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
  }
};


class IResponse {
  constructor(data) {
    this.data = data;
  }
  then(fn) {
    //todo
  }
  fail(fn) {
    //todo
  }
  response() {
    return this.data;
  }
}
//do
class SuccesfullResponse extends IResponse {
  then(fn) {
    return fn(this);
  }
  fail(fn) {
    return this;
  }
}
//do not
class ErrorResponse extends IResponse {
  then(fn) {
    return this;
  }
  fail(fn) {
    return fn(this);
  }
}

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

function findOfferById(state) {
  if (!repository[state.data.id]){
    notify("Offer not found: " + state.id);
    return new ErrorResponse({
      "code": 404,
      "body": {
        "errors": ["The id you provided cannot be found"]
      }
    });
  }
  else
    return new SuccesfullResponse({
      "code": 200,
      "body": repository[state.data.id]
    });
}

function json(state) {
  return new SuccesfullResponse(JSON.stringify(state.data));
}


function getOfferById(requestId) {
  return validateId(requestId)
  .then(findOfferById)
  .then(json)
  .fail(json)
  .response();
}





describe('functional pipelines (with either) 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();