Intermediate JavaScript:
Encapsulation

Description

Encapsulation is an important part of Object-Oriented programming, but we don't need objects to achieve it. This lesson covers using scope and IIFEs to make truly private variables in JavaScript without classes or prototypes.




 Slides

Encapsulation in JavaScript

encapsulation) is an important principle of software design

essentially it means:

Keep similar things together; keep different things apart.

Two related and overlapping concepts are information hiding and decomposition).

Why encapsulate?

  • keep code and the data it uses together
  • limit state mutations (to a single file or a contiguous block of code)
  • limit reach of bugs
  • increase cohesion of data & the code that loves it
  • decrease coupling of unrelated program features
  • focus on one thing at a time
    • so you know where in your code to look

"Do one thing, and do it well" is the encapsulator's motto

Encapsulation is for Humans

Remember, the computer doesn't care about your fancy design, but humans will, so keep it simple.

OO?

Encapsulation is an important part of object-oriented design

but it's not the only part (there's also inheritance, polymorphism, interfaces, etc.)

and you can use encapsulation on its own

Encapsulation Technique 1: closure scope

Here's a factory method that creates & returns a circle object:

function createCircle(radius) {
    return {
        area: function() {
            return radius * radius * Math.PI;
        }
    }
};

let circle = createCircle(3);

console.log("Area is " + circle.area());
console.log("Radius is " + circle.radius); // THIS WILL FAIL
  • In the above code, radius is only visible inside the scope of the function createCircle.

    • This is normal; any parameter or local variable is only visible inside its function.
    • It's also visible inside the area function since that function is defined inside the function createCircle.
  • area is a property on the hash (JS object) that is returned by createCircle

    • This means that area is visible to the caller of createCircle
  • So, area is exposed but radius is hidden. In other words...

radius is encapsulated inside circle

Encapsulation Technique 2: private variables

private variables are encapsulated inside a closure:

function createCircle(radius) {
    let diameter = radius * 2;

    return {
        area: function() {
            return radius * radius * Math.PI;
        },
        circumference: function() {
            return diameter * Math.PI;
        }
    }
};

let circle = createCircle(3);
console.log("Area is " + circle.area());

Note that only area and circumference are exposed via a pointer to the circle; diameter and radius are private.

Encapsulation Technique 3: IIFE

  • The following cryptic syntax is very common in JS
  • It's essentially the same as the previous code, but combining two steps into one
  • It's called an IIFE = Immediately Invoked Function Expression
(function(){ })();

Expanded:

( function() { ... }  ) ();
   ↑           ↑         ↑
  define       ↑       invoke
              execute   

The trick is, when you invoke the function, you generate a new scope for that invocation's closure.

You then preserve that scope by returning a pointer to an object that you created inside that closure.

Encapsulation with IIFE Example

Example:

let circle = (function(radius) {
    let diameter = radius * 2;

    return {
        area: function() {
            return radius * radius * Math.PI;
        },
        circumference: function() {
            return diameter * Math.PI;
        }
    }
})(3);
console.log("Area is " + circle.area());

https://developer.mozilla.org/en-US/docs/Glossary/IIFE


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