closure.js

let employeeSalary = function(initialSalary) {
  var salary = initialSalary;
  function changeBy(amount) {
    salary += amount;
  }
  return {
    raise: function() {
      changeBy(5000);
    },
    lower: function() {
      changeBy(-5000);
    },
    currentAmount: function() {
      return salary;
    }
  };
};

let Dennis = employeeSalary(10000);
let Susan = employeeSalary(12000);
console.log(`Dennis' salary ${Dennis.currentAmount()}`); // Output: Dennis' salary 10000
console.log(`Susan's salary ${Susan.currentAmount()}`); // Output: Susan's salary 12000
Dennis.raise();
console.log(`Dennis' salary increased to ${Dennis.currentAmount()}`); // Output: Dennis' salary increased to 15000
Susan.lower();
console.log(`Susan's salary decreased to ${Susan.currentAmount()}`); // Output: Susan's salary decreased to 7000
Dennis.changeBy(); // TypeError: Dennis.changeBy is not a funciton