Javascript : Protect Properties Within an Object

Programming May 30 ,2020

Object bird had a public property name. It is considered public because it can be accessed and changed outside of bird's definition.

let bird = {
  name: "Donald",
  numLegs: 2
};

The name property can be accessed as below :

bird.name = "Duffy";

Therefore, any part of your code can easily change the name of bird to any value. Think about things like passwords and bank accounts being easily changeable by any part of your codebase. That could cause a lot of issues.

 

The simplest way to make this public property private is by creating a variable within the constructor function. This changes the scope of that variable to be within the constructor function versus available globally. This way, the variable can only be accessed and changed by methods also within the constructor function.

function Bird() {
  let hatchedEgg = 10; // private variable

  /* publicly available method that a bird object can use */
  this.getHatchedEggCount = function() { 
    return hatchedEgg;
  };
}
let ducky = new Bird();
ducky.getHatchedEggCount(); // returns 10

Here getHatchedEggCount is a privileged method, because it has access to the private variable hatchedEgg. This is possible because hatchedEgg is declared in the same context as getHatchedEggCount. In JavaScript, a function always has access to the context in which it was created. This is called closure.