It's like script-setup syntax:

import { ref, defineComponent } from 'vue'

const Test = defineComponent({
    setup() {
        const msg = ref('I am Brave')

        const changeMsg = () => {
            console.log('aa')
            msg.value = 'I will get everything I want'
        } 

        return () => (
            <div>
                <span>{msg.value}</span>
                <button onClick={changeMsg}>btn</button>
            </div>
        );
    },
});

export default Test