notes: react usestate not mutating object

Just a footgun I ran into while working on the Threadlet dashboard.

If you do the following it will not work as expected:

function f() {
    const [a, setA] = useState({x: 1 , y: 2})
    setA((old) => {
        old.x = 4
        return old
    })
}

You should always return a wholly new object.

function f() {
    const [a, setA] = useState({x: 1 , y: 2})
    setA((old) => {
        n = {...old, x: 4}
        return n
    })
}