What Is Component State?
Most components should simply take in props and render. But, components also offer state, and it is used to store information/data about the component that can change over time. Typically the change comes as a result of user events or system events (i.e., as a response to user input, a server request, or the passage of time).
According to the React documentation state should:
Contain data that a component's event handlers may change to trigger a UI update. In real apps this data tends to be very small and JSON-serializable. When building a stateful component, think about the minimal possible representation of its state, and only store those properties in this.state. Inside of render() simply compute any other information you need based on this state. You'll find that thinking about and writing applications in this way tends to lead to the most correct application, since adding redundant or computed values to state means that you need to explicitly keep them in sync rather than rely on React computing them for you.