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Objects are collections of related data and functionality. We store that functionality in methods on our objects:

const goat = {
  dietType: 'herbivore',
  makeSound() {
    console.log('baaa');
  }
};

In our goat object we have a .makeSound() method. We can invoke the .makeSound() method on goat.

goat.makeSound(); // Prints baaa

Nice, we have a goat object that can print baaa to the console. Everything seems to be working fine. What if we wanted to add a new method to our goat object called .diet() that prints the goat‘s dietType?

const goat = {
  dietType: 'herbivore',
  makeSound() {
    console.log('baaa');
  },
  diet() {
    console.log(dietType);
  }
};
goat.diet(); 
// Output will be "ReferenceError: dietType is not defined"

That’s strange, why is dietType not defined even though it’s a property of goat? That’s because inside the scope of the .diet() method, we don’t automatically have access to other properties of the goat object.

Here’s where the this keyword comes to the rescue. If we change the .diet() method to use the this, the .diet() works! :

const goat = {
  dietType: 'herbivore',
  makeSound() {
    console.log('baaa');
  },
  diet() {
    console.log(this.dietType);
  }
};

goat.diet(); 
// Output: herbivore

The this keyword references the calling object which provides access to the calling object’s properties. In the example above, the calling object is goat and by using this we’re accessing the goat object itself, and then the dietType property of goat by using property dot notation.

Let’s get comfortable using the this keyword in a method.

Instructions

Let’s create a new object to practice using this.

In main.js there is an object robot, add a property of model and assign to it a value of '1E78V2'. Add another property, energyLevel and assign to it a value of 100.

Inside the robot object, add a method named provideInfo. Inside the body of provideInfo(), return the following string by using interpolation:

I am MODEL and my current energy level is ENERGYLEVEL.

Replace ‘MODEL’ and ‘ENERGYLEVEL’ with the calling object’s model and energyLevel property. Remember, to get the access to the calling object’s properties inside a method, you have to use the this keyword!

Now to check .provideInfo() has access to the internal properties of robot. Log the result of calling .provideInfo() method on robot to the console.

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