Testing a Stack structure with Mocha and Chai

<!DOCTYPE html>
<html>

  <head>
    <link data-require="mocha@*" data-semver="1.13.0" rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/mocha/1.13.0/mocha.css" />
    <script data-require="mocha@*" data-semver="1.13.0" src="//cdnjs.cloudflare.com/ajax/libs/mocha/1.13.0/mocha.js"></script>
    <script data-require="mocha-chai@1.9.0" data-semver="1.9.0" src="//cdnjs.cloudflare.com/ajax/libs/chai/1.9.0/chai.js"></script>
    <link rel="stylesheet" href="style.css" />
    <script src="stack.js"></script>
  </head>

  <body>
    <div id="mocha"></div>
    <script>mocha.setup('bdd')</script>
    <script src="stack-spec.js"></script>
    <script>mocha.run()</script>
</body>

</html>

// Code goes here
function Stack(){
  this.data = [];
  
  this.push = function(element) {
    this.data.push(element);
  }
  
  this.pop = function() {
    var index = this.data.length-1
    var top = this.data[index];
    this.data.splice(index, 1);
    return top;
  }  

}

/* Styles go here */

Testing a Stack structure with Mocha and Chai

var  expect = chai.expect

describe('Stack', function() {
  it('creates a Stack', function() {
     expect(new Stack()).to.exist 
  });
  
  it('has a data array', function() {
    var stack = new Stack();
    expect(stack.data).to.exist;
    expect(stack.data).to.be.an('array');
  }); 

  it('push an element to the stack', function() {
    var stack = new Stack()
    stack.push(1);
    expect(stack.data).to.have.lengthOf(1);
  });   
  
  it('pop an element from the stack and return it', function() {
    var stack = new Stack();
    stack.push(1);
    var element = stack.pop();
    expect(element).to.be.equal(1);
    expect(stack.data).to.have.lengthOf(0);
  }); 
  
  it('push and pop from the top', function() {
    var stack = new Stack();
    stack.push(1);
    stack.push(2);    
    expect(stack.pop()).to.be.equal(2);
    expect(stack.pop()).to.be.equal(1);
  });    
});