This is and implementations of a Queue in javascript using a double linked list

Queue.js

class Queue {
    constructor(){
        this.tail = null
        this.head = null
        this.length = 0;
    }

    shift(){
        let temp = this.head
        if (this.length < 2){
            this.head = null;
            this.tail = null;
        }
        else{
            temp.next.prev = null;
            this.head = temp.next
        }
        this.length--;
        return temp.value
    }

    pop(){
        let temp = this.tail;
        if (this.length < 2){
            this.head = null;
            this.tail = null;
        }
        else{
            temp.prev.next = null;
            this.tail = temp.prev;
        }
        this.length--;
        return temp.value;
    }

    unshift(value){

        let array = Array.isArray(value) ? value: [value]

        for(let i = 0; i < array.length; i++){

            let val = array[array.length-i-1]
            let n = new Node(val);
            if (this.length==0) {
            this.head = n
            this.tail = n
            }
            else{
                let temp = this.head
                n.next = temp
                temp.prev = n
                this.head = n
            }
            this.length++
    }
        return this.length;
    }

    push(val){
        let n = new Node(val);
        if  (this.length==0){
            this.head = n
            this.tail = n
        }
        else{
            let temp = this.tail
            n.prev = temp
            temp.next = n
            this.tail = n
        }
        return this.length++;
    }
    
    extend(list){

        list.forEach( elem => this.push(elem) );
        return this.length;

    }

    valueOf(){
        return this.length!=0;
    }

    toString(){
        let r = []
        let curr = this.head
        while (curr){
            r.push(curr.value);
            curr = curr.next
        }
        return r;
    }

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