Using this in js

usingThis.js

/**
* Use of this in js
*
* Unlike other languages, this works a bit differently in js
* A simple way to identify what is the value of this would be:
*
*/

function someConstructor () {
    this.someProperty = 1
    this.getThatProperty = function () {
       return this.someProperty
    }
}

var instance = new someConstructor ()

console.log(instance)
console.log(instance.getThatProperty())

/** notice how you can use this inside getThatProperty() method
* why: 
* getThatProperty is a method on someConstructor, 
* so the value of this will be the object calling the function.
*
*    this.getThatProperty or instance.getThatProperty 
*    passes the context for this. 
*/

// LESSON: this => object calling the function, if no object, this = window

var a = [1, 2, 3, 4]

a.map(function () {
  console.log(this)
})

/** array.map is called by array, arrays are objects so why this didn't work?
*    because the anonymous function is not being called by the array there is no '.' operator there!
*/

Array.prototype.test = function () {
  console.log(this) // here this is the object calling the function so... Array
}

// Now test is a method that can be applied on all arrays

a.test() // this will print the same array!