if my feature has 2 rules, one is rollout rule and second is a/b experiment, how do I get assignment value for a/b experiment?

A bit hard to diagnose without more information about your setup, but generally speaking you can try either using our Dev tools or debug logging to trace through how each rule is evaluated. GB Dev Tools: https://chrome.google.com/webstore/detail/growthbook-devtools/opemhndcehfgipokneipaafbglcecjia Or to enable debug mode, set

growthbook.debug = true

immediately after invoking the SDK.

The first matching rule applies. If the A/B experiment is after the rollout then it should only applies for those it wasn't rolled out for. It is possible you might want to instead get rid of the rollout rule, and in the experiment rule, only having it "include x% of users in the experiment". You can start the x% small to make sure there are no bugs and gradually increase as you become more confident.