A slightly involved example for students to learn how closure works

Closure Example

'use strict';

const WeightGoals = {
  QUICK_LOSS: 1,
  GRADUAL_LOSS: 2,
  MAINTENANCE: 3,
  BULK: 4,
};

const Gender = {
  MALE: 1,
  FEMALE: 2,
};

function calorieCounter(gender, weightGoal) {
  let caloricBase;

  switch (gender) {
    case Gender.MALE:
      caloricBase = 2100;
      break;
    case Gender.FEMALE:
      caloricBase = 1800;
      break;
    default:
      throw new RangeError('gender must be male or female');
  }

  switch (weightGoal) {
    case WeightGoals.QUICK_LOSS:
      caloricBase -= 600;
      break;
    case WeightGoals.GRADUAL_LOSS:
      caloricBase -= 300;
      break;
    case WeightGoals.MAINTENANCE:
      break;
    case WeightGoals.BULK:
      caloricBase += 600;
      break;
    default:
      throw new RangeError('Invalid weight goal');
  }

  function addMeal(calories) {
    console.log(`Consumed ${calories} calories.`);
    caloricBase -= calories;
  }

  function addExercise(calories) {
    console.log(`Burned ${calories} calories.`);
    caloricBase += calories;
  }

  function caloriesRemaining() {
    console.log(`${caloricBase} calories remaining`);
  }

  return {
    addMeal,
    addExercise,
    caloriesRemaining,
  };
}


// Execution

// Notice that the caloricBase value will be remembered by the counter1 object
const counter1 = calorieCounter(Gender.MALE, WeightGoals.BULK);
counter1.addMeal(500);
counter1.addMeal(300);
counter1.addExercise(250);
counter1.caloriesRemaining();

// We can create a second instance of the counter and track a completely different caloricBase value
const counter2 = calorieCounter(Gender.FEMALE, WeightGoals.GRADUAL_LOSS);
counter2.addMeal(150);
counter2.addExercise(200);
counter2.addMeal(600);
counter2.caloriesRemaining();

// The original counter remains unchanged
counter1.caloriesRemaining();

console.log(counter1.caloricBase); // undefined: caloricBase is hidden to external scope