Why are constructors used in this video if they haven't even been introduced?

I just had a look at the FEWD track. It looks like the course Object-Oriented JavaScript actually comes after courses on AJAX and Asynchronous Programming, which is a major oversight. Object Oriented programming is a particular way, or paradigm for programming in which the major components of a program are composed of objects, which have methods (functions attached to the object) and properties (variables encapsulated in the object). A lot of the things you're working with in asynchronous programming, like the XMLHttpRequest object and promises are objects. Objects are made out of classes, which are like blueprints that can generate as many instances as you want, you simply use the constructor for the class. E.g:

class Cat 
{
    constructor(name) 
    {
        this.name = name;
    }

    sayHello() 
    {
        console.log(`Meow, my name is ${this.name});
    }
}

const fuzzy = new Cat('Fuzzy');

fuzzy.sayHello();
// Meow, my name is Fuzzy

This is just a really basic tutorial. You should probably take the Object-Oriented JavaScript course and then the Object-Oriented JavaScript: Challenge course before doing anything else. These two courses will definitely challenge you, but take it at a comfortable speed and be patient. At the end you will have leveled up as a developer.

You might also want to let the Treehouse staff know about the OOP not being taught at the correct point in the track. Good luck!

My last comment might not have made one point clear. The constructor is the function you call to instantiate a class, or in other words, create an object from the class. And again, the class is a blue print for the object. So you write:

new Cat('Fuzzy');

The new keyword lets JavaScript know you're making a new object from the class. You can do this with things like arrays, for instance. Arrays are just a kind of object. Usually people use array-literals [1, 2, 3] but you could write:

const myArray = new Array(1, 2, 3);

myArray;
// [1, 2, 3]

Everything in JavaScript pretty much is an object and can be worked with in this sort of way, though for a long of things its not convenient and literals work better.