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# 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.
_**Note:** We've [removed these two filters](https://gitlab.com/gitlab-org/gitlab-ce/blob/8-5-stable/config/initializers/haml.rb)
in 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();
```
However, there's a feature of GitLab's Markdown processing that [automatically
adds anchors to header elements][ToC Processing], with the `id` attribute being
automatically generated based on the content of the header.
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 with the following Markdown:
```markdown
## JS My Selector
```
Which gets converted to the following HTML:
```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)
|
