mySpec.js

// the basic structure
describe("the class you want to test", function() {
  describe("a function you want to test", function() {
    it("should do something", function() {
      // your nice test code here
    })
  })
})

// some more practical
describe("Car", function() {
  describe("constructor", function() {
    it("should throw exception if no speed limit is passed", function() {
      // call the constructor and don't pass a speed limit!
    })
  })
})