Double-ended queue (typescript)

deque.ts

// Linked data structure part
interface Node<T> {
  previous: Node<T>,
  value: T,
  next: Node<T>
}
// Double-ended queue structure
class Deque<T> {
  private first: Node<T> = null;
  private last: Node<T> = null;
  private size: number = 0;

  public get length() {
    return this.size;
  }
  public pushBack(value: T) {
    // Update last
    const last = this.last;
    this.last = {previous: last, value: value, next: null};
    if(last !== null)
      last.next = this.last;
    // Update first
    if(this.first === null)
      this.first = this.last;
    // Update size
    this.size++;
    // Return new size
    return this.size;
  }
  public pushFront(value: T) {
    // Update first
    const first = this.first;
    this.first = {previous: null, value: value, next: first};
    if(first !== null)
      first.previous = this.first;
    // Update last
    if(this.last === null)
      this.last = this.first;
    // Update size
    this.size++;
    // Return new size
    return this.size;
  }

  public popBack() {
    // Check possibility
    if(this.size === 0)
      return null;
    // Update last
    const entry = this.last;
    this.last = entry.previous;
    if(this.last !== null)
      this.last.next = null;
    // Update first
    if(this.first === entry)
      this.first = null;
    // Update size
    this.size--;
    // Return value of removed entry
    return entry.value;
  }
  public popFront() {
    // Check possibility
    if(this.size === 0)
      return null;
    // Update first
    const entry = this.first;
    this.first = entry.next;
    if(this.first !== null)
      this.first.previous = null;
    // Update last
    if(this.last === entry)
      this.last = null;
    // Update size
    this.size--;
    // Return value of removed entry
    return entry.value;
  }
}