Make a Linked list-JS的單向鏈結

/**
 * .
 * append, removeAt, indexOf, remove, toString, size, getHead, insert, isEmpty
 */
const DoubleLinked = function () {
    const Node = function (element) {
        this.element = element;
        this.next = null;
        this.prev = null;
    }
    let length = 0;
    let head, tail;
    this.append = (element) => {
        let node = new Node(element);
        if(!head){
            head = node;
            tail = node;
            return head.element;
        }else{
            tail.next = node;
            node.prev = tail;
            tail = node;
            return tail.element;
        }
    }
    this.insert = (position, element) => {
        if (position > -1 && position <= length) {
            const node = new Node(element);
            let current = head;
            let previous;

            if (position === 0) {
                if (!head) {
                    head = node;
                    tail = node;
                } else {
                    node.next = current;
                    current.prev = node;
                    head = node;
                }
            } else if (position === length) {
                current = tail;
                current.next = node;
                node.prev = current;
                tail = node;
            } else {
                let index = 0;
                while (index++ < position) {
                    previous = current;
                    current = current.next;
                }
                previous.next = node;
                node.prev = previous;
                node.next = current;
                current.prev = node;
            }
            length++;
            return head;
        } else {
            return false;
        }
    }
    this.removeAt = (position) => {
        if (position > -1 && position <= length) {
            let current = head;
            let previous;
            if (position === 0) {
                if (head) {
                    head = current.next;
                    // head.prev = null;
                } else {
                    return false;
                }
            } else if (position === length) {
                tail = tail.prev;
                tail.next = null;
            } else {
                let index = 0;
                while (index++ < position) {
                    previous = current;
                    current = current.next;
                }
                previous.next = current.next;
                current.next.prev = previous;
            }
            length--;
            return current.element;
        } else {
            return false
        }
    };
    this.indexOf = (element) => {
        let current = head;
        let index = 0;
        while (current) {
            if (current.element === element) {
                return index;
            } else {
                return false;
            }
            index++;
            current = current.next;
        }
    };
    this.remove = (element) => {
        let position = this.indexOf(element);
        if (position >= 0) {
            return this.removeAt(position);
        } else {
            return null;
        }
    }
    this.toString = () => {
        if(head){
            let current = head;
            let string = '';
            while(current){
                string += current;
                current = current.next;
            }
        }else{
            return null;
        }
    };
    this.isEmpty = () => {
        return length === 0;
    };
    this.getHead = () => {
        return head;
    };
    this.getTail = () => {
        return tail;
    };
    this.getSize = () => {
        return hength;
    };
}

const assert = require('assert');
describe('DoubleLinked', () => {
    let a = new DoubleLinked();
    describe('#insert()', () => {
        it('Given link of Empty when enter position and element then check value.', () => {
            let test = a.insert(0, "a").element;
            assert.equal("a", test, "The result: " + test);
        });
    });
    describe('#append()', () => {
        it('Should allow append to last one.', () => {
            let test = a.append("b");
            assert.equal("b", test, "The result: " + test);
        });
    });
    describe('#indexOf()', () => {
        it('Should return index of element.', () => {
            let test = a.indexOf("a");
            assert.equal(0, test, "The result: " + test);
        })
    });
    describe('#removeAt()', () => {
        it('Should through position to remove.', () => {
            let test = a.removeAt(0);
            assert.equal("a", test, "The result: " + test);
        })
    });
    describe('#remove()', () => {
        it('Should through element to remove.', () => {
            let test = a.remove('b');
            assert.equal("b", test, "The result: " + test);
        })
    });
})