为什么类型 SFC<AnchorProps> 不能分配给类型 SFC<{}>？

Question

以下代码在使用 TypeScript 编译时会抛出错误。

import React, { SFC } from "react"
import { Link } from "react-router-dom"
import { HashLink } from "react-router-hash-link"
import MarkdownToJSX from "markdown-to-jsx"

interface AnchorProps {
  baseUrl: string
  relativeUrl: string
  href: string
}

const Anchor: SFC<AnchorProps> = function(props) {
  /*
  FYI, props.baseUrl and props.relativeUrl is used by code I didn’t include in
  this example to keep things simple.
  */
  if (
    props.href.match(/^http(s)?:\/\//) ||
    props.href.match(/^mailto:/) ||
    props.href.match(/^tel:/)
  ) {
    return (
      <a href={props.href} rel="noopener noreferrer" target="_blank">
        {props.children}
      </a>
    )
  } else if (props.href.match(/^#/)) {
    return (
      <HashLink to={props.href} smooth>
        {props.children}
      </HashLink>
    )
  } else {
    return <Link to={props.href}>{props.children}</Link>
  }
}

const Markdown = function() {
  return (
    <MarkdownToJSX
      options={{
        overrides: {
          a: {
            component: Anchor,
            props: {
              baseUrl: "/privacy-guides",
              relativeUrl: "",
            },
          },
        },
      }}
    >
      # This is [markdown](markdown)
    </MarkdownToJSX>
  )
}

export default Markdown

/Users/sunknudsen/Sites/sunknudsen/sunknudsen-website/src/Test.tsx
TypeScript error in /Users/sunknudsen/Sites/sunknudsen/sunknudsen-website/src/Test.tsx(40,13):
No overload matches this call.
  Overload 1 of 2, '(props: Readonly<MarkdownProps>): Markdown', gave the following error.
    Type 'FunctionComponent<AnchorProps>' is not assignable to type 'string | SFC<{}> | ComponentClass<{}, any>'.
      Type 'FunctionComponent<AnchorProps>' is not assignable to type 'SFC<{}>'.
        Types of parameters 'props' and 'props' are incompatible.
          Type '{ children?: ReactNode; }' is not assignable to type 'PropsWithChildren<AnchorProps>'.
            Type '{ children?: ReactNode; }' is missing the following properties from type 'AnchorProps': baseUrl, relativeUrl, href
  Overload 2 of 2, '(props: MarkdownProps, context?: any): Markdown', gave the following error.
    Type 'FunctionComponent<AnchorProps>' is not assignable to type 'string | SFC<{}> | ComponentClass<{}, any>'.
      Type 'FunctionComponent<AnchorProps>' is not assignable to type 'SFC<{}>'.  TS2769

    38 |         overrides: {
    39 |           a: {
  > 40 |             component: Anchor,
       |             ^
    41 |             props: {
    42 |               baseUrl: "/privacy-guides",
    43 |               relativeUrl: "",

我不明白为什么。

Answer 1

这似乎是markdown-to-jsx 类型定义的限制。

具体来说，MarkdownToJSX.options.overrides 属于ComponentOverride 类型，ComponentOverride 属于这种类型：

export type ComponentOverride = string | React.ComponentClass | React.SFC | {
    component: string | React.ComponentClass | React.SFC;
    props?: any;
};

该类型不允许您将泛型参数传递给React.SFC，这意味着它采用其默认泛型参数：

type SFC<P = {}> = FunctionComponent<P>;

默认的泛型参数是一个空对象{}。

由于您的代码试图将 AnchorProps 类型分配给空对象类型，因此编译器会抱怨。

这在运行时是否会成为问题取决于底层 JavaScript 在运行时的工作方式。 可能可以这样投射：

const Markdown = function() {
  return (
    <MarkdownToJSX
      options={{
        overrides: {
          a: {
            component: Anchor,
            props: {
              baseUrl: "/privacy-guides",
              relativeUrl: ""
            }
            // We have tested that this works at runtime,
            // we we know more than the compiler knows.
          } as ComponentOverride
        }
      }}
    >
      # This is [markdown](markdown)
    </MarkdownToJSX>
  );
};