javascript - When to use JSX.Element vs ReactNode vs ReactElement?

Question

I am currently migrating a React application to TypeScript. So far, this works pretty well, but I have a problem with the return types of my render functions, specifically in my functional components.

I have always used JSX.Element as the return type, now this doesn't work any more if a component decides to not render anything, i.e. returns null, since null is not a valid value for JSX.Element. This was the beginning of my journey. I searched the web and found that you should use ReactNode instead, which includes null and a few other things that can happen.

However, when creating a functional component, TypeScript complains about the ReactNode type. Again, after some searching I found, that for functional components you should use ReactElement instead. However, if I do so, the compatibility issue is gone, but now TypeScript again complains about null not being a valid value.

To cut a long story short, I have three questions:

1. What is the difference between JSX.Element, ReactNode and ReactElement?
2. Why do the render methods of class components return ReactNode, but functional components return ReactElement?
3. How do I solve this with respect to null?

Answer 1

What is the difference between JSX.Element, ReactNode and ReactElement?

A ReactElement is an object with a type and props.

 type Key = string | number

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

A ReactNode is a ReactElement, a ReactFragment, a string, a number or an array of ReactNodes, or null, or undefined, or a boolean:

type ReactText = string | number;
type ReactChild = ReactElement | ReactText;

interface ReactNodeArray extends Array<ReactNode> {}
type ReactFragment = {} | ReactNodeArray;

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

JSX.Element is a ReactElement, with the generic type for props and type being any. It exists, as various libraries can implement JSX in their own way, therefore JSX is a global namespace that then gets set by the library, React sets it like this:

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

By example:

 <p> // <- ReactElement = JSX.Element
   <Custom> // <- ReactElement = JSX.Element
     {true && "test"} // <- ReactNode
  </Custom>
 </p>

Why do the render methods of class components return ReactNode, but function components return ReactElement?

Indeed, they do return different things. Components return:

 render(): ReactNode;

And functions are "stateless components":

 interface StatelessComponent<P = {}> {
    (props: P & { children?: ReactNode }, context?: any): ReactElement | null;
    // ... doesn't matter
}

This is actually due to historical reasons.

How do I solve this with respect to null?

Type it as ReactElement | null just as react does. Or let Typescript infer the type.

_{source for the types}