How can i update a specific value of a recoil state object

I have this recoil state object:

export const LivePolygon = atom({
    key: "LivePolygon",
    default: {
        radii: ['', ''],
        coordinates: ['', ''],
        tilt: ['']
      },
});

And on another file i import it like this:

import { LivePolygon } from "../TheFileOfLivePolygon";

const [liveP, setLiveP] = useRecoilState(LivePolygon);

Now i want to update a specific value of it (from the other file, where it's being imported to).

For Example, if i want to update the object radii in the second cell to be equal to 5.

With a simple variable i'd do it like this:

liveP.radii[1] = 5

How can i do it here? I saw a few questions about it, but non of them helped with this case.

Solution

Here's a commented example of how to update an object value in a Recoil atom. Just make sure not to mutate any objects while updating it (the same is true for any React state).

body { font-family: sans-serif; }
button, pre { font-size: 1rem; }
pre { font-family: monospace; }

<div id="root"></div><script src="https://unpkg.com/react@18.1.0/umd/react.development.js"></script><script src="https://unpkg.com/react-dom@18.1.0/umd/react-dom.development.js"></script><script src="https://unpkg.com/recoil@0.7.2/umd/recoil.min.js"></script><script src="https://unpkg.com/@babel/standalone@7.17.11/babel.min.js"></script>
<script type="text/babel" data-type="module" data-presets="env,react">

// import * as ReactDOM from 'react-dom/client';
// import {atom, RecoilRoot, useRecoilState} from 'recoil';

// This Stack Overflow snippet demo uses UMD modules instead of the above import statments
const {atom, RecoilRoot, useRecoilState} = Recoil;

// It won't make any difference whether you define this atom
// here in this module or define it in another module and import it:
const livePolyAtom = atom({
  key: 'livePolygon',
  default: {
    radii: [0, 0],
    coordinates: [0, 0],
    tilt: [0],
  },
});

function App () {
  const [livePoly, setLivePoly] = useRecoilState(livePolyAtom);

  const updateRadiiValue = () => {
    // Make sure not to mutate any objects when returning the new object state:
    setLivePoly(lp => {
      const radii = [...lp.radii];
      radii[1] = 5;
      return {...lp, radii};
    });
  };

  return (
    <div>
      <button onClick={updateRadiiValue}>Update radii value</button>
      <pre><code>{JSON.stringify(livePoly, null, 2)}</code></pre>
    </div>
  );
}

const reactRoot = ReactDOM.createRoot(document.getElementById('root'));

reactRoot.render(
  <RecoilRoot>
    <App />
  </RecoilRoot>
);

</script>