The contract between getPrefix() and prefixWord() is violated but this is not detected by the specs

utils_contract_violated_with_return_type_inconsistency.js

// utils/get_prefix.js
// Changed the return value type from string to object
export const getPrefix = () => ({ en: 'mega', fr: 'méga' });

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

// spec/utils/get_prefix.js
import getPrefix from '../../utils/get_prefix';

// Updated the test on getPrefix() to pass
describe('getPrefix()', => {
  it('should return a prefix object', =>
    expect(getPrefix).to.equal({ en: 'mega', fr: 'méga' }));
});

// spec/utils/prefix_word.js
import prefixWord from '../../utils/prefix_word';

// Forgot to update the stub on getPrefix()
const getPrefix = sinon.stub().returns('mega');

// The test on prefixWord() still passes but should fail
describe('prefixWord()', => {
  it('should prefix the supplied word', =>
    expect(prefixWord(getPrefix, 'Word')).to.equal('megaWord'));
});