The contract between getPrefix() and prefixWord() is enforced by the specs

utils_contract_respected_and_enforced.js

// utils/get_prefix.js
export const getPrefix = (lang) => {
  if (lang === 'fr') return 'méga';
  return 'mega';
};

// utils/prefix_word.js
export const prefixWord = (prefixGetter, wordToPrefix) => prefixGetter() + wordToPrefix;

// spec/fixtures/prefix_fixtures.js
export const prefixFixtures = {
  en: { args: 'en', result: 'mega' },
  fr: { args: 'en', result: 'méga' },
};


// spec/utils/get_prefix_spec.js
import getPrefix from '../../utils/get_prefix';
import prefixFixtures from '../fixtures/prefix_fixtures';

const { args, result } = prefixFixtures.en;

describe('getPrefix()', => {
  it('should return mega', () =>
    expect(getPrefix(...args)).to.equal(result));
});

// spec/utils/prefix_word_spec.js
import sinon from 'sinon';

import prefixWord from '../../utils/prefix_word';
import prefixFixtures from '../../utils/prefix_fixtures';

const { args, result } = prefixFixtures.en;
const getPrefix = sinon.stub().returns(result);

describe('prefixWord()', => {
  it('should call getPrefix with the correct args', () =>
    expect(prefixWord.args).to.deep.equal([args]));

  it('should prefix the supplied word', =>
    expect(prefixWord(getPrefix, 'Word')).to.equal('megaWord'));
});