feat: Make value optional if no defaultVariant is set

[marcozabel]

This PR

Makes the resolution value optional. If no targeting rules match and no defaultVariant is set it should be possible to return reason DEFAULT and no value to fall back to the code default.

Related Issues

open-feature/flagd#1856

Notes

Follow-up Tasks

How to test

[gemini-code-assist]

Summary of Changes

Hello @marcozabel, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the flexibility of flag evaluation by making the value field in ResolutionDetails optional. This change enables flagd to return a DEFAULT reason without a concrete value when no targeting rules are met and no default variant is specified, allowing client-side applications to implement their own fallback logic.

Highlights

• Optional Value in ResolutionDetails: The value property within the ResolutionDetails type has been changed from required to optional (value?: U). This allows for scenarios where a flag evaluation might not return a specific value, such as when no targeting rules match and no default variant is set.

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[gemini-code-assist]

Code Review

This pull request makes the value property on ResolutionDetails optional. This is a significant breaking change that allows providers to not return a value when no targeting rule matches and no default variant is set. While this is a valid feature, the pull request is incomplete as it doesn't update any of the consuming code to handle this change. This will lead to build failures and runtime errors in various parts of the SDKs, such as the React and Angular packages. The necessary follow-up changes to handle the optional value should be included in this PR.

[gemini-code-assist]

Making value optional is a significant breaking change that will affect consumers of ResolutionDetails and EvaluationDetails. For example, in packages/react/src/evaluation/use-feature-flag.ts, functions like useBooleanFlagValue will no longer be type-safe:

// packages/react/src/evaluation/use-feature-flag.ts:110
export function useBooleanFlagValue(
  flagKey: string,
  defaultValue: boolean,
  options?: ReactFlagEvaluationOptions,
): boolean {
  return useBooleanFlagDetails(flagKey, defaultValue, options).value;
}

With value being optional (boolean | undefined), this function will fail to compile as it's typed to return boolean. This will be the case for all use*FlagValue hooks in the React package. Similar issues will arise in other packages like the Angular SDK.

To address this, all call sites that access .value directly and expect a non-undefined value must be updated to handle the optionality. For example, by using the provided defaultValue:

// in packages/react/src/evaluation/use-feature-flag.ts:115
return useBooleanFlagDetails(flagKey, defaultValue, options).value ?? defaultValue;

This pull request should be expanded to include the necessary changes across the entire codebase to correctly handle this new optional property.