Add clientside feature flag test docsclientside-feature-flag-test-docs

1 files changed, 16 insertions, 2 deletions

diff --git a/doc/development/feature_flags.md b/doc/development/feature_flags.md
index 3271f9a7fb3..4d8919596f8 100644
--- a/doc/development/feature_flags.md
+++ b/doc/development/feature_flags.md
@@ -26,8 +26,8 @@ the time, you should execute `/chatops run feature set my_feature_flag 50`.
 
 ## Feature flags for user applications
 
-This document only covers feature flags used in the development of GitLab 
-itself. Feature flags in deployed user applications can be found at 
+This document only covers feature flags used in the development of GitLab
+itself. Feature flags in deployed user applications can be found at
 [Feature Flags](https://docs.gitlab.com/ee/user/project/operations/feature_flags.html)
 
 ## Developing with feature flags
@@ -103,6 +103,8 @@ that checking for `gon.features.vim_bindings` would not work.
 
 ### Specs
 
+#### RSpec
+
 In the test environment `Feature.enabled?` is stubbed to always respond to `true`,
 so we make sure behavior under feature flag doesn't go untested in some non-specific
 contexts.
@@ -114,6 +116,18 @@ feature flag. You can stub a feature flag as follows:
 stub_feature_flags(my_feature_flag: false)
 ```
 
+#### Karma / Jest
+
+You can mock a feature flag by setting the boolean flag property on `gon.features`,
+which is reset `beforeEach` test to an empty object.
+
+```js
+it('works great when newFeature is enabled', () => {
+   Object.assign(gon.features, { newFeature: true });
+   // ...
+});
+```
+
 ## Enabling a feature flag (in development)
 
 In the rails console (`rails c`), enter the following command to enable your feature flag