React Hooks useState+useEffect+event gives stale state

value is stale in the event handler because it gets its value from the closure where it was defined. Unless we re-subscribe a new event handler every time value changes, it will not get the new value.

Solution 1: Make the second argument to the publish effect [value]. This makes the event handler get the correct value, but also causes the effect to run again on every keystroke.

Solution 2: Use a ref to store the latest value in a component instance variable. Then, make an effect which does nothing but update this variable every time value state changes. In the event handler, use the ref, not value.

const [value, setValue] = useState(initialValue);
const refValue = useRef(value);
useEffect(() => {
    refValue.current = value;
});
const handleEvent = (msg, data) => {
    console.info("Value in event handler: ", refValue.current);
};

https://reactjs.org/docs/hooks-faq.html#what-can-i-do-if-my-effect-dependencies-change-too-often

Looks like there are some other solutions on that page which might work too. Much thanks to @Dinesh for the assistance.

Updated Answer.

The issue is not with hooks. Initial state value was closed and passed to EventEmitter and was used again and again.

It's not a good idea to use state values directly in handleEvent. Instead we need to pass them as parameters while emitting the event.

import React, { useState, useEffect } from "react";
import { Controlled as CodeMirror } from "react-codemirror2";
import "codemirror/lib/codemirror.css";
import EventEmitter from "events";

let ee = new EventEmitter();

const initialValue = "initial value";

function App(props) {
  const [value, setValue] = useState(initialValue);
  const [isReady, setReady] = useState(false);

  // Should get the latest value
  function handleEvent(value, msg, data) {
    // Do not use state values in this handler
    // the params are closed and are executed in the context of EventEmitter
    // pass values as parameters instead
    console.info("Value in event handler: ", value);
    document.getElementById("result").innerHTML = value;
  }

  // Get value from server on component creation (mocked)
  useEffect(() => {
    setTimeout(() => {
      setValue("value from server");
      setReady(true);
    }, 1000);
  }, []);

  // Subscribe to events on component creation
  useEffect(
    () => {
      if (isReady) {
        ee.on("some_event", handleEvent);
      }
      return () => {
        if (!ee.off) return;
        ee.off(handleEvent);
      };
    },
    [isReady]
  );

  function handleClick(e) {
    ee.emit("some_event", value);
  }

  return (
    <React.Fragment>
      <CodeMirror
        value={value}
        options={{ lineNumbers: true }}
        onBeforeChange={(editor, data, newValue) => {
          setValue(newValue);
        }}
      />
      <button onClick={handleClick}>EventEmitter (works now)</button>
      <div id="result" />
    </React.Fragment>
  );
}

export default App;

Here is a working codesandbox