feat(components): add Anchor component

src/components/Anchor/Anchor.docs.mdx

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+import { Status, Props, Story } from '../../../.storybook/components';
+import Anchor from '.';
+
+# Anchor
+
+<Status.Stable />
+
+Anchor is used to display a link or button that visually looks like a hyperlink.
+
+<Story id="typography-anchor--as-link" />
+<Props of={Anchor} />
+
+## Usage guidelines
+
+- **Do** use the `mega` size as the default for body text
+- **Do** open the link in the same window unless it leads to an external source.
+- **Do not** use in combination with the button component, use the tertiary button variant instead
+
+### As button
+
+<Story id="typography-anchor--as-button" />

src/components/Anchor/Anchor.spec.tsx

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+/**
+ * Copyright 2020, SumUp Ltd.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import React from 'react';
+
+import {
+  create,
+  render,
+  renderToHtml,
+  axe,
+  RenderFn,
+  act,
+  userEvent
+} from '../../util/test-utils';
+
+import { Anchor, AnchorProps } from './Anchor';
+
+describe('Anchor', () => {
+  function renderAnchor(renderFn: RenderFn, props: AnchorProps) {
+    return renderFn(<Anchor {...props} />);
+  }
+
+  const baseProps = { children: 'Anchor' };
+
+  describe('styles', () => {
+    it('should render as a `span` when neither href nor onClick is passed', () => {
+      const actual = renderAnchor(create, baseProps);
+      expect(actual).toMatchSnapshot();
+    });
+
+    it('should render as an `a` when an href (and onClick) is passed', () => {
+      const props = {
+        ...baseProps,
+        href: 'https://sumup.com',
+        onClick: jest.fn()
+      };
+      const actual = renderAnchor(create, props);
+      expect(actual).toMatchSnapshot();
+    });
+
+    it('should render as a `button` when an onClick is passed', () => {
+      const props = { ...baseProps, onClick: jest.fn() };
+      const actual = renderAnchor(create, props);
+      expect(actual).toMatchSnapshot();
+    });
+  });
+
+  describe('business logic', () => {
+    it('should call the onClick handler when rendered as a link', () => {
+      const props = {
+        ...baseProps,
+        href: 'https://sumup.com',
+        onClick: jest.fn(event => event.preventDefault()),
+        'data-testid': 'anchor'
+      };
+      const { getByTestId } = renderAnchor(render, props);
+
+      act(() => {
+        userEvent.click(getByTestId('anchor'));
+      });
+
+      expect(props.onClick).toHaveBeenCalledTimes(1);
+    });
+
+    it('should call the onClick handler when rendered as a button', () => {
+      const props = {
+        ...baseProps,
+        onClick: jest.fn(),
+        'data-testid': 'anchor'
+      };
+      const { getByTestId } = renderAnchor(render, props);
+
+      act(() => {
+        userEvent.click(getByTestId('anchor'));
+      });
+
+      expect(props.onClick).toHaveBeenCalledTimes(1);
+    });
+  });
+
+  describe('accessibility', () => {
+    it('should meet accessibility guidelines', async () => {
+      const props = {
+        ...baseProps,
+        href: 'https://sumup.com',
+        onClick: jest.fn()
+      };
+      const wrapper = renderAnchor(renderToHtml, props);
+      const actual = await axe(wrapper);
+      expect(actual).toHaveNoViolations();
+    });
+  });
+});

src/components/Anchor/Anchor.story.tsx

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+/**
+ * Copyright 2020, SumUp Ltd.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import React, { FunctionComponent } from 'react';
+
+import Anchor from '.';
+import docs from './Anchor.docs.mdx';
+
+export default {
+  title: 'Typography/Anchor',
+  component: Anchor,
+  parameters: {
+    docs: { page: docs },
+    jest: ['Anchor']
+  }
+};
+
+export const AsLink: FunctionComponent = () => (
+  <Anchor href="https://opensource.sumup.com">
+    {`View SumUp's OSS projects`}
+  </Anchor>
+);
+
+export const AsButton: FunctionComponent = () => (
+  <Anchor onClick={() => alert('Hello')}>Say hello</Anchor>
+);

src/components/Anchor/Anchor.tsx

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+/**
+ * Copyright 2020, SumUp Ltd.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import React, { HTMLProps, ReactNode, ReactElement } from 'react';
+import { css } from '@emotion/core';
+
+import styled, { StyleProps } from '../../styles/styled';
+import { focusOutline } from '../../styles/style-helpers';
+import Text from '../Text';
+import { TextProps } from '../Text/Text';
+import { useComponents } from '../ComponentsContext';
+
+interface BaseProps extends TextProps {
+  children: ReactNode;
+}
+
+type LinkElProps = Omit<HTMLProps<HTMLAnchorElement>, 'size'>;
+type ButtonElProps = Omit<HTMLProps<HTMLButtonElement>, 'size'>;
+
+export type AnchorProps = BaseProps & LinkElProps & ButtonElProps;
+
+type ReturnType = ReactElement<any, any> | null;
+
+const baseStyles = ({ theme }: StyleProps) => css`
+  display: inline-block;
+  text-decoration: underline;
+  text-decoration-skip-ink: auto;
+  border: 0;
+  outline: none;
+  background: none;
+  padding: 0;
+  margin-top: 0;
+  margin-left: 0;
+  margin-right: 0;
+  color: ${theme.colors.p700};
+
+  &:hover,
+  &:active {
+    color: ${theme.colors.p900};
+    cursor: pointer;
+  }
+
+  &:visited {
+    color: ${theme.colors.v700};
+
+    &:hover,
+    &:active {
+      color: ${theme.colors.v900};
+    }
+  }
+
+  &:focus {
+    ${focusOutline({ theme })};
+    border-radius: ${theme.borderRadius.giga};
+  }
+`;
+
+const BaseAnchor = styled(Text)<AnchorProps>(baseStyles);
+
+/**
+ * The Anchor is used to display a link or button that visually looks like
+ * a hyperlink. Based on the Text component, so it also supports its props.
+ */
+export function Anchor(props: BaseProps & LinkElProps): ReturnType;
+export function Anchor(props: BaseProps & ButtonElProps): ReturnType;
+export function Anchor(props: AnchorProps): ReturnType {
+  const { Link } = useComponents();
+  const AnchorLink = BaseAnchor.withComponent(Link);
+
+  if (!props.href && !props.onClick) {
+    return <Text as="span" {...props} />;
+  }
+
+  if (props.href) {
+    return <AnchorLink {...props} />;
+  }
+
+  return <BaseAnchor as="button" {...props} />;
+}