Javascript queue implementation with linkedlist

class Node {
    constructor(value){
        this.value = value;
        this.next = null;
    }
}


class Queue{
    constructor(){
        this.first = null;
        this.last = null;
        this.length = 0;
    }

    enqueue(value){
        const newNode = new Node(value)
        if(this.length === 0){
            this.first = newNode;
            this.last = newNode;
        } else {
            this.last.next = newNode;
            this.last = newNode;
        }
        this.length++;
        return this;
    }
}

const myQueue = new Queue();
myQueue.enqueue('a')
myQueue.enqueue('b')

Here I am implementing Queue with linkedlists. In else block of enqueue() method i am not assigning anything to this.first i am only assigning to this.last

How my this.first if changing.

Please have a look.

How this.first is changing without even touching it.

Actually the answer is correct but, I am not able to understand the logic.