Allow Link to render any valid component type

package.json

@@ -49,6 +49,7 @@
     "react": "^16.8.0",
     "react-bodymovin": "2.0.0",
     "react-dom": "^16.8.1",
+    "react-is": "16.8.6",
     "styled-system": "5.0.5"
   },
   "devDependencies": {

pages/components/docs/Link.md

@@ -4,6 +4,8 @@ The Link component styles anchor tags with default blue styling and hover text d
 
 In special cases where you'd like a `<button>` styled like a `Link`, use `<Link as='button'>`. Make sure to provide a click handler with `onClick`.
 
+**Important:** When using the `as` prop, be sure to always render an accessible element type, like `a`, `button`, `input`, or `summary`.
+
 ## Default example
 
 ```.jsx
@@ -16,10 +18,10 @@ Link components get `COMMON` and `TYPOGRAPHY` system props. Read our [System Pro
 
 ## Component props
 
-| Name | Type | Default | Description |
-| :- | :- | :-: | :- |
-| href | String | | URL to be used for the Link |
-| underline | Boolean | false | Adds underline to the Link |
-| as | String | 'a' | Can be 'a', 'button', 'input', or 'summary'
+| Name      | Type    | Default | Description                                 |
+| :-------- | :------ | :-----: | :------------------------------------------ |
+| href      | String  |         | URL to be used for the Link                 |
+| underline | Boolean |  false  | Adds underline to the Link                  |
+| as        | String  |   'a'   | Can be 'a', 'button', 'input', or 'summary' |
 
 export const meta = {displayName: 'Link'}

src/Link.js

@@ -1,8 +1,9 @@
 import PropTypes from 'prop-types'
 import styled from 'styled-components'
-import {TYPOGRAPHY, COMMON} from './constants'
-import theme from './theme'
 import {system} from 'styled-system'
+import {COMMON, TYPOGRAPHY} from './constants'
+import theme from './theme'
+import elementType from './utils/elementType'
 
 const buttonStyles = {
   display: 'inline-block',
@@ -29,7 +30,7 @@ const Link = styled.a`
     text-decoration: underline;
     ${hoverColor};
   }
-  ${props => (props.is === 'button' ? buttonStyles : '')};
+  ${props => (props.as === 'button' ? buttonStyles : '')};
   ${TYPOGRAPHY} ${COMMON};
 `
 
@@ -39,7 +40,7 @@ Link.defaultProps = {
 }
 
 Link.propTypes = {
-  as: PropTypes.oneOf(['a', 'button', 'input', 'summary']),
+  as: elementType,
   href: PropTypes.string,
   theme: PropTypes.object,
   underline: PropTypes.bool,

src/__tests__/Link.js

@@ -26,6 +26,14 @@ describe('Link', () => {
     expect(render(<Link as="button" />).type).toEqual('button')
   })
 
+  it('respects the "as" prop with non-string component types', () => {
+    function ExampleLink({children}) {
+      return <a className="example-link">{children}</a>
+    }
+
+    expect(render(<Link as={ExampleLink}>Example</Link>)).toMatchSnapshot()
+  })
+
   it('respects hoverColor prop', () => {
     expect(render(<Link hoverColor="blue.4" />)).toMatchSnapshot()
   })
@@ -34,4 +42,8 @@ describe('Link', () => {
     expect(render(<Link fontStyle="italic" />)).toHaveStyleRule('font-style', 'italic')
     expect(render(<Link as="i" fontStyle="normal" />)).toHaveStyleRule('font-style', 'normal')
   })
+
+  it('applies button styles when rendering a button element', () => {
+    expect(render(<Link as="button" />)).toMatchSnapshot()
+  })
 })

src/__tests__/__snapshots__/Link.js.snap

@@ -1,5 +1,37 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`Link applies button styles when rendering a button element 1`] = `
+.c0 {
+  -webkit-text-decoration: none;
+  text-decoration: none;
+  display: inline-block;
+  padding: 0;
+  font-size: inherit;
+  white-space: nowrap;
+  cursor: pointer;
+  -webkit-user-select: none;
+  -moz-user-select: none;
+  -ms-user-select: none;
+  user-select: none;
+  background-color: transparent;
+  border: 0;
+  -webkit-appearance: none;
+  -moz-appearance: none;
+  appearance: none;
+  color: #0366d6;
+}
+
+.c0:hover {
+  -webkit-text-decoration: underline;
+  text-decoration: underline;
+}
+
+<button
+  className="c0"
+  color="blue.5"
+/>
+`;
+
 exports[`Link passes href down to link element 1`] = `
 .c0 {
   -webkit-text-decoration: none;
@@ -55,3 +87,11 @@ exports[`Link respects hoverColor prop 1`] = `
   color="blue.5"
 />
 `;
+
+exports[`Link respects the "as" prop with non-string component types 1`] = `
+<a
+  className="example-link"
+>
+  Example
+</a>
+`;

src/utils/elementType.js

@@ -0,0 +1,14 @@
+import {isValidElementType} from 'react-is'
+
+// This function is a temporary workaround until we can get
+// the official PropTypes.elementType working (https://git.io/fjMLX).
+// PropTypes.elementType is currently `undefined` in the browser.
+function elementType(props, propName, componentName) {
+  if (props[propName] && !isValidElementType(props[propName])) {
+    return new Error(
+      `Invalid prop '${propName}' supplied to '${componentName}': the prop is not a valid React component`
+    )
+  }
+}
+
+export default elementType