Closure.md

Closure

Table of Contents:

• Definition.
• Example 1.
• Example 2.
• Example 3.

Definition:

• Closure = a function that has a reference (=can access) a private variable.

Example 1:

function Person(pName) {
  this.name = pName;
  }
const me = new Person('Islam');
me.name;//'Islam'

Now let's do it another way

function Person(pName) {
  var name = pName;
  }
const me = new Person('Islam');
me.name;//undefined

Instead we could use a closure to have a reference to this private name variable.

function Person(pName) {
var name = pName;
return function() {
  return this.name = pName;
  }
}
const obj = Person('Islam');
const me = new obj();
me.name;//'Islam'

Example 2

function counterFunction() {
    var counter = 0;
    return function () {return counter += 1;}
}
const add = counterFunction();
add();//1
add();//2
add();//3

The add() refer to the returned anonymous function which has reference to the private counter variable and increment it by 1 each time the add() invoked. If you wrote add; in console it will return the returned anonymous function.

add;//ƒ () {return counter += 1;}

Example 3

function invokeTwice(callback) {
   callback();
   callback();
}

const dog = {
  age: 5,
  growOneYear: function () {
    this.age += 1;
  }
};

Invoking the function invokeTwice(dog.growOneYear);//undefined will not increment age property of the dog object. This is because this inside a regular function is scoped to the global object window when the function is invoked. A solution to that is using a closure function to have a refernce to age property inside the dog object and then the method can increment its value.

invokeTwice(function() {
dog.growOneYear()});
dog.age;//7