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27.

Optimizing the "Commons" Chunk

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Here's how I like to configure the CommonsChunkPlugin. Set minChunks to Infinity. Then change the name option to layout:

115 lines | webpack.config.js
// ... lines 1 - 30
module.exports = {
// ... lines 32 - 93
plugins: [
// ... lines 95 - 105
new webpack.optimize.CommonsChunkPlugin({
// "layout" is an entry file
// anything included in layout, is not included in other output files
name: 'layout',
minChunks: Infinity
})
],
// ... line 113
};

That name is important. Scroll up: layout is the name of one of our entries!

115 lines | webpack.config.js
// ... lines 1 - 30
module.exports = {
entry: {
// ... lines 33 - 34
layout: './assets/js/layout.js',
},
// ... lines 37 - 93
plugins: [
// ... lines 95 - 105
new webpack.optimize.CommonsChunkPlugin({
// "layout" is an entry file
// anything included in layout, is not included in other output files
name: 'layout',
minChunks: Infinity
})
],
// ... line 113
};

Because of this, anything that is required in layout.js will not be included in other files. Yep, since layout.js requires jquery, it will live in the compiled layout.js, but not in login.js or rep_log.js:

12 lines | assets/js/layout.js
// ... lines 1 - 2
import $ from 'jquery';
import 'bootstrap-sass';
// ... lines 5 - 12

And since bootstrap-sass is used here, it also won't be output in any other built file.

Our layout.js file now serves two purposes. First, to collect all the common modules that we don't want duplicated across multiple entries. And second, as a way for us to execute any global JavaScript we might have.

Ok, find your Webpack terminal and stop it. Then, clear out the build directory:

rm -rf web/build/*

to delete that vendor.js file. Now, Restart webpack:

./node_modules/.bin/webpack --watch

Yep, no vendor.js. In base.html.twig, we can remove the extra vendor.js script tag:

104 lines | app/Resources/views/base.html.twig
// ... lines 1 - 94
{% block javascripts %}
// ... lines 96 - 97
<script src="{{ asset('build/vendor.js') }}"></script>
// ... line 99
{% endblock %}
// ... lines 101 - 104

Try it out: refresh! Yes! Everything works!

The Webpack Manifest

Look inside the built layout.js file. Right on top, you can see the Webpack bootstrap: a collection of functions that the rest of the built code use to help organize all the Webpack module-loading magic. Before we used CommonsChunkPlugin, this appeared at the top of every output file. But now - as you can see in the built login.js - it's not there anymore. Webpack is already smart enough to know that since layout.js is our common chunk and will be included on every page, the Webpack bootstrap code doesn't need to be repeated. Clever Bootstrap!

But... there's one small catch. Isn't there always one small catch? The Webpack bootstrap code includes something called the manifest. Internally, Webpack gives each module a number id, and the manifest contains those ids. Sometimes, those ids change.

Normally, we don't care! These are all internal Webpack details! But... if the module ids change... then the manifest changes... and that means that the contents of layout.js change.

Let me say it a different way: because of the module ids in the manifest, if I make a change to, say, login.js, it may cause the built layout.js file to change. Why is that a problem? Caching.

We're going to talk more about caching and versioning later. But for now, let me just say this: you don't want the contents of a built file to change unless it actually needs to change. Why make your user re-download a fresh layout.js file... if nothing important changed inside it?

Extracting the Manifest

Anyways, the fix is to move that Webpack bootstrap code out of layout.js... so it can happily remain unchanged even when the internal module ids change. To do that, open webpack.config.js and go to the CommonsChunkPlugin. Change the name option to an array, with layout and a new entry called manifest:

119 lines | webpack.config.js
// ... lines 1 - 30
module.exports = {
// ... lines 32 - 93
plugins: [
// ... lines 95 - 105
new webpack.optimize.CommonsChunkPlugin({
name: [
// ... lines 108 - 109
'layout',
// ... line 111
'manifest'
],
// ... line 114
})
],
// ... line 117
};

I'll move my first comment down a bit, and then add a new comment above manifest:

Dumps the manifest in a separate file.

119 lines | webpack.config.js
// ... lines 1 - 30
module.exports = {
// ... lines 32 - 93
plugins: [
// ... lines 95 - 105
new webpack.optimize.CommonsChunkPlugin({
name: [
// "layout" is an entry file
// anything included in layout, is not included in other output files
'layout',
// dumps the manifest into a separate file
'manifest'
],
// ... line 114
})
],
// ... line 117
};

Let's see what this does! Restart Webpack:

./node_modules/.bin/webpack --watch

Ok, we still have a layout.js file... but now we also have a tiny manifest.js! Thanks to this change, layout.js is just our code. And the tiny manifest.js contains the Webpack bootstrap. If the ids change now, the user will only need to re-download that small file.

Of course, to get this to work... we need to add another script tag! Open base.html.twig. Add a second script tag pointing to manifest.js:

104 lines | app/Resources/views/base.html.twig
// ... lines 1 - 94
{% block javascripts %}
// ... lines 96 - 97
<script src="{{ asset('build/manifest.js') }}"></script>
<script src="{{ asset('build/layout.js') }}"></script>
{% endblock %}
// ... lines 101 - 104

Phew! I know, that was confusing. But our setup rocks! Thanks to CommonsChunkPlugin, anything in layout.js, will not be duplicated in the other files.

Next! Let's learn about the webpack-dev-server.