JSX.Element vs ReactElement vs ReactNode

#javascript #react #webdev #beginners

These three types usually confuse novice React developers. It seems like they are the same thing, just named differently.
But it's not quite right.

`JSX.Element` vs `ReactElement`

Both types are the result of React.createElement()/jsx() function call.

They are both objects with:

type
props
key
a couple of other "hidden" properties, like ref, $$typeof, etc

`ReactElement`

ReactElement type is the most basic of all. It's even defined in React source code using flow!

// ./packages/shared/ReactElementType.js

export type ReactElement = {|
  $$typeof: any,
  type: any,
  key: any,
  ref: any,
  props: any,
  // ReactFiber
  _owner: any,

  // __DEV__
  _store: {validated: boolean, ...},
  _self: React$Element<any>,
  _shadowChildren: any,
  _source: Source,
|};

This type is also defined in DefinitelyTyped package.

interface ReactElement<P = any, T extends string | JSXElementConstructor<any> = string | JSXElementConstructor<any>> {
  type: T;
  props: P;
  key: Key | null;
}

`JSX.Element`

It's more generic type. The key difference is that props and type are typed as any in JSX.Element.

declare global {
  namespace JSX {
    interface Element extends React.ReactElement<any, any> { }
    // ...
  }
}

This gives flexibility in how different libraries implement JSX.
For example, Preact has its own implementation with different API.

`ReactNode`

ReactNode type is a different thing. It's not a return value of React.createElement()/jsx() function call.

const Component = () => {
  // Here it's ReactElement
  return <div>Hello world!</div>
}

// Here it's ReactNode
const Example = Component();

React node itself is a representation of the virtual DOM. So ReactNode is the set of all possible return values of a component.

type ReactChild = ReactElement | ReactText;

type ReactFragment = {} | Iterable<ReactNode>;

interface ReactPortal extends ReactElement {
  key: Key | null;
  children: ReactNode;
}

type ReactNode =
  | ReactChild
  | ReactFragment
  | ReactPortal
  | boolean
  | null
  | undefined;

What to use for `children`?

Generally speaking, ReactNode is the correct way to type the children prop. It gives the most flexibility while maintaining the proper type checking.

But it has a caveat, because ReactFragment allows a {} type.

const Item = ({ children }: { children: ReactNode }) => {
  return <li>{children}</li>;
}

const App = () => {
  return (
    <ul>
      // Run-time error here, objects are not valid children!
      <Item>{{}}</Item>
    </ul>
  );
}