var expect = chai.expect;
describe('Initializing', function() {
  it('with new', function() {
    expect(new Moon({el: "#initialize"}) instanceof Moon).to.equal(true);
  });
});

describe('Data', function() {
  var dataApp = new Moon({
    el: "#data",
    data: {
      msg: "Hello Moon!"
    }
  });
  it('while initializing', function() {
    expect(document.getElementById("data").innerHTML).to.equal("Hello Moon!");
  });
  it('when setting', function() {
    dataApp.set('msg', 'New Value');
  });
  expect(document.getElementById("data").innerHTML).to.equal("New Value");
});

var expect = chai.expect;
describe('Initializing', function() {
  it('with new', function() {
    expect(new Moon({el: "#initialize"}) instanceof Moon).to.equal(true);
  });
});

describe('Data', function() {
  var dataApp = new Moon({
    el: "#data",
    data: {
      msg: "Hello Moon!"
    }
  });
  it('while initializing', function() {
    expect(document.getElementById("data").innerHTML).to.equal("Hello Moon!");
  });
  it('when setting', function() {
    dataApp.set('msg', 'New Value');
    expect(document.getElementById("data").innerHTML).to.equal("New Value");
  });
});

put expect inside