Why when I'm setting Data with Axios it doesn't work

I’m learning React and I’m facing my first problem. I try to set some Data to use it but I failed.
Here is what I do :

const [randomCharacters, setRandomCharacters] = useState([]);

const randomNumber = (minimum,maximum, numberOfCharactersWanted) => {

        let characters = [];

        for (let i = 1; i <= numberOfCharactersWanted; ++i){
            let randomNumber = (Math.random() * (maximum - minimum + 1) ) << 0;
            if (!characters.includes(randomNumber)){
                characters.push(randomNumber);
            } else {
                randomNumber = (Math.random() * (maximum - minimum + 1) ) << 0;
                characters.push(randomNumber);
            }

        }
        return characters.join();
    }

    useEffect(() =>{
        axios.get('https://rickandmortyapi.com/api/character/'+ randomNumber(0,totalCharacters, 6))
            .then((response) => {
                response.data.map((res) => (
                    setRandomCharacters(res.id)

                ))
                //setRandomCharacters(response.data)
            })
    })

TotalCharacter is equal to 826 and when I fetch, I get the goo url, for example =>
‘https://rickandmortyapi.com/api/character/23,45,89,567,432,9’ but when I fetch the randomCharacters, I can see state overriding over and over instand of getting an array of value and I don’t understand why, any idea ?

>Solution :

change return characters.join(); to return characters.join('');

in your randomNumber function.

reason why, a blank .join() uses , as the default joining value.