# Frontend Development Guidelines

This document describes various guidelines to ensure consistency and quality
across GitLab's frontend team.

## Overview

GitLab is built on top of [Ruby on Rails][rails] using [Haml][haml] with
[Hamlit][hamlit]. Be wary of [the limitations that come with using
Hamlit][hamlit-limits]. We also use [SCSS][scss] and plain JavaScript with
[ES6 by way of Babel][es6].

The asset pipeline is [Sprockets][sprockets], which handles the concatenation,
minification, and compression of our assets.

[jQuery][jquery] is used throughout the application's JavaScript, with
[Vue.js][vue] for particularly advanced, dynamic elements.

### Architecture

The Frontend Architect is an expert who makes high-level frontend design choices
and decides on technical standards, including coding standards, and frameworks.

When you are assigned a new feature that requires architectural design,
make sure it is discussed with one of the Frontend Architecture Experts.

This rule also applies if you plan to change the architecture of an existing feature.

These decisions should be accessible to everyone, so please document it on the Merge Request.

You can find the Frontend Architecture experts on the [team page][team-page].

You can find documentation about the desired architecture for a new feature built with Vue.js in [here][vue-section].

### Realtime

When writing code for realtime features we have to keep a couple of things in mind:
1. Do not overload the server with requests.
1. It should feel realtime. 

Thus, we must strike a balance between sending requests and the feeling of realtime. Use the following rules when creating realtime solutions.

1. The server will tell you how much to poll by sending `X-Poll-Interval` in the header. Use that as your polling interval. This way it is easy for system administrators to change the polling rate. A `X-Poll-Interval: -1` means you should disable polling, and this must be implemented. 
1. A response of `HTTP 429 Too Many Requests`, should disable polling as well. This must also be implemented.
1. Use a common library for polling.
1. Poll on active tabs only. Use a common library to find out which tab currently has eyes on it. Please use [Focus](https://gitlab.com/andrewn/focus). Specifically [Eyeballs Detector](https://gitlab.com/andrewn/focus/blob/master/lib/eyeballs-detector.js).
1. Use regular polling intervals, do not use backoff polling, or jitter, as the interval will be controlled by the server.
1. The backend code will most likely be using etags. You do not and should not check for status `304 Not Modified`. The browser will transform it for you.

### Vue

For more complex frontend features, we recommend using Vue.js. It shares
some ideas with React.js as well as Angular.

To get started with Vue, read through [their documentation][vue-docs].

#### How to build a new feature with Vue.js
**Components, Stores and Services**

In some features implemented with Vue.js, like the [issue board][issue-boards]
or [environments table][environments-table]
you can find a clear separation of concerns:

```
new_feature
├── components
│   └── component.js.es6
│   └── ...
├── store
│  └── new_feature_store.js.es6
├── service
│  └── new_feature_service.js.es6
├── new_feature_bundle.js.es6
```
_For consistency purposes, we recommend you to follow the same structure._

Let's look into each of them:

**A `*_bundle.js` file**

This is the index file of your new feature. This is where the root Vue instance
of the new feature should be.

The Store and the Service should be imported and initialized in this file and provided as a prop to the main component.

Don't forget to follow [these steps.][page_specific_javascript]

**A folder for Components**

This folder holds all components that are specific of this new feature.
If you need to use or create a component that will probably be used somewhere
else, please refer to `vue_shared/components`.

A good thumb rule to know when you should create a component is to think if
it will be reusable elsewhere.

For example, tables are used in a quite amount of places across GitLab, a table
would be a good fit for a component.
On the other hand, a table cell used only in on table, would not be a good use
of this pattern.

You can read more about components in Vue.js site, [Component System][component-system]

**A folder for the Store**

The Store is a class that allows us to manage the state in a single
source of truth.

The concept we are trying to follow is better explained by Vue documentation
itself, please read this guide: [State Management][state-management]

**A folder for the Service**

The Service is used only to communicate with the server.
It does not store or manipulate any data.
We use [vue-resource][vue-resource-repo] to
communicate with the server.

The [issue boards service][issue-boards-service]
is a good example of this pattern.

## Performance

### Resources

- [WebPage Test][web-page-test] for testing site loading time and size.
- [Google PageSpeed Insights][pagespeed-insights] grades web pages and provides feedback to improve the page.
- [Profiling with Chrome DevTools][google-devtools-profiling]
- [Browser Diet][browser-diet] is a community-built guide that catalogues practical tips for improving web page performance.

### 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.

In cases where libraries are only used on a few specific pages, we use
"page-specific JavaScript" to prevent the main `application.js` file from
becoming unnecessarily large.

Steps to split page-specific JavaScript from the main `application.js`:

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. In `graphs_bundle.js` add the line `//= require_tree .`, this adds all other files in the directory to the bundle.
1. Add any necessary libraries to `app/assets/javascripts/lib/`, all files directly descendant from this directory will be precompiled as separate assets, in this case `chart.js` would be added.
1. Add the new "bundle" file to the list of precompiled assets in
`config/application.rb`.
  - For example: `config.assets.precompile << "graphs/graphs_bundle.js"`.
1. Move code reliant on these libraries into the `graphs` directory.
1. In the relevant views, add the scripts to the page with the following:

```haml
- content_for :page_specific_javascripts do
  = page_specific_javascript_tag('lib/chart.js')
  = page_specific_javascript_tag('graphs/graphs_bundle.js')
```

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].

### Minimizing page size

A smaller page size means the page loads faster (especially important on mobile
and poor connections), the page is parsed more quickly by the browser, and less
data is used for users with capped data plans.

General tips:

- Don't add new fonts.
- 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 does 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.

## Accessibility

### Resources

[Chrome Accessibility Developer Tools][chrome-accessibility-developer-tools]
are useful for testing for potential accessibility problems in GitLab.

Accessibility best-practices and more in-depth information is available on
[the Audit Rules page][audit-rules] for the Chrome Accessibility Developer Tools.

## Security

### Resources

[Mozilla’s HTTP Observatory CLI][observatory-cli] and the
[Qualys SSL Labs Server Test][qualys-ssl] are good resources for finding
potential problems and ensuring compliance with security best practices.

<!-- Uncomment these sections when CSP/SRI are implemented.
### Content Security Policy (CSP)

Content Security Policy is a web standard that intends to mitigate certain
forms of Cross-Site Scripting (XSS) as well as data injection.

Content Security Policy rules should be taken into consideration when
implementing new features, especially those that may rely on connection with
external services.

GitLab's CSP is used for the following:

- Blocking plugins like Flash and Silverlight from running at all on our pages.
- Blocking the use of scripts and stylesheets downloaded from external sources.
- Upgrading `http` requests to `https` when possible.
- Preventing `iframe` elements from loading in most contexts.

Some exceptions include:

- Scripts from Google Analytics and Piwik if either is enabled.
- Connecting with GitHub, Bitbucket, GitLab.com, etc. to allow project importing.
- Connecting with Google, Twitter, GitHub, etc. to allow OAuth authentication.

We use [the Secure Headers gem][secure_headers] to enable Content
Security Policy headers in the GitLab Rails app.

Some resources on implementing Content Security Policy:

- [MDN Article on CSP][mdn-csp]
- [GitHub’s CSP Journey on the GitHub Engineering Blog][github-eng-csp]
- The Dropbox Engineering Blog's series on CSP: [1][dropbox-csp-1], [2][dropbox-csp-2], [3][dropbox-csp-3], [4][dropbox-csp-4]

### Subresource Integrity (SRI)

Subresource Integrity prevents malicious assets from being provided by a CDN by
guaranteeing that the asset downloaded is identical to the asset the server
is expecting.

The Rails app generates a unique hash of the asset, which is used as the
asset's `integrity` attribute. The browser generates the hash of the asset
on-load and will reject the asset if the hashes do not match.

All CSS and JavaScript assets should use Subresource Integrity. For implementation details,
see the documentation for [the Sprockets implementation of SRI][sprockets-sri].

Some resources on implementing Subresource Integrity:

- [MDN Article on SRI][mdn-sri]
- [Subresource Integrity on the GitHub Engineering Blog][github-eng-sri]

-->

### Including external resources

External fonts, CSS, and JavaScript should never be used with the exception of
Google Analytics and Piwik - and only when the instance has enabled it. Assets
should always be hosted and served locally from the GitLab instance. Embedded
resources via `iframes` should never be used except in certain circumstances
such as with ReCaptcha, which cannot be used without an `iframe`.

### Avoiding inline scripts and styles

In order to protect users from [XSS vulnerabilities][xss], we will disable inline scripts in the future using Content Security Policy.

While inline scripts can be useful, they're also a security concern. If
user-supplied content is unintentionally left un-sanitized, malicious users can
inject scripts into the web app.

Inline styles should be avoided in almost all cases, they should only be used
when no alternatives can be found. This allows reusability of styles as well as
readability.

## Style guides and linting

See the relevant style guides for our guidelines and for information on linting:

- [SCSS][scss-style-guide]
- JavaScript - We defer to [AirBnb][airbnb-js-style-guide] on most style-related
conventions and enforce them with eslint. See [our current .eslintrc][eslintrc]
for specific rules and patterns.

## Testing

Feature tests need to be written for all new features. Regression tests
also need to be written for all bug fixes to prevent them from occurring
again in the future.

See [the Testing Standards and Style Guidelines](testing.md) for more
information.

### Running frontend tests

`rake karma` runs the frontend-only (JavaScript) tests.
It consists of two subtasks:

- `rake karma:fixtures` (re-)generates fixtures
- `rake karma:tests` actually executes the tests

As long as the fixtures don't change, `rake karma:tests` is sufficient
(and saves you some time).

Please note: Not all of the frontend fixtures are generated. Some are still static
files. These will not be touched by `rake karma:fixtures`.

## Design Patterns

### Singletons

When exactly one object is needed for a given task, prefer to define it as a
`class` rather than as an object literal. Prefer also to explicitly restrict
instantiation, unless flexibility is important (e.g. for testing).

```javascript
// bad

gl.MyThing = {
  prop1: 'hello',
  method1: () => {}
};

// good

class MyThing {
  constructor() {
    this.prop1 = 'hello';
  }
  method1() {}
}

gl.MyThing = new MyThing();

// best

let singleton;

class MyThing {
  constructor() {
    if (!singleton) {
      singleton = this;
      singleton.init();
    }
      return singleton;
  }

  init() {
    this.prop1 = 'hello';
  }

  method1() {}
}

gl.MyThing = MyThing;

```

### Manipulating the DOM in a JS Class

When writing a class that needs to manipulate the DOM guarantee a container option is provided.
This is useful when we need that class to be instantiated more than once in the same page.

Bad:
```javascript
class Foo {
  constructor() {
    document.querySelector('.bar');
  }
}
new Foo();
```

Good:
```javascript
class Foo {
  constructor(opts) {
    document.querySelector(`${opts.container} .bar`);
  }
}

new Foo({ container: '.my-element' });
```
You can find an example of the above in this [class][container-class-example];

## Supported browsers

For our currently-supported browsers, see our [requirements][requirements].


## Gotchas

### Spec errors due to use of ES6 features in `.js` files

If you see very generic JavaScript errors (e.g. `jQuery is undefined`) being
thrown in Karma, Spinach, or Rspec tests but can't reproduce them manually,
you may have included `ES6`-style JavaScript in files that don't have the
`.js.es6` file extension. Either use ES5-friendly JavaScript or rename the file
you're working in (`git mv <file.js> <file.js.es6>`).

### Spec errors due to use of unsupported JavaScript

Similar errors will be thrown if you're using JavaScript features not yet
supported by our test runner's version of webkit, whether or not you've updated
the file extension. Examples of unsupported JavaScript features are:

- Array.from
- Array.find
- Array.first
- Object.assign
- Async functions
- Generators
- Array destructuring
- For Of
- Symbol/Symbol.iterator
- Spread

Until these are polyfilled or transpiled appropriately, they should not be used.
Please update this list with additional unsupported features or when any of
these are made usable.

### Spec errors due to JavaScript not enabled

If, as a result of a change you've made, a feature now depends on JavaScript to
run correctly, you need to make sure a JavaScript web driver is enabled when
specs are run. If you don't you'll see vague error messages from the spec
runner, and an explosion of vague console errors in the HTML snapshot.

To enable a JavaScript driver in an `rspec` test, add `js: true` to the
individual spec or the context block containing multiple specs that need
JavaScript enabled:

```ruby

# For one spec
it 'presents information about abuse report', js: true do
    # assertions...
end

describe "Admin::AbuseReports", js: true do
   it 'presents information about abuse report' do
        # assertions...
    end
   it 'shows buttons for adding to abuse report' do
        # assertions...
    end
end
```

In Spinach, the JavaScript driver is enabled differently. In the `*.feature`
file for the failing spec, add the `@javascript` flag above the Scenario:

```
@javascript
Scenario: Developer can approve merge request
    Given I am a "Shop" developer
    And I visit project "Shop" merge requests page
    And merge request 'Bug NS-04' must be approved
    And I click link "Bug NS-04"
    When I click link "Approve"
    Then I should see approved merge request "Bug NS-04"

```


[rails]: http://rubyonrails.org/
[haml]: http://haml.info/
[hamlit]: https://github.com/k0kubun/hamlit
[hamlit-limits]: https://github.com/k0kubun/hamlit/blob/master/REFERENCE.md#limitations
[scss]: http://sass-lang.com/
[es6]: https://babeljs.io/
[sprockets]: https://github.com/rails/sprockets
[jquery]: https://jquery.com/
[vue]: http://vuejs.org/
[vue-docs]: http://vuejs.org/guide/index.html
[web-page-test]: http://www.webpagetest.org/
[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
[chrome-accessibility-developer-tools]: https://github.com/GoogleChrome/accessibility-developer-tools
[audit-rules]: https://github.com/GoogleChrome/accessibility-developer-tools/wiki/Audit-Rules
[observatory-cli]: https://github.com/mozilla/http-observatory-cli
[qualys-ssl]: https://www.ssllabs.com/ssltest/analyze.html
[secure_headers]: https://github.com/twitter/secureheaders
[mdn-csp]: https://developer.mozilla.org/en-US/docs/Web/Security/CSP
[github-eng-csp]: http://githubengineering.com/githubs-csp-journey/
[dropbox-csp-1]: https://blogs.dropbox.com/tech/2015/09/on-csp-reporting-and-filtering/
[dropbox-csp-2]: https://blogs.dropbox.com/tech/2015/09/unsafe-inline-and-nonce-deployment/
[dropbox-csp-3]: https://blogs.dropbox.com/tech/2015/09/csp-the-unexpected-eval/
[dropbox-csp-4]: https://blogs.dropbox.com/tech/2015/09/csp-third-party-integrations-and-privilege-separation/
[mdn-sri]: https://developer.mozilla.org/en-US/docs/Web/Security/Subresource_Integrity
[github-eng-sri]: http://githubengineering.com/subresource-integrity/
[sprockets-sri]: https://github.com/rails/sprockets-rails#sri-support
[xss]: https://en.wikipedia.org/wiki/Cross-site_scripting
[scss-style-guide]: scss_styleguide.md
[requirements]: ../install/requirements.md#supported-web-browsers
[issue-boards]: https://gitlab.com/gitlab-org/gitlab-ce/tree/master/app/assets/javascripts/boards
[environments-table]: https://gitlab.com/gitlab-org/gitlab-ce/tree/master/app/assets/javascripts/environments
[page_specific_javascript]: https://docs.gitlab.com/ce/development/frontend.html#page-specific-javascript
[component-system]: https://vuejs.org/v2/guide/#Composing-with-Components
[state-management]: https://vuejs.org/v2/guide/state-management.html#Simple-State-Management-from-Scratch
[vue-resource-repo]: https://github.com/pagekit/vue-resource
[issue-boards-service]: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/javascripts/boards/services/board_service.js.es6
[airbnb-js-style-guide]: https://github.com/airbnb/javascript
[eslintrc]: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/.eslintrc
[team-page]: https://about.gitlab.com/team
[vue-section]: https://docs.gitlab.com/ce/development/frontend.html#how-to-build-a-new-feature-with-vue-js
[container-class-example]: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/javascripts/mini_pipeline_graph_dropdown.js
