react useEffect when configuring dependency list to [] - equivalent to to componentMount

 This basically means that the component will load the first time and do it things 

useEffect(() => {

     console.log("effect called.");

   }, []);

Probably relevant to other scenario as well - this shows an example but maybe not realistic one - that if you forget to clean, there could be some side effects.  

useEffect(() => {

    console.log("calling effects");

    const itv = setInterval(() =>

    {

      setCount(count + 1)

    }, 3000);

    return () => {

       clearInterval(itv);

    }

   }, [count]);