How to change text on mouse event in React

You should use only one react’s state in Class Component, in that way can access the setState and Update it. A state is an object so it can contain inside of it more variables, arrays, or even more objects. just hold inside the same state, variable for tekst

and update it like you update the first state, like so:

 this.state = {
            index: 0,
            imgList: [img0, img1, img2, img3],
            tekst: 'pies',
        }

And then update the state whenever you need, like so:

 this.setState({
                tekst: "Pies domowy - udomowiony gatunek ssaka drapieżnego z rodziny psowatych, traktowany przez niektóre ujęcia systematyczne za podgatunek wilka."
            })