gitlab-org--gitlab-foss/doc/development/gotchas.md

# Gotchas

The purpose of this guide is to document potential "gotchas" that contributors
might encounter or should avoid during development of GitLab CE and EE.

## Don't `describe` symbols

Consider the following model spec:

```ruby
require 'rails_helper'

describe User do
  describe :to_param do
    it 'converts the username to a param' do
      user = described_class.new(username: 'John Smith')

      expect(user.to_param).to eq 'john-smith'
    end
  end
end
```

When run, this spec doesn't do what we might expect:

```sh
spec/models/user_spec.rb|6 error|  Failure/Error: u = described_class.new NoMethodError: undefined method `new' for :to_param:Symbol
```

### Solution

Except for the top-level `describe` block, always provide a String argument to
`describe`.

## Don't `rescue Exception`

See ["Why is it bad style to `rescue Exception => e` in Ruby?"][Exception].

_**Note:** This rule is [enforced automatically by
Rubocop](https://gitlab.com/gitlab-org/gitlab-ce/blob/8-4-stable/.rubocop.yml#L911-914)._

[Exception]: http://stackoverflow.com/q/10048173/223897

## Don't use inline CoffeeScript in views

Using the inline `:coffee` or `:coffeescript` Haml filters comes with a
performance overhead.

We've [removed these two filters entirely](https://gitlab.com/gitlab-org/gitlab-ce/blob/8-5-stable/config/initializers/haml.rb)
through an initializer.

### Further reading

- Pull Request: [Replace CoffeeScript block into JavaScript in Views](https://git.io/vztMu)
- Stack Overflow: [Performance implications of using :coffescript filter inside HAML templates?](http://stackoverflow.com/a/17571242/223897)

## ID-based CSS selectors need to be a bit more specific

Normally, because HTML `id` attributes need to be unique to the page, it's
perfectly fine to write some JavaScript like the following:

```javascript
$('#js-my-selector').hide();
```

But there's a feature of GitLab's Markdown processing that will automatically
add `id` attributes underneath header elements in order to make them linkable.
The content of the header is ["dasherized"][ToC Processing] and used in the `id`
attribute.

Unfortunately, this feature makes it possible for user-generated content to
create a header element with the same `id` attribute we're using in our
selector, potentially breaking the JavaScript behavior. A user could break the
above example JavaScript with the following Markdown:

```markdown
## JS My Selector
```

Which gets converted to the following HTML after processing:

```html
<h2>
  <a id="js-my-selector" class="anchor" href="#js-my-selector" aria-hidden="true"></a>
  JS My Selector
</h2>
```

[ToC Processing]: https://gitlab.com/gitlab-org/gitlab-ce/blob/8-4-stable/lib/banzai/filter/table_of_contents_filter.rb#L31-37

### Solution

The current recommended fix for this is to make our selectors slightly more
specific:

```javascript
$('div#js-my-selector').hide();
```

### Further reading

- Issue: [Merge request ToC anchor conflicts with tabs](https://gitlab.com/gitlab-org/gitlab-ce/issues/3908)
- Merge Request: [Make tab target selectors less naive](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2023)
- Merge Request: [Make cross-project reference's clipboard target less naive](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2024)