Testable Tailwind CSS plugins

January 4th, 2023

A great thing about Tailwind CSS plugins being written in JavaScript is that they can be tested using tools like Jest.

Here's an example from https://github.com/opdavies/tailwindcss-plugin-jest-example (it may need updating to work with the latest Tailwind versions or to use the latest best practices):

function run(options = {}) {
  return postcss(
    tailwindcss({
      corePlugins: false,
      plugins: [plugin(options)],
    }),
  ).process("@tailwind utilities;", {
    from: undefined,
  });
}

expect.extend({
  toMatchCss: cssMatcher,
});

test("it generates the correct classes with no variants", () => {
  const output = `
    .test {
      display: block
    }
  `;

  run().then((result) => {
    expect(result.css).toMatchCss(output);
  });
});

test("it generates the correct classes with variants", () => {
  const output = `
    .test {
      display: block
    }
    .hover\\:test:hover {
      display: block
    }
    .focus\\:test:focus {
      display: block
    }
  `;

  run({ variants: ["hover", "focus"] }).then((result) => {
    expect(result.css).toMatchCss(output);
  });
});

Within the test, Tailwind can be run using PostCSS and generates styles based on a provided configuration, which is then checked against some expected output. If the generated styles match what was expected, the tests pass and the plugin is working as expected.