An added note: I checked the notes for the release, and it doesn't look like there were any breaking changes that concern us on the move from 3 to 4.

Size Change: -5.87 kB (0%)

Total Size: 819 kB

_{compressed-size-action}

An added note: I checked the notes for the release, and it doesn't look like there were any breaking changes that concern us on the move from 3 to 4.

Reference: https://github.com/kpdecker/jsdiff/blob/master/release-notes.md#v400---january-5th-2019

The new version is also slightly smaller and tree-shakeable

According to Bundlephobia, because it doesn't declare sideEffects: false, we're not able to take as much advantage of exports as we would otherwise. I wonder for these cases if we can forcibly override this for packages we assume to be safely without side-effects, but which don't declare so themselves. I see at webpack/webpack#6065 (comment) and by corresponding documentation we might be able to do this in Webpack. My naive attempt at this doesn't seem to make any difference, though it's unclear if I've made a mistake.

diff --git a/webpack.config.js b/webpack.config.js
index 340728cfe1..b2a8f962ee 100644
--- a/webpack.config.js
+++ b/webpack.config.js
@@ -5,7 +5,7 @@ const { DefinePlugin } = require( 'webpack' );
 const CopyWebpackPlugin = require( 'copy-webpack-plugin' );
 const postcss = require( 'postcss' );
 const { get, escapeRegExp, compact } = require( 'lodash' );
-const { basename, sep } = require( 'path' );
+const { basename, sep, resolve } = require( 'path' );
 
 /**
  * WordPress dependencies
@@ -54,6 +54,10 @@ module.exports = {
 	},
 	module: {
 		rules: compact( [
+			{
+				include: resolve( 'node_modules', 'diff' ),
+				sideEffects: false,
+			},
 			mode !== 'production' && {
 				test: /\.js$/,
 				use: require.resolve( 'source-map-loader' ),

With or without this change, I see 390kb for block-editor chunk. Still better than it is on master (395kb).

According to Bundlephobia, because it doesn't declare sideEffects: false, we're not able to take as much advantage of exports as we would otherwise.

Thanks for looking into this, @aduth!

It appears that the library has some more fundamental issues than just a missing sideEffects declaration. As far as I can tell, its index files aren't correctly generated for tree-shaking, with lib/index.es6.js (the module file) being a gigantic single export, and lib/index.js (the main file) possibly not being in the correct format for webpack to grok, even with sideEffects: false (I manually edited its package.json to be sure).

Here are some experiments, with sideEffects: false:

import { diffChars } from 'diff' -> 16.47KB / 6.44KB gzipped
import { diffChars } from 'diff/lib/index.js' -> 19.57KB / 6.12KB gzipped
import { diffChars } from 'diff/lib/diff/character' -> 2.64KB / 1.16KB gzipped

The latter doesn't need sideEffects: false, so I've switched to it in the PR, which means we get some nice savings 🙂

Rebased on master to see if the github-actions size results make more sense. I assume that it's comparing vs master rather than comparing vs the branching point of the PR?

Edit: Looking at the updated results, that does appear to be the case. They make a lot more sense now.

I'm not sure why the Build artifacts check is complaining that there are uncommitted changes; neither npm install nor npm run docs:build seem to produce any changed files for me.

I'm not sure why the Build artifacts check is complaining that there are uncommitted changes; neither npm install nor npm run docs:build seem to produce any changed files for me.

I can see local changes, but only after first removing node_modules.

diff --git a/package-lock.json b/package-lock.json
index 172654b86d..5905049454 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -31146,9 +31146,9 @@
 			}
 		},
 		"mkdirp-classic": {
-			"version": "0.5.2",
-			"resolved": "https://registry.npmjs.org/mkdirp-classic/-/mkdirp-classic-0.5.2.tgz",
-			"integrity": "sha512-ejdnDQcR75gwknmMw/tx02AuRs8jCtqFoFqDZMjiNxsu85sRIJVXDKHuLYvUUPRBUtV2FpSZa9bL1BUa3BdR2g==",
+			"version": "0.5.3",
+			"resolved": "https://registry.npmjs.org/mkdirp-classic/-/mkdirp-classic-0.5.3.tgz",
+			"integrity": "sha512-gKLcREMhtuZRwRAfqP3RFW+TK4JqApVBtOIftVgjuABpAtpxhPGaDcfvbhNvD0B8iD1oUr/txX35NjcaY6Ns/A==",
 			"dev": true
 		},
 		"mkdirp-promise": {

@aduth I had the same issue, I commited this change to master

Thanks, @youknowriad! I rebased on master again to see if it picks up your changes.

Everything looks green now 👍

Since I'm working around tree-shaking issues as well now, I'm re-requesting a review in case you want to take another look, @aduth

Thank you for the reviews, @aduth! 👍