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 getOfferById(id) {
  var response = {};
  if (typeof id !== 'string')
    response = {
      "code": 400,
      "body": {
        "errors": ["The id you provided is invalid"]
      }
    };
  else if (repository[id]) {
    response = {
      "code": 200,
      "body": repository[id]
    };
  } else {
    notify("Offer not found: " + id);
    response = {

      "code": 404,
      "body": {
        "errors": ["The id you provided cannot be found"]
      }
    };
  }
  return JSON.stringify(response);
}




describe('ifs 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();