Calculate Height/Depth Of Binary Tree Javascript

I am just a beginner, so if the error is something too obvious, I apologize .

My two questions are:

1. What is this.root in our school’s provided code;
2. How can I implement the .height method in order to measure the depth of a Tree.

The explanation:
We were provided with this code in the class:

function BinarySearchTree(value) {
  this.value = value;
  this.right = null;
  this.left = null;
}

BinarySearchTree.prototype.add = function(value) {
  let newLeaf = new BinarySearchTree(value)

  if(value > this.value){
    this.right === null? this.right = newLeaf : this.right.add(value)
  } else {
    this.left === null? this.left = newLeaf : this.left.add(value)
  }
};

And we were supposed to write a method to calculate the height/depth of a binary tree. Now, while practicing, I’ve seen something odd. Upon creation of a new node of an empty binary tree, the first node ends up being completely empty, while it proceeds to create a new node on the left side of the first empty one. Well, not empty, but whose value is undefined. Is this a desired behavior?

let newTree = new BinarySearchTree
>undefined

newTree.add(7)
>undefined

newTree.add(3)
>undefined

newTree.add(5)
>undefined

newTree
>BinarySearchTree {value: undefined, right: null, left: BinarySearchTree}
  left: BinarySearchTree {value: 7, right: null, left: BinarySearchTree}
  right: null
  value: undefined
  [[Prototype]]: Object

Now, considering the tests are passing for .add method, obviously I may be wrong in this situation, since this is the code provided to us by the teacher in the class.
This is the code I keep finding online and the reason I am not getting far with my code for .heigth method is because I am unable to implement this.root:

function Node(val){
  this.value = val;
  this.left = null;
  this.right = null;
}


function BinarySearchTree(){
  this.root = null;
}

How should I proceed with the .height method?
If it helps, here are the tests:

describe('Binary Search Tree', function() {
  var binarySearchTree;

  beforeEach(function() {
    binarySearchTree = new BinarySearchTree(5);
  });

  it('should have methods named "add", "contains", "depthFirstPre", "depthFirstIn", "depthFirstPost", "breadthFirst"', function() {
    expect(binarySearchTree.add).to.be.a("function");
  });

  it('should add values at the correct location in the tree', function(){
    binarySearchTree.add(2);
    binarySearchTree.add(3);
    binarySearchTree.add(7);
    binarySearchTree.add(6);
    expect(binarySearchTree.left.right.value).to.equal(3);
    expect(binarySearchTree.right.left.value).to.equal(6);
  });

  it('height method should return correct height', function() {
    binarySearchTree.left = new BinarySearchTree(3);
    binarySearchTree.left.left = new BinarySearchTree(1);
    expect(binarySearchTree.height()).to.eql(2);

    binarySearchTree.left.left.right = new BinarySearchTree(2);
    expect(binarySearchTree.height()).to.eql(3);

    binarySearchTree.left.left.left = new BinarySearchTree(0);
    expect(binarySearchTree.height()).to.eql(3);

    binarySearchTree.right = new BinarySearchTree(8);
    expect(binarySearchTree.height()).to.eql(3);
  });
}

Again, I apologize for a long question. I was trying to write all the relevant information regarding my problem.
Happy holidays!