A proficient React javascript developer must possess strong knowledge of ReactJS library fundamentals, JavaScript, HTML/CSS, and component-based architecture. They should be experienced in state management with libraries like Redux, proficient in front-end development tools, and familiar with version control systems such as Git. Additionally, they should have skills in debugging, testing, responsive design principles, and problem-solving, besides a commitment to continuous learning to update themselves with the latest trends and advancements in ReactJS development.