Doesn’t read commonmark when included in unit testing

I create a util function that uses commonmark to convert markdown to HTML. And then I tried to write a test case for that function, it throws error saying that commonmark is not defined.

Here is my code:

markdown.test.js

const {markdownToHtml} = require('../../services/util/markdown');
const assert = require('assert');

describe('Commonmark plugin', () => {
  it('normal text returns HTML string', () => {
    assert.strictEqual(markdownToHtml('Hello world').toBe('<p>Hello world</p>\n'));
  });
});

markdown.js

const markdownToHtml = (text) => {
  const reader = new commonmark.Parser();
  const writer = new commonmark.HtmlRenderer({safe: true});

  const parsed = reader.parse(text); // parsed is a 'Node' tree
  // transform parsed if you like...
  const result = writer.render(parsed); // result is a String
  return result;
};

module.exports = {markdownToHtml};

I get the following error when I run the test:
ReferenceError: commonmark is not defined

Start with services/util/markdown.js and markdown.test.js, then check how commonmark is exposed to modules in this project. Run the focused test and confirm the utility can construct the parser and renderer and return the expected HTML for “Hello world”.