face-detection-app | detect faces using Python and OpenCV Face | Computer Vision library

Your component is defined as const Detector = props => { making it a function component. Function components don't have a "this" and don't have methods like setState or componentDidMount. There are two ways you can solve your problem.

1. Either you make a component that inherits from React.Component. See the docs. When you do this, this will be available, as well as other component methods like this.setState.
2. Or you use hooks like useState to manage the state inside your components. docs

Which one you choose depends on preference, although hooks is the "newer" way of doing things.