ReactJS is one of the most popular JavaScript-libraries, which is used broadly in app development. The goal of this thesis was to explore the main features of ReactJS and how to use it. The thesis investigated the main concepts of ReactJS like component-based approach, virtual DOM model, JSX-syntax, and state management. The thesis also presents the introduction of ReactJS, and how components work and as well their relations through the practical section of the thesis. In the practical section of the bachelor’s thesis a simple task list-application was created, in which functional components were used and for the state management React Hooks were used. In the task list application, it is possible to add, edit or delete tasks and as well mark them as completed. The end result of this thesis delivered an information package of the main features of ReactJS and its usage in application development. Another result was also the task list application demo-version which utilized ReactJS library in a practical application.