SOURCE

function ListNode(val) {
    this.val = val;
    this.next = null;
}

function LinkList() {
    this.length = 0;
    this.head = null;
}

LinkList.prototype.getElementAt = function(index) {
    if(index < 0 || this.length) return null;

    let cur = this.head;
    while(index--) {
        cur = cur.next;
    }
    return cur;
};

LinkList.prototype.find = function(val) {
    let cur = this.head;
    while(cur) {
        if(cur.val === val) return cur;
        cur = cur.next
    }
    return null;
};

LinkList.prototype.append = function(val) {
    let node = new ListNode(val);

    if(!this.head) {
        this.head = node;
    }else{
        let cur = this.getElementAt(this.length -1);
        cur.next = node;
    }
    this.length++;
};

LinkList.prototype.insert = function(index, val) {
    if(index < 0 || index > this.length) return false;

    let node = new ListNode(val);

    if(index === 0) {
        node.next = this.head;
        this.head = node;
    }else{
        let pre = this.getElementAt(index-1);
        node.next = pre.next;
        pre.next = node;
    }
    this.length++;
    return true
};

LinkList.prototype.removeAt = function(index) {
    if(index < 0 || index >= this.length) return null;

    let cur = this.head;

    if(index === 0) {
        this.head = cur.next;
    }else{
        let pre = this.getElementAt(index-1);
        cur = pre.next;
        pre.next = cur.next;
    }

    this.length--;
    return cur.val;
};

LinkList.prototype.indexOf = function(val) {
    let cur = this.head;

    for(let i=0;i<this.length;i++) {
        if(cur.val === val) return i;
        cur = cur.next;
    }

    return -1;
};

LinkList.prototype.remove = function(val) {
    let index = this.indexOf(val);
    return this.removeAt(index);
};

LinkList.prototype.isEmpty = function(val) {
    return !this.length;
};

LinkList.prototype.size = function(val) {
    return this.length;
};

LinkList.prototype.getHead = function(val) {
    return this.head;
};

LinkList.prototype.clear = function(val) {
    this.head = null;
    this.length = 0;
};

LinkList.prototype.join = function(val) {
    let cur = this.head;
    let str = '';
    while(cur) {
        str += cur.val;
        if(cur.next) str += string;
        cur = cur.next;
    }
    return str;
};