Frontend FAQ

Rules of Frontend FAQ

  1. You talk about Frontend FAQ. Please share links to it whenever applicable, so more eyes catch when content gets outdated.
  2. Keep it short and simple. Whenever an answer needs more than two sentences it does not belong here.
  3. Provide background when possible. Linking to relevant source code, issue / epic, or other documentation helps to understand the answer.
  4. If you see something, do something. Please remove or update any content that is outdated as soon as you see it.


1. How do I find the Rails route for a page?

Check the 'page' data attribute

The easiest way is to type the following in the browser while on the page in question:

Find here the source code setting the attribute.

Rails routes

The rake routes command can be used to list all the routes available in the application, piping the output into grep, we can perform a search through the list of available routes. The output includes the request types available, route parameters and the relevant controller.

bundle exec rake routes | grep "issues"

2. modal_copy_button vs clipboard_button

The clipboard_button uses the copy_to_clipboard.js behavior, which is initialized on page load, so if there are vue-based clipboard buttons that don't exist at page load (such as ones in a GlModal), they do not have the click handlers associated with the clipboard package.

modal_copy_button was added that manages an instance of the clipboard plugin specific to the instance of that component, which means that clipboard events are bound on mounting and destroyed when the button is, mitigating the above issue. It also has bindings to a particular container or modal ID available, to work with the focus trap created by our GlModal.

3. A gitlab-ui component not conforming to Pajamas Design System

Some Pajamas Design System components implemented in gitlab-ui do not conform with the design system specs because they lack some planned features or are not correctly styled yet. In the Pajamas website, a banner on top of the component examples indicates that:

This component does not yet conform to the correct styling defined in our Design System. Refer to the Design System documentation when referencing visuals for this component.

For example, at the time of writing, this type of warning can be observed for all form components. It, however, doesn't imply that the component should not be used.

GitLab always asks to use <gl-*> components whenever a suitable component exists. It makes codebase unified and more comfortable to maintain/refactor in the future.

Ensure a Product Designer reviews the use of the non-conforming component as part of the MR review. Make a follow up issue and attach it to the component implementation epic found within the Components of Pajamas Design System epic.

4. My submit form button becomes disabled after submitting

If you are using a submit button inside a form and you attach an onSubmit event listener on the form element, this piece of code adds a disabled class selector to the submit button when the form is submitted. To avoid this behavior, add the class js-no-auto-disable to the button.

5. Should I use a full URL (i.e. gon.gitlab_url) or a full path (i.e. gon.relative_url_root) when referencing backend endpoints?

It's preferred to use a full path over a full URL because the URL uses the hostname configured with GitLab which may not match the request. This causes CORS issues like this Web IDE one.


// bad :(
// If gitlab is configured with hostname ``
// This will cause CORS issues if I request from `localhost`
axios.get(joinPaths(gon.gitlab_url, '-', 'foo'))

// good :)
axios.get(joinPaths(gon.relative_url_root, '-', 'foo'))

Also, please try not to hardcode paths in the Frontend, but instead receive them from the Backend (see next section). When referencing Backend rails paths, avoid using *_url, and use *_path instead.


-# Bad :(
#js-foo{ data: { foo_url: some_rails_foo_url } }

-# Good :)
#js-foo{ data: { foo_path: some_rails_foo_path } }

6. How should the Frontend reference Backend paths?

We prefer not to add extra coupling by hardcoding paths. If possible, add these paths as data attributes to the DOM element being referenced in the JavaScript.


// Bad :(
// Here's a Vuex action that hardcodes a path :(
export const fetchFoos = ({ state }) => {
  return axios.get(joinPaths(gon.relative_url_root, '-', 'foo'));

// Good :)
function initFoo() {
  const el = document.getElementById('js-foo');

  // Path comes from our root element's data which is used to initialize the store :)
  const store = createStore({
    fooPath: el.dataset.fooPath

    render(h) {
      return h(Component);

// Vuex action can now reference the path from its state :)
export const fetchFoos = ({ state }) => {
  return axios.get(state.settings.fooPath);

7. How can I test the production build locally?

Sometimes it's necessary to test locally what the frontend production build would produce, to do so the steps are:

  1. Stop webpack: gdk stop webpack.
  2. Open gitlab.yaml located in your gitlab installation folder, scroll down to the webpack section and change dev_server to enabled: false.
  3. Run yarn webpack-prod && gdk restart rails-web.

The production build takes a few minutes to be completed; any code changes at this point are displayed only after executing the item 3 above again.

To return to the normal development mode:

  1. Open gitlab.yaml located in your gitlab installation folder, scroll down to the webpack section and change back dev_server to enabled: true.
  2. Run yarn clean to remove the production assets and free some space (optional).
  3. Start webpack again: gdk start webpack.
  4. Restart GDK: gdk restart rails-web.

8. Babel polyfills

Introduced in GitLab 12.8.

GitLab has enabled the Babel preset-env option useBuiltIns: 'usage', which adds the appropriate core-js polyfills once for each JavaScript feature we're using that our target browsers don't support. You don't need to add core-js polyfills manually.

GitLab adds non-core-js polyfills for extending browser features (such as the GitLab SVG polyfill), which allow us to reference SVGs by using <use xlink:href>. Be sure to add these polyfills to app/assets/javascripts/commons/polyfills.js.

To see what polyfills are being used:

  1. Navigate to your merge request.

  2. In the secondary menu below the title of the merge request, click Pipelines, then click the pipeline you want to view, to display the jobs in that pipeline.

  3. Click the compile-production-assets job.

  4. In the right-hand sidebar, scroll to Job Artifacts, and click Browse.

  5. Click the webpack-report folder to open it, and click index.html.

  6. In the upper left corner of the page, click the right arrow {angle-right} to display the explorer.

  7. In the Search modules field, enter gitlab/node_modules/core-js to see which polyfills are being loaded and where:

    Image of webpack report