This

JavaScript does not have analogous classes to Object Oriented languages.

JavaScript has objects and functions.

Skill++

Technically, a function is just a special type of object.

When a function is invoked, this is assigned a value. The value it is assigned can vary based on how the function was invoked.

When dealing with object methods, this will be bound to the object which the method is a property of (i.e. the invoking object):

let dog = {};
dog.name = "Sparky";

// 'dog' is the invoking object, 'this' references 'dog'
dog.sayName = function() {
  return this.name;
}

dog.sayName(); // 'Sparky'

It is important to note that this will only be bound to the invoking object if a function is called. * Thus, attempting to assign a value to another property in an object literal with this will produce undefined (or a radically unexpected result). For example:

let dog = {
  fname : 'Sparky',
  lname : 'Le Grange',
  fullName : this.fname + " " + this.lname
};

dog.fullName; // undefined undefined

In the above example, this is actually referencing the global object (in this case window). The reason it is undefined is because window does not have fname or lname properties.

Given that the identity of this can be confusing, some style guides choose to reassign the value of this to a variable name inside of functions for clarity:

let dog = {
  fname : 'Sparky',
  lname : 'Le Grange',
  // Method, when invoked will assign `this` as invoking object
  getFullName : function() {
    let thisDog = this;
    return thisDog.fname + " " + thisDog.lname;
  }
}

dog.getFullName(); // Sparky Le Grange

Skill Drill

1. Create a variable named tree.

2. Give tree an age property, and set it equal to some number.

3. Create a method on tree named ageOneYear, which uses this to update the age property on tree. Call ageOneYear and check the age property.

4. Create another method on tree named isDead. isDead should set the age property to NaN if the tree is older than 100. Call isDead within ageOneYear. Use this.

5. In a while loop, until the tree's age is NaN, call ageOneYear and print the age to console.

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