linkedlist

function createNode(value) {
  return {
    value: value,
    next: null
  }
};

function createLinkedList() {
  return {
    head: null,
    tail: null,
    length: 0,
    
    push(value) {
      const node = createNode(value);
      
      if (this.head === null) {
        this.head = node;
        this.tail = node;
        this.length++;
        return node;
      }
      
      this.tail.next = node;
      this.tail = node;
      this.length++;
      return node;      
    },
    
    pop() {
      if (this.isEmpty()) {
        return null;
      }
      const node = this.tail;
      if (this.head === this.tail) {
        this.head = null;
        this.tail = null;
        this.length--;
        return node;
      }
      
      let current = this.head;
      let penultimate;
      while (current){
        if (current.next === this.tail) {
          penultimate = current;
          break;
        }
        current = current.next;
      }
      penultimate.next = null;
      this.tail = penultimate;
      this.length--;
      return node;      
    },
    
    get(index) {
      if (index < 0 || index > this.length) {
        return null;
      }
      
      if (index === 0) {
        return this.head;
      }
      
      let current = this.head;
      let i = 0;
      while (i < index) {
        i++;
        current = current.next;
      }
      return current;
    },
    
    delete(index) {
      if (index < 0 || index > this.length) {
        return null;
      }
      if (index === 0) {
        const deleted = this.head;
        
        this.head = this.head.next;
        this.length--;
        return deleted;
      }
      
      let current = this.head;
      let previous
      let i = 0;
      while (i < index) {
        i++;
        previous = current;
        current = current.next;
      }
      
      const deleted = current;
      previous.next = current.next;
      this.length--;
      return deleted;
    },
    
    isEmpty() {
      return this.length === 0;
    },
    print() {
      const values = [];
      let current = this.head;
      while (current) {
        values.push(current.value);
        current = current.next;
      }
      return values.join(' => ');
    }
  }
}

const list = createLinkedList();
const values = ['a', 'b', 'c', 'd', 'e'];
const nodes = values.map(v => list.push(v));

console.log(list.isEmpty());

console.log(list.print())
//"a => b => c => d => e"

list.pop();
console.log(list.print())

console.log(list.head)
console.log(list.tail)

console.log(list.get(2))
list.delete(2)
console.log(list.print())