score:4

Accepted answer

first, welcome! you should have probably made it more clear that this question is about react.

now to the answer, every state modification would be made using setstate, so your addfavourites function should be like this:

addfavourites() {
    let data = this.state.data.map((e) => {
        e.favourite = false;
        return e;
    });
    this.setstate({ data: data });
}

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