How to persist slot when invoking AWS SDK LexRuntime

I am trying to write a unit test suite for an Amazon Lex chatbot. However, I can only test the first prompt of the conversation. For example:

User: "I'd like to download a mobile app" -> Response: "Which product do you want to use?"
User: "rate watch" -> Response: "Which platform are you using?"

According to param's specification of PostText method in AWS SDK Node.js API Reference, the only way we can persist information about conversation is by passing sessionAttributes. Even so, I am confused of how to persist slots attributes so Amazon Lex can recognize what the next state of conversation is. Any help is appreciated.