cucumber-js-demo/ts/features/steps/composition.ts

import { When, Then } from 'cucumber';
import { expect } from 'chai';

When('the step definitions are written in {word}', function (language: string) {
  this.language = language;
});

Then("I don't get a warning about undefined steps", function () {
  expect(this.language).to.equal('TypeScript');
});