Where should functions in function components go?

Question

I'm trying to convert this cool <canvas> animation I found here into a React reusable component. It looks like this component would require one parent component for the canvas, and many children components for the function Ball().

It would probably be better for performance reasons to make the Balls into stateless components as there will be many of them. I'm not as familiar with making stateless components and wondered where I should define the this.update() and this.draw functions defined in function Ball().

Do functions for stateless components go inside the component or outside? In other words, which of the following is better?

1:

const Ball = (props) => {
    const update = () => {
        ...
    }

    const draw = () => {
        ...
    }

    return (
       ...
    );
}

2:

function update() {
     ...
}

function draw() {
     ...
}

const Ball = (props) => {
    return (
       ...
    );
}

What are the pros and cons of each and is one of them better for specific use cases such as mine?

Answer 1

If you want to use props or state of a component in a function, that should be defined in the component with useCallback.

function Component(props){
  const onClick=useCallback(()=>{
     // Do some things with props or state
  },[])

  return <Something {...{onClick}} />
}

On the other hand, if you don't want to use props or state in a function, define that outside of the component.

const computeSomethings=()=>{
   // Do some things with params or side effects
}

function Component(props){
  return <Something onClick={computeSomethings} />
}

For HTML tags you don't need useCallback because that will handle in the react side and will not be assigned to HTML.

function Component(props){
  const onClick=()=>{
     // Do some things with props or state
  }

  return <div {...{onClick}} />
}

Edit: Functions in hooks

For using function in hooks, for example useEffect, my suggestion is defining a function inside useEffect. If you're worried about DRY, make your function pure call it in hook and give your params to it. What about hooks deps? You should/could add all of your params to hooks deps, but useEffect just needs deps which should affect for them changes.

Answer 2

The first thing to note is that stateless functional components cannot have methods: You shouldn't count on calling update or draw on a rendered Ball if it is a stateless functional component.

In most cases you should declare the functions outside the component function so you declare them only once and always reuse the same reference. When you declare the function inside, every time the component is rendered the function will be defined again.

There are cases in which you will need to define a function inside the component to, for example, assign it as an event handler that behaves differently based on the properties of the component. But still you could define the function outside Ball and bind it with the properties, making the code much cleaner and making the update or draw functions reusable:

// you can use update somewhere else
const update = (propX, a, b) => { ... };
    
const Ball = props => (
  <Something onClick={update.bind(null, props.x)} />
);

If you're using hooks, you can use useCallback to ensure the function is only redefined when any of its dependencies change (props.x in this case):

const Ball = props => {
  const onClick = useCallback((a, b) => {
    // do something with a, b and props.x
  }, [props.x]);

  return (
    <Something onClick={onClick} />
  );
}

This is the wrong way:

const Ball = props => {
  function update(a, b) {
    // props.x is visible here
  }
    
  return (
    <Something onClick={update} />
  );
}

When using useCallback, defining the update function in the useCallback hook itself or outside the component becomes a design decision more than anything: You should take into account if you're going to reuse update and/or if you need to access the scope of the component's closure to, for example, read/write to the state. Personally I choose to define it inside the component by default and make it reusable only if the need arises, to prevent over-engineering from the start. On top of that, reusing application logic is better done with more specific hooks, leaving components for presentational purposes. Defining the function outside the component while using hooks really depends on the grade of decoupling from React you want for your application logic.

Another common discussion about useCallback is whether to always use it for every function or not. That is, treat is as opt-in or always recommendable. I would argue to always use useCallback: I've seen many bugs caused by not wrapping a function in useCallback and not a single scenario where doing so affects the performance or logic in any way. In most cases, you want to keep a reference while the dependencies don't change, so you can use the function itself as a dependency for other effects, memos or callback. In many cases the callback will be passed as a prop to other elements, and if you memoized it with useCallback you won't change the props (thus re-render) other components independently of how cheap or costly that would be. I've seen many thousands of functions declared in components and not a single case in which using useCallback would have any down side. On the other hand most functions not memoized with useCallback would eventually be changed to do so, causing serious bugs or performance issues if the developer doesn't recognize the implications of not doing so. Technically there is a performance hit by using useCallback, as you would be creating and additional function but it is negligible compared to the re-declaration of the function that always has to happen either you use useCallback or not and the overall footprint of React and JavaScript. So, if you are really concerned about the performance impact of useCallback versus not using it, you should be questioning yourself if React is the right tool for the job.

Answer 3

You can place functions inside stateless functional components:

function Action() {
    function handlePick(){
        alert("test");
    }

    return (
        <div>
            <input type="button" onClick={handlePick} value="What you want to do ?" />
        </div>
    )
}

But it's not a good practice as the function handlePick() will be defined every time the component is rendered.

It would be better to define the function outside the component:

function handlePick(){
    alert("test");
}

function Action() {
    return (
        <div>
            <input type="button" onClick={handlePick} value="What you want to do ?" />
        </div>
    )
}

Answer 4

import React, { useState } from 'react';
function Example() {
  // Declare a new state variable, which we'll call "count"
  const [count, setCount] = useState(0);
  const a = () => {
    setCount(count + 1);
  };
  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={a}>Click me</button>
    </div>
  );
}
export default Example;

Answer 5

We can use the React hook useCallback as below in a functional component:

const home = (props) => {
    const { small, img } = props
    const [currentInd, setCurrentInd] = useState(0);
    const imgArrayLength = img.length - 1;
    useEffect(() => {
        let id = setInterval(() => {
            if (currentInd < imgArrayLength) {
                setCurrentInd(currentInd => currentInd + 1)
            }
            else {
                setCurrentInd(0)
            }
        }, 5000);
        return () => clearInterval(id);
    }, [currentInd]);
    const onLeftClickHandler = useCallback(
        () => {
            if (currentInd === 0) {

            }
            else {
                setCurrentInd(currentInd => currentInd - 1)
            }
        },
        [currentInd],
    );

    const onRightClickHandler = useCallback(
        () => {
            if (currentInd < imgArrayLength) {
                setCurrentInd(currentInd => currentInd + 1)
            }
            else {

            }
        },
        [currentInd],
    );
    return (
        <Wrapper img={img[currentInd]}>
            <LeftSliderArrow className={currentInd > 0 ? "red" : 'no-red'} onClick={onLeftClickHandler}>
                <img src={Icon_dir + "chevron_left_light.png"}></img>
            </LeftSliderArrow>
            <RightSliderArrow className={currentInd < imgArrayLength ? "red" : 'no-red'} onClick={onRightClickHandler}>
                <img src={Icon_dir + "chevron_right_light.png"}></img>
            </RightSliderArrow>
        </Wrapper>);
}

export default home;

I'm getting 'img' from it's parent and that is an array.