I wonder about differences between using `useEffect()` with empty dependency array and not using `useEffect()` itself

Question

I know useEffect() with no array run the callback only at the first render.

Then what is the differences between useEffect(()=>{},[]) and no useEffect().

I mean between this:

function myComponent() {
  // some states, variables

  useEffect(() => {
    // do something on mount <= this is my current concern.
  }, [/* empty */]);

  // return Element ...
}

and this:

function myComponent() {
  // some states, variables

  // do something on mount <= this is my current concern.
  
  // return Element ...
}

Answer 1

In React, a component re-renders whenever there is a change in it's state or one of it's props.
The reason it behaves like this is so that it would be possible to "react" to a change in the mentioned data, and to reflect UI changes accordingly.

Every time the component re-renders, so does any logic within it that is not cached (functions, variables, etc..)

useEffect helps us with reacting to a change in the state or props that are stated in it's dependency array.
It gives us to option to automatically run a callback function in such an event/s.

useEffect with an empty dependency array, will run only a single time when the component is mounted.

So in this example -

function myComponent() {
  // some states, variables

  useEffect(() => {
    // do something on mount <= this is my current concern.
  }, [/* empty */]);

  // return Element ...
}

The callback function inside the useEffect will run only once, when the component is first "brought to life".
Subsequent renders will not invoke this.

While in this example -

function myComponent() {
  // some states, variables

  // do something on mount <= this is my current concern.
  
  // return Element ...
}

Whatever you put in there will run every time the component re-renders.
Whether this is ok or not depends on your use-case and what function are you trying to run, if it's "cheap" to run or not, etc..