Edit in JSFiddle

// Configure Mocha, telling both it and chai to use BDD-style tests. 
mocha.setup("tdd");
var assert = chai.assert;



let repository = {
  "200": {
    "offer-id": 200,
    "percentage": 15
  },
  "400": {
    "offer-id": 400,
    "percentage": 22
  }
};


function validateId(state) {
  if (typeof state.id !== 'string') {
    state.response = {
      "code": 400,
      "body": {
        "errors": ["The id you provided is invalid"]
      }
    };
  }
  return state;
}

function findOfferById(state) {
  //precondition
  if (state.response) return state;
  if (!repository[state.id])
    state.response = {
      "code": 404,
      "body": {
        "errors": ["The id you provided cannot be found"]
      }
    };
  else
    state.response = {
      "code": 200,
      "body": repository[state.id]
    };
  return state;
}

function json(state) {
  return JSON.stringify(state.response);
}



function getOfferById(requestId) {
  return run({
      id: requestId
    },
    [
      validateId,
      findOfferById,
      json
    ]);
}






describe('functional pipeline (with flag) endpoint should', function() {
  it('return an offer 200', function() {
    assert.equal("{\"code\":200,\"body\":{\"offer-id\":200,\"percentage\":15}}", getOfferById("200"));
  });
  it('return a validation error 400 when not found', function() {
    assert.equal("{\"code\":400,\"body\":{\"errors\":[\"The id you provided is invalid\"]}}", getOfferById(400));
  });
  it('return a not found 404', function() {
    assert.equal("{\"code\":404,\"body\":{\"errors\":[\"The id you provided cannot be found\"]}}", getOfferById("404"));
  });
});
// Run all our test suites.  Only necessary in the browser.
mocha.run();