How can I check if the component is unmounted in a functional component?

Here is some pseudo code how you can use useEffect to see if a component is mounted.

It uses useEffect to listen to someService when it receives a message it checks if the component is mounted (cleanup function is also called when component unmounts) and if it is it uses setServiceMessage that was created by useState to set messages received by the service:

import { useState, useEffect } from 'react';
import someService from 'some-service';

export default props => {
  const userId = props.userId;
  const [serviceMessage, setServiceMessage] = useState([]);
  useEffect(
    () => {
      const mounted = { current: true };
      someService.listen(
        //listen to messages for this user
        userId, 
        //callback when message is received
        message => {
          //only set message when component is mounted
          if (mounted.current) {
            setServiceMessage(serviceMessage.concat(message));
          }
      });
      //returning cleanup function
      return () => {
        //do not listen to the service anymore
        someService.stopListen(userId);
        //set mounted to false if userId changed then mounted
        //  will immediately be set to true again and someService
        //  will listen to another user's messages but if the 
        //  component is unmounted then mounted.current will 
        //  continue to be false
        mounted.current = false;
      };
    },//<-- the function passed to useEffects
    //the function passed to useEffect will be called
    //every time props.userId changes, you can pass multiple
    //values here like [userId,otherValue] and then the function
    //will be called whenever one of the values changes
    //note that when this is an object then {hi:1} is not {hi:1}
    //referential equality is checked so create this with memoization
    //if this is an object created by mapStateToProps or a function
    [userId]
  );
};