Button.js

import React from 'react';
import { string, bool, func } from 'prop-types';
import { StyledButton } from './styled';

const Button = ({
  size,
  text,
}) => (
  <StyledButton
    size={size}
    text={text}
  >
    {text}
  </StyledButton>
);

EXPButton.propTypes = {
  size: string,
  text: string,
};

EXPButton.defaultProps = {
  size: '',
  text: '',
};

export default Button;

Button.test.js

import React from 'react';
import { shallow } from 'enzyme';

import Button from '../../components/Button/Button';

describe('Component: Button', () => {
  const minProps = {
    text: '',
    size: '',
  };
  
  it('renders a button in size of "small" with text in it', () => {
    const wrapper = shallow(
      <Button {...minProps} size="small" text="Join us" />
    );
    expect(wrapper.prop('size')).toBe('small');
    expect(wrapper.prop('text')).toBe('Join us');
  });
});