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JavaScript

jamesjones21
jamesjones21
9,260 Points
Posted by jamesjones21
jamesjones21
9,260 Points

mind is blown about 'this' keyword in Javascript

Hi,

I have the following object which calcAge property is set to a method (so it will be similar to a function expression.

The code below where I console.log(this) it will refer to the current object.

 const john = {
      firstName: 'John',
      lastName: 'Smith',
      birthYear: 1990,
      family: ['jane', 'bob', 'emily'],
      job: 'teacher',
      isMarried: false,
      calcAge: function(){
          console.log(this);
          //return 2018 - this.birthYear;
      },

   };

But when I change it to the ES6 arrow function it is then pointing to the global scope which is the window:

 const john = {
      firstName: 'John',
      lastName: 'Smith',
      birthYear: 1990,
      family: ['jane', 'bob', 'emily'],
      job: 'teacher',
      isMarried: false,
      calcAge: () => {
          console.log(this);
          //return 2018 - this.birthYear;
      },

   };

Can someone please attempt to explain this?

Kind regards,

James

2 Answers

Richard Verbraak
Richard Verbraak
7,739 Points
Richard Verbraak
Richard Verbraak
7,739 Points

As someone who just struggled to learn about the this keyword. I found this helpful topic about ES6 Arrow functions and the this keyword.

https://medium.freecodecamp.org/learn-es6-the-dope-way-part-ii-arrow-functions-and-the-this-keyword-381ac7a32881

If I'm reading correctly, arrow functions use lexical scoping. The this keyword doesn't bind to it's parent so to speak (the object) but to it's function. So it get's the window object instead.

Feel free to correct me here.

jamesjones21
jamesjones21
9,260 Points
jamesjones21
jamesjones21
9,260 Points

Just read that now but found a YouTube video on it which explains it all. If arrow function is used in the object then it binded to the window which means it was binded to the window when the function was created. How mad is this language, 🤣

Here is the link: https://youtu.be/eOI9GzMfd24

6 mins onwards you'll see he puts an example which I kinda get k It. Just have to know when and where to use them

Richard Verbraak
Richard Verbraak
7,739 Points
Richard Verbraak
Richard Verbraak
7,739 Points

Ah thanks for the video. I literally just started learning about that keyword today and was so confused the first 5 hours haha. Then I see your post and it made me scratch my head again hahah. But atleast we both got it now!

jamesjones21
jamesjones21
9,260 Points
jamesjones21
jamesjones21
9,260 Points

this is my current practice to understand, so this will be a good session to get my head around it. Which I am hoping to use this within my current small project and change some of syntax to refer to this.

https://github.com/bladedevleoper/wage-planner

jamesjones21
jamesjones21
9,260 Points
jamesjones21
jamesjones21
9,260 Points

here is some of my understanding from off top of my head:

/*--------------------------------------------------------------------------------------------------*/
//function expression with the arrow syntax

//this will refer to the global scope
//will refer to the lexical scope
var arrowFunction = () => {

    console.log('This is referring to the global scope (window)');

};

/*-------------------------------------------------------------------------------------------------*/
// Normal Function Expressions

//this will also refer to the global scope
//will refer to the lexical scope
var normalFunction = function () {

        console.log(`${this} This is referring to the global scope (window)`);

};

//outputs window
arrowFunction();
//outputs window
normalFunction();


/*-------------------------------------------------------------------------------------------------*/
//Object Literal

var myObject = {

    //this method will refer to the global scope (window)
    //arrow functions will loose their value of this when the function is created (lexical scope)
    whereAmI1: () => {

        return `${this} is referring to the global scope Window`;
    },

    //this will refer to the current object
    whereAmI2: function () {

        return `${this} is referring to the current Object`;
    },


};

//outputs window
console.log(myObject.whereAmI1());
//outputs current object
console.log(myObject.whereAmI2());


/*-------------------------------------------------------------------------------------------------*/
//Normal Function

//this will also refer to the global scope
function showMyFunction()
{

    var x = this;
    return x;
}

//outputs window
console.log(showMyFunction());

/*------------------------------------------------------------------------------------------------*/
//function using strict mode

//this won't work in use strict, it will return undefined as the function is not declared
function useStrictMode()
{
    'use strict';
    return this;
}

//outputs undefined as not declared
console.log('Using strict mode ' + useStrictMode());

//since we have now declared it, it returns the function (object)
var show = new useStrictMode();

//outputs current object when using scrict mode
console.log(show);
/*--------------------------------------------------------------------------------------------------*/
Steven Parker
Steven Parker
231,184 Points
Steven Parker
Steven Parker
231,184 Points

Arrow functions are not always suitable replacements for conventional functions. One way they are different is that they do not establish a value for "this" like conventional functions do. For this reason (and perhaps others), MDN suggests that "Arrow function expressions are ill suited as methods".

For more details on the differences, see the MDN page on Arrow Functions.

jamesjones21
jamesjones21
9,260 Points
jamesjones21
jamesjones21
9,260 Points

So they are more suitable as function expression? Which is why they were created.

Steven Parker
Steven Parker
231,184 Points
Steven Parker
Steven Parker
231,184 Points

Well, they are function expressions. What MDN is suggesting is that they might not be the best choice for class method implementation.