Declarative Single Page Web Application (SPA) builder system and method are used to build complex, fully functional SPA without undue coding. A SPA builder contains a set of rules that facilitate the building of a SPA. The set of rules in the SPA builder includes a set of user interface elements and a finite state machine model with a set of data dependency rules, a set of user interface rendering rules, and a set of action execution data flow rules. The SPA builder also utilizes mock data that allows user to build and verify user interaction flows of the SPA at design time.