javascript - 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

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 of 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 one of its dependencies (props.x in this case) changes:

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 our 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.