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OptimizelyDecision for the JavaScript (Browser) SDK

Describes the OptimizelyDecision object for the JavaScript (Browser), which is returned from the decide methods in Optimizely Feature Experimentation.


SDK v4.5 and higher

Object model

The following shows the interface of the OptimizelyDecision object.

interface OptimizelyDecision {
  variationKey: string | null;
  enabled: boolean;
  variables: { [variableKey: string]: unknown };
  ruleKey: string | null;
  flagKey: string;
  userContext: OptimizelyUserContext;
  reasons: string[];

The following table describes the fields of the OptimizelyDecision object:

enabledBooleanWhether the flag is enabled or not. Optimizely Feature Experimentation never returns null for this field; it returns false when it fails to make a decision due to an error.
variablesmapKey-value pairs of all the flag variables. When there are no variables defined for the flag or SDK cannot get variables, this field returns an empty map. When SDK fails to get variables for a decision, the Reasons field includes the error messages regardless of the Include Reasons option.
variationKeyString (nullable)When SDK fails to make a decision, this field returns null and the Reasons field includes the log messages if the Include Reasons option is set.
In legacy projects, there is an edge case: 1. no messages in the Reasons field and 2. null value in the Variation Key field means that a variation cannot be found (for example, because the flag is part of a rollout).
ruleKeyString (nullable)The key of the flag rule that the user bucketed into for this decision.
When a valid rule key is not available, returns a copy of the experiment key.
Returns null if neither a valid rule key nor experiment key is found.
flagKeyStringThe key of the flag for which the decision was made.
userContextOptimizelyUserContextThe user associated with the call to the Decide method.
reasonsArrayAn array of relevant error and log messages, in chronological order. Optimizely Feature Experimentation returns messages in this field:
- when errors are detected while making decisions.
- when reasons for decisions are enabled in the decide options
- an empty array when there are no messages.

See the following section for more information.


The returned decision object includes error and log messages in the Reasons field.

Error reasons

For the sake of consistency with other decision results, Optimizely Feature Experimentation returns a decision with null fields instead of throwing errors or exceptions.

Critical errors are always included in the reasons field of the returned decision object regardless of the include reasons option you set on the Decide method.

ErrorPossible reasons
null variationKey- Optimizely Feature Experimentation SDK not configured properly yet
- User context is not valid
- No flag was found for key
empty variables- Invalid variables for OptimizelyJSON
- Variable value for key is invalid or wrong type

Log reasons

Optimizely Feature Experimentation collects log messages in the Reasons field of the returned OptimizelyDecision object in the order in which they are logged.

The following types of log messages are only included in the Reasons field if you set the Include Reasons option on the Decide method:

  • Log messages related to bucketing users based on audiences or attributes.
  • Log messages related to bucketing users based on traffic percentages.


The language and platform source files containing the implementation for JavaScript (Browser) are available on GitHub.