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 Error {
  constructor(message) {
    this.message = message;
  }
}
class ValidationError extends Error {}
class NotFoundError extends Error {}

function validId(id) {
  if (typeof id !== 'string') throw new ValidationError("The id you provided is invalid");
  return id;
}

function findOfferById(id) {
  if (!repository[id]) {
    notify("Offer not found: " + id);
    throw new NotFoundError("The id you provided cannot be found");
  }
  return repository[id];
}

function getOfferById(requestId) {
  let response = null;
  try {
    const id = validId(requestId);
    const offer = findOfferById(id);
    response = {
      code: 200,
      body: offer
    };
  } catch (err) {
    if (err instanceof ValidationError) {
      response = {
        code: 400,
        body: {
          errors: [err.message]
        }
      };
    } else if (err instanceof NotFoundError) {
      response = {
        code: 404,
        body: {
          errors: [err.message]
        }
      };
    }
  }
  return JSON.stringify(response);
}




describe('exceptions 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();