Support styling custom link components

src/components/Link/Link.test.tsx

@@ -87,6 +87,8 @@ describe('Link component', () => {
         'custom-attr',
         'customVal'
       )
+      expect(getByTestId('customComponent')).toHaveClass('usa-link')
+      expect(getByTestId('customComponent')).toHaveClass('custom-class')
     })
 
     it('handles unstyled prop', () => {

src/components/Link/Link.tsx

@@ -1,15 +1,18 @@
 import React from 'react'
 import classnames from 'classnames'
 
-type StyledLinkProps = React.PropsWithChildren<{
+type StyledLinkProps = {
   variant?: 'external' | 'unstyled'
-}>
+  className?: string
+}
 
-type LinkProps = StyledLinkProps & JSX.IntrinsicElements['a']
+type LinkProps = React.PropsWithChildren<StyledLinkProps> &
+  JSX.IntrinsicElements['a']
 
 type AsCustomProps<T> = {
   asCustom: keyof JSX.IntrinsicElements | React.FunctionComponent<T>
-} & React.PropsWithChildren<T>
+} & StyledLinkProps &
+  React.PropsWithChildren<T>
 
 type PossibleLinkProps<T> = LinkProps | AsCustomProps<T>
 
@@ -19,41 +22,49 @@ function isAsCustomProps<T>(
   return (props as AsCustomProps<T>).asCustom !== undefined
 }
 
+function linkClasses(
+  variant: 'external' | 'unstyled' | undefined,
+  className: string | undefined
+) {
+  const unstyled = variant === 'unstyled'
+  const isExternalLink = variant === 'external'
+
+  return unstyled
+    ? className
+    : classnames(
+        'usa-link',
+        {
+          'usa-link--external': isExternalLink,
+        },
+        className
+      )
+}
+
 export function Link<FCProps = LinkProps>(
   props: PossibleLinkProps<FCProps>
 ): React.ReactElement {
-  // 2 ways to use this
-  // 1. default element "a", styled using default LinkProps
-  // 2. asCustom element or FunctionComponent, styled using custom
-  //    FCProps.  The custom FCProps might have a `variant` that has a
-  //    different meaning in the custom component, so we have to leave
-  //    styling up to the custom component
-
   if (isAsCustomProps<FCProps>(props)) {
-    const { asCustom, children, ...remainingProps } = props
+    const { variant, className, asCustom, children, ...remainingProps } = props
     // 1. We know props is AsCustomProps<FCProps>
     // 2. We know AsCustomProps<FCProps> is
-    //    FCProps & { children: ..., asCustom: ... }
-    // 3. Therefore we know that removing those two props leaves us
+    //    FCProps & { variant: ..., className: ..., children: ..., asCustom: ... }
+    // 3. Therefore we know that removing those props leaves us
     //    with FCProps
     //
-    // Not sure why TypeScript can't figure that out
     const linkProps: FCProps = (remainingProps as unknown) as FCProps
-    return React.createElement(asCustom, linkProps, children)
+    const classes = linkClasses(variant, className)
+    return React.createElement(
+      asCustom,
+      {
+        className: classes,
+        ...linkProps,
+      },
+      children
+    )
   } else {
     const { children, variant, className, ...linkProps } = props
-    const unstyled = variant === 'unstyled'
-    const isExternalLink = variant === 'external'
-
-    const classes = unstyled
-      ? className
-      : classnames(
-          'usa-link',
-          {
-            'usa-link--external': isExternalLink,
-          },
-          className
-        )
+
+    const classes = linkClasses(variant, className)
     return (
       <a className={classes} {...linkProps}>
         {children}