Javascript this

A lot of people find this keyword in Javascript very confusing. Depending on where is it used and defined it can have different value.

This In Functions

In normal Javascript functions this refers to global object. It will be window object in browser and global object in node.

function simpleFunction () {
  console.log(this === window); 
  //prints true to console
}

If strict mode will be enabled - this inside normal function will be undefined.

function withStrict () {
  'use strict';
  console.log(this);
  // prints undefined to console
}

This In Constructor Functions

When a function is invoked with new keyword then the function is known as constructor function and returns a new instance. In such cases, the value of “this” refers to newly created instance.

function Person() {
    this.name = "Vlad"

    this.logName = function(){
        console.log(this.name)
    }
}

const person = new Person();
person.logName() // Will output `Vlad`

This In Methods In Javascript object fields can be simple values like string or number but it can also be a function.

In this case this function will be a method of an object. And when object method is invoked - this will refer to that paren object.

const user = {
    name: 'Vlad',
    logName: function(){
        console.log(this.name)
        // will log `Vlad`
    }
}

But method can loose reference to it’s parent object. In this example if we assign logName to an external variable and then call it - this inside of it will be global object (or undefined in strict mode).

const user = {
    name: 'Vlad',
    logName: function(){
        'use strict'
        console.log(this)
        console.log(this.name)
    }
}

const brokenLogName = user.logName
brokenLogName() // Will give `TypeError: this is undefined`

This In Browser Event Handlers

In event handlers callbacks, this refers to the HTML element that received the event:

document.querySelector('#button').addEventListener('click', function(e) {
  console.log(this) //HTMLElement
}