Code Refactoring for making more testable

dependencyInjection.js

var Meal = mongoose.model()

class MealService{
  changeCalories(id, calories){
    Meal.update(id, {
      $set: {calories}
    })
  }
}

//Test Code
it("should update calories", ()=>{
  
  const mealService = new MealService()
  
  mealService.changeCalories(1, 1000)
  /* It will mutate external state, on which we don't have controll everytime*/
})

//With dependency injection

class MealService{
  constructor(mealModel){
    this.mealModel = mealModel
  }
  changeCalories(id, calories){
    Meal.update(id, {
      $set: {calories}
    })
  }
}

//Test Code
it("should update calories", ()=>{
  const MealMockModel = {
    update: ()=>(id, calories) {
      /*
      OUR ASSERTIONS
      */
    }
  }
  const mealService = new MealService(MealMockModel)
  
  mealService.changeCalories(1, 1000)
  /* It will mutate external state, on which we don't have controll everytime*/
})

stateless.js

// Do not use explicit state

SomeService.getCaloriesPerGram() {
  return this.calroies/this.mass
}

//Test Code
it("should return calories per gram", ()=>{
  /*
  Run some code for setting calories and mass in the object
  Everytime we will change how calories and mass are initiated, we may need to change the test as well
  */
  
  assert.eq(SomeService.getCaloriesPerGram(), 100);
})


//instead

SomeService.getCaloriesPerGram(calories, mass) {
  return calroies/mass
}

//Test Code

it("should return calories per gram", ()=>{
  /*
  No need to change everytime.
  */
  
  assert.eq(SomeService.getCaloriesPerGram(10000, 100), 100);
})