Helpful setup function patterns for testing

How a setup pattern can simplify testing components

I ran into this pattern at work, and I love it. It makes tests straightforward to read. This way, each test, is only testing one prop at a time, and the setup is ultra-clear. If you're updating a single prop, the test itself declares that. Let's imagine the following component (stolen from the footer of my personal site but simplified).

The goal will be to have a component that renders three links, and a view more button, or more than 3 links and, a show less button.

import React from 'react';

const Footer = ({ links }) => {
  // handles the number of links to show  
  const [linksLength, setlinksLength] = React.useState(2);
  // handles the button text
  const buttonText = linksLength === 2 ? 'View more' : 'View less';
  // handles the links to show, if the number > 2 it 
  const socialLinks = links.filter((item, index) => index <= socialLength);

  const handleClick = () => {
    setSocialLength(socialLength === links.length ? 2 : links.len)
  }

  return (
      <div>
        <h4>Site Links</h4>
        <ul>
          {socialLinks.map(({ siteLink, siteName }) => 
            (
              <li key={siteLink}>
                <a href={siteLink}>
                  {siteName}
                </a>
              </li>
            ))}
        </ul>
        <button onClick={() => handleClick()}>{buttonText}</button>
      </div>
    )

There are a million ways to write that but suppose that's how we did it. I'd write three tests to write for this I think.

toggles button text between "view more" and "view less"

three links are rendered by default

more than three links are rendered on click

maybe a 4th for no button when there are fewer than 3 links

With the setup function this test looks something like this:

describe('<Footer/>', () => { // this is a style I like just preference
    const setup = overrides => {    
      const props = {
      links: ['link-one', 'link-two', 'link-three', 'link-four']
      ...overrides
      }

      const R = render(<Footer {...props}/>)

      return {
      ...R,
      props
    }
  }

  it('toggles button text between "view more" and "view less"', () => {
    const {
      queryByText, getByText, getByRole
    } = setup();

    expect(getByText(/View more/i)).toBeInTheDocument();
    expect(queryByText(/View less/i)).not.toBeInTheDocument();
    userEvent.click(getByRole('button'));
    expect(getByText(/View less/i)).toBeInTheDocument();
    expect(queryByText(/View more/i)).not.toBeInTheDocument();
  })
})

Generally, I'm weakly opposed to multiple expects in a block but that case makes sense.

Next we handle showing the default.

it('shows three links by default', () => {
    const mockLinks = ['One', 'Two', 'Three', 'Four'];
    const { queryByText } = setup({links: mockLinks});

    expect(queryByText(/Four/i)).not.toBeInTheDocument();
  })

For me, this test is super clean. I can know everything important about this inside the "it block".

I suppose some folks would dislike this next test since it's basically the same. Reasonable arguments could be made about duplication, or perhaps how you don't need to have a test that tests super similar things. I like this test duplication.

it('more than three links are rendered on click', () => {
    const mockLinks = ['One', 'Two', 'Three', 'Four'];
    const {
      queryByText, getByText, getByRole
    } = setup({links: mockLinks});


    expect(queryByText(/Four/i)).not.toBeInTheDocument();
    userEvent.click(getByRole('button'));
    expect(getByText(/Four/i)).toBeInTheDocument();
  })

Look how clean! One thing to note here is that I'm getting by role button. A few months ago I would have used a getByText(/View more/i) for that. But suppose marketing decides they want to change that to Show more. Your test, for how many links show, fails. It's a little thing, but I think it's nicer. Also, it forces you to make a small react component, rather than some 300 line monstrosity.

If we want, we can add that fourth test (which would fail if we don't fix the example code).

it('shows no button when there are fewer than 3 links', () => {
    const mockLinks = ['One', 'Two'];
    const { queryByRole } = setup({links: mockLinks});

    expect(queryByRole('button')).not.toBeInTheDocument();

    // This is also okay I just wouldn't do it now
    // expect(queryByText(/View more/i)).not.toBeInTheDocument();
  })

This simple little pattern has been helpful for me. I typically write more testable code because of this setup function. I'm glad I learned it and it's useful for me. If it's you're kinda thing, you can use this extension. It's not particularly fancy but I made it to make that easier for myself. Maybe it helps you. Message me if this makes sense or you have some other testing tips. I think testing is probably my favorite part of writing code these days.