Feature Flag best practices

[severinbeauvais]

Best practices and lessons learned during use of feature flags in UIs...

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Copy existing code
Use local default/failsafe values
Sample boolean flag
Sample array flag
Targeting
Enabling a FF that uses targeting rules

[severinbeauvais]

Copy existing code

The following code initializes the Launch Darkly library and provides two helpful functions:

• a function to update the LD user (for user-based targeting)
• a function to get a specified FF

https://github.com/bcgov/business-filings-ui/blob/main/src/utils/feature-flags.ts

[severinbeauvais]

Use local default/failsafe flag values

Your app needs to work "safely" if the feature flags cannot be fetched from Launch Darkly.

[severinbeauvais]

Sample boolean flag

LD config:

UI code:

if (!!getFeatureFlag('download-summary-enabled') {
   // your code here
}

Note the use of !! to convert type "any" to "boolean".

[severinbeauvais]

Sample array flag

LD config:

UI code:

if (!!getFeatureFlag('supported-dissolution-entities')?.includes('BEN'))) {
   // your code here
}

Note the use of !! to convert type "any" to "boolean".
Note the use of conditional chaining (?) in case return value is null/undefined.

[severinbeauvais]

Targeting

This feature allows some users to see one variation of a feature flag (eg, ON) while remaining users see another variatio of a flag (eg, OFF).

This can be configured using a simple targeting rule (eg, a list of emails) or using a segment (eg, a list of users).

Note

LaunchDarkly does not record a user correctly, and therefore does not provide targeted feature flags,
if the user has “do not track” enabled in their browser.

If this is working correctly, you will see the user reported in LaunchDarkly -> Users.

[severinbeauvais]

Enabling a FF that uses targeting rules

"Targeting" and "whether the flag is enabled or disabled" are the same thing, therefore if you turn off targeting then you are disabling the flag, and if you turn on the flag then you are enabling targeting (with whatever rules are in place, such as certain emails only). THEREFORE, to enable the flag for all users, all rules have to be deleted.