This

This is a special variable in JavaScript. It is instantiated when you call a function,and it's value will change depending on how you call the function.

This in the global scope

If we open a console and simply type this, we get back "window".

console.log(this)
          //Window

The default value for this is the global scope. If we do nothing else, this will be the global scope.

This within a function

Likewise, the value of this within a global function will also be the global scope:

var a = function() {
            console.log(this);
          }
          a();
          // Window

This in an object

If we are calling a function as an attribute of an object, this is set to that object. This is always the object we are operating inside of.

The simple heuristic is that this is the object immediately preceding the dot.

var car = {
            speedUp: function() {
              console.log(this);
            }
          }
          car.speedUp();
          // outputs the car
          
          var speedUp = car.speedUp;
          var speed = 0;
          speedUp();
          // outputs the global scope object Window
          
          var bus = {
            speedUp: car.speedUp
          }
          bus.speedUp();
          // outputs the bus

This in a closure

People often confuse this. The value of this is related to JavaScript as an object oriented language. It is not affected by the closure.

var fnc = function() {
            var inner_fnc = function() {
              console.log(this)
            }
            inner_fnc();
          }
          fnc();
          // Window

Setting this with apply

We can also manually set the value of this to any value we like. This trick is employed by several frameworks to make your life easier.

For example, jQuery will set the value of this to be the DOM node that received an event. Angular will set the value of this to the controller when you call a function on that controller's $scope object.

var myFunc = function() {
            console.log(this);
          }
          var myObj = {
            test:"cucumbers"
          }
          
          myFunc.apply(myObj,[]);
          // outputs myObj

Exercises - Guess the Value of This

Question 1

var x = function() {
            console.log(this);
          };
          x();

Question 2

var x = {
            y: function() {
              console.log(this);
            }
          }
          x.y();

Question 3

var x = function() {
            console.log(this);
          }
          var y = {
            x: x
          }
          y.x();

Question 4

var x = {
            y: function() {
              console.log(this);
            }
          };
          var a = {
            b:x.y
          };
          a.b();

Question 5

(function() {
            var x = function() {
              console.log(this);
            }
            x();
          })();

Question 6

(function() {
            var x = function() {
              console.log(this);
            }
            y = {};
            y.x = x;
            y.x();
          })();

Question 7

(function() {
            var x = function() {
              console.log(this);
            };
            x.apply({a:'b'});
          })();

Question 8

(function() {
            var Cat = function() {
              console.log(this);
            };
            var x = new Cat();
          })();

Exercise - Fix the broken code

The following code is broken. There are a few ways it could be fixed, we are going to fix it by storing the value of "this" in a variable which we will call "that".

var noise = "woof";
          
          var cats = {
            noise: 'Meow',
            sayMiow: function() {
              setInterval(
                function() {
                  console.log(this.noise);
                },
                1000
              );
            }
          };
          
          cats.sayMiow();