A very simple example:

import React from "react";
import CodeMirror from '@uiw/react-codemirror';
import { javascript } from '@codemirror/lang-javascript';

function Test() {
  const [value, setValue] = React.useState("console.log('hello world!');");
  const onChange = React.useCallback((val: string) => {
    setValue(val);
    console.log("val:", val);
    console.log("value:", value);
  }, []);
  return <CodeMirror value={value} height="200px" extensions={[javascript({ jsx: true })]} onChange={onChange} />;
}

Where the value state should be set to val in onChange but it never seems to be.