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rewrite docs for page-specific-javascript

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Mike Greiling 2018-03-12 15:19:19 -05:00
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doc/development/fe_guide/performance.md
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@ -49,39 +49,95 @@ properties once, and handle the actual animation with transforms.
### Page-specific JavaScript
Certain pages may require the use of a third party library, such as [d3][d3] for
the User Activity Calendar and [Chart.js][chartjs] for the Graphs pages. These
libraries increase the page size significantly, and impact load times due to
bandwidth bottlenecks and the browser needing to parse more JavaScript.
Webpack has been configured to automatically generate entry point bundles based
on the file structure within `app/assets/javascripts/pages/*`. The directories
within the `pages` directory are meant to correspond to Rails controllers and
actions. These auto-generated bundles will be automatically included on the
corresponding pages.
In cases where libraries are only used on a few specific pages, we use
"page-specific JavaScript" to prevent the main `main.js` file from
becoming unnecessarily large.
For example, if you were to visit [gitlab.com/gitlab-org/gitlab-ce/issues](https://gitlab.com/gitlab-org/gitlab-ce/issues),
you would be accessing the `app/controllers/projects/issues_controller.rb`
controller with the `index` action. If a corresponding file exists at
`pages/projects/issues/index/index.js`, it will be compiled into a webpack
bundle and included on the page.
Steps to split page-specific JavaScript from the main `main.js`:
> **Note:** Previously we had encouraged the use of
> `content_for :page_specific_javascripts` within haml files, along with
> manually generated webpack bundles. However under this new system you should
> not ever need to manually add an entry point to the `webpack.config.js` file.
1. Create a directory for the specific page(s), e.g. `graphs/`.
1. In that directory, create a `namespace_bundle.js` file, e.g. `graphs_bundle.js`.
1. Add the new "bundle" file to the list of entry files in `config/webpack.config.js`.
- For example: `graphs: './graphs/graphs_bundle.js',`.
1. Move code reliant on these libraries into the `graphs` directory.
1. In `graphs_bundle.js` add CommonJS `require('./path_to_some_component.js');` statements to load any other files in this directory. Make sure to use relative urls.
1. In the relevant views, add the scripts to the page with the following:
In addition to these page-specific bundles, the code within `main.js` and
`commons/index.js` are imported on _all_ pages.
```haml
- content_for :page_specific_javascripts do
= webpack_bundle_tag 'lib_chart'
= webpack_bundle_tag 'graphs'
```
#### Important Tips and Considerations:
The above loads `chart.js` and `graphs_bundle.js` for this page only. `chart.js`
is separated from the bundle file so it can be cached separately from the bundle
and reused for other pages that also rely on the library. For an example, see
[this Haml file][page-specific-js-example].
- If you are unsure what controller and action corresponds to a given page, you
can find this out by checking `document.body.dataset.page` while on any page
within gitlab.com.
- Since `main.js` and `commons/index.js` are imported on all pages, it is
important to not add anything to these bundles unless it is truly needed
_everywhere_. This includes ubiquitous libraries like `vue`, `axios`, and
`jQuery`, as well as code for the main navigation and sidebar.
- Page-specific javascript entry points should be as lite as possible. These
files are exempt from tests, and should be used primarily for instantiation
and dependency injection of classes and methods that live in modules outside
of the entry point script. Just import, read the DOM, instantiate, and
nothing else.
- **DO NOT ASSUME** that the DOM has been fully loaded and available when an
entry point script is run. If you require that some code be run after the
DOM has loaded, you must attach an event handler to the `DOMContentLoaded`
event with:
```javascript
import initMyWidget from './my_widget';
document.addEventListener('DOMContentLoaded', () => {
initMyWidget();
});
```
- If a class or a module is specific to a particular route, try to locate it
close to the entry point it will be used. For instance, if `my_widget.js` is
only imported within `pages/widget/show/index.js`, you should place the
module at `pages/widget/show/my_widget.js` and import it with a relative path
(e.g. `import initMyWidget from './my_widget';`).
- If a class or module is used by multiple routes, place it within a shared
directory at the closest common parent directory for the entry points that
import it. For example, if `my_widget.js` is imported within both
`pages/widget/show/index.js` and `pages/widget/run/index.js`, then place the
module at `pages/widget/shared/my_widget.js` and import it with a relative
path if possible `../shared/my_widget`.
- For GitLab Enterprise Edition, page-specific entry points will override their
Community Edition counterparts with the same name, so if
`ee/app/assets/javascripts/pages/foo/bar/index.js` exists, it will take
precedence over `app/assets/javascripts/pages/foo/bar/index.js`. If you want
to minimize duplicate code, you can import one entry point from the other.
This is not done automatically to allow for flexibility in overriding
functionality.
### Code Splitting
> *TODO* flesh out this section once webpack is ready for code-splitting
For any code that does not need to be run immediately upon page load, (e.g.
modals, dropdowns, and other behaviors that can be lazy-loaded), you can split
your module into asynchronous chunks with dynamic import statements. These
imports return a Promise which will be resolved once the script has loaded:
```javascript
import(/* webpackChunkName: 'emoji' */ '~/emoji')
.then(/* do something */)
.catch(/* report error */)
```
Please try to use `webpackChunkName` when generating these dynamic imports as
it will provide a deterministic filename for the chunk which can then be cached
the browser.
More information is available in [webpack's code splitting documentation](https://webpack.js.org/guides/code-splitting/#dynamic-imports).
### Minimizing page size
@ -95,7 +151,8 @@ General tips:
- Prefer font formats with better compression, e.g. WOFF2 is better than WOFF, which is better than TTF.
- Compress and minify assets wherever possible (For CSS/JS, Sprockets and webpack do this for us).
- If some functionality can reasonably be achieved without adding extra libraries, avoid them.
- Use page-specific JavaScript as described above to dynamically load libraries that are only needed on certain pages.
- Use page-specific JavaScript as described above to load libraries that are only needed on certain pages.
- Use code-splitting dynamic imports wherever possible to lazy-load code that is not needed initially.
- [High Performance Animations][high-perf-animations]
-------
@ -112,8 +169,5 @@ General tips:
[pagespeed-insights]: https://developers.google.com/speed/pagespeed/insights/
[google-devtools-profiling]: https://developers.google.com/web/tools/chrome-devtools/profile/?hl=en
[browser-diet]: https://browserdiet.com/
[d3]: https://d3js.org/
[chartjs]: http://www.chartjs.org/
[page-specific-js-example]: https://gitlab.com/gitlab-org/gitlab-ce/blob/13bb9ed77f405c5f6ee4fdbc964ecf635c9a223f/app/views/projects/graphs/_head.html.haml#L6-8
[high-perf-animations]: https://www.html5rocks.com/en/tutorials/speed/high-performance-animations/
[flip]: https://aerotwist.com/blog/flip-your-animations/