React + TypeScript only allow children of specific type

Question

I run into a trouble with trying to allow only specific children to be passed to a component.

Here are my components, HeaderLink

import { SFC, ReactElement } from "react";

import "./Header.scss";

export interface IHeaderLinkProps {
  url: string;
  label: string;
}

const HeaderLink: SFC<IHeaderLinkProps> = ({ url, label }): ReactElement => (
  <li className="nav-item">
    <a href={url}>{label}</a>
  </li>
);

export default HeaderLink;

and Header

import { SFC, ReactElement } from "react";
import { IHeaderLinkProps } from "./HeaderLink";

import "./Header.scss";

interface IHeaderProps {
  children: ReactElement<IHeaderLinkProps> | ReactElement<IHeaderLinkProps>[];
}

const Header: SFC<IHeaderProps> = ({ children }): ReactElement => {
  return (
    <header className="header">
      <div className="container">
        <div className="row">
          <div className="col">
            <img
              className="header__logo"
              src={"/logo.png"}
              alt="monqrime-logo"
            />

            <ul className="header__nav">{children}</ul>
          </div>
        </div>
      </div>
    </header>
  );
};

So the problem is that I want to allow only HeaderLink components to be passed as children to the Header, but my current solution still allows me to put everything as a children.

Answer 1

I would probably declare IHeaderProps.children as:

children: React.ReactElement<IHeaderProps> | React.ReactElement<IHeaderProps>[];

To account for the possibility of having both a single and multiple children.

In any case, what you want is not possible. See:

• How do I restrict the type of React Children in TypeScript, using the newly added support in TypeScript 2.3?

• React Component children typecheck with typescript

What you could do instead is declare a prop, let's say links?: IHeaderLinkProps[], to pass down the props you need to create those HeaderLinks, rather than their JSX, and render them inside Header:

interface IHeaderProps {
  children?: never;
  links?: IHeaderLinkProps[];
}

...

const Header: React.FC<IHeaderProps> = ({ links }) => {
  return (
    ...

    <ul className="header__nav">
      { links.map(link => <HeaderLink key={ link.url } { ...link } />) }
    </ul>

    ...
  );
};