precompiled files' or 'precompiled JavaScript' as part of the description to match the terms we use in our install docs?

[colinrotherham]

Thanks @36degrees I'll hold on for @claireashworth

I like 'precompiled JavaScript' but which bit would you swap out?

[claireashworth]

This looks fine to me. You could shuffle the text around to mention the polyfills not running sooner, but this para is short so not an essential change.

edit: I assume Ollie's suggestion would be here - We've fixed errors in IE8 caused by updates to our precompiled JavaScript...

[colinrotherham]

@claireashworth Fab, I've pushed again

@36degrees I've switched to JavaScript minifier UglifyJS "precompiled JavaScript" as all the necessary info on both UglifyJS and terser can be found in the PRs—that alright?

Fixes

We've fixed errors in IE8 caused by updates to our precompiled JavaScript. The issue prevented some polyfills from running but was limited to the release-v4.4.1.zip and release-v4.4.0.zip assets on GitHub releases:

• #3137: Enable UglifyJS compatibility workarounds
• #3013: Swap JavaScript minifier from UglifyJS to terser