Stack implementation using TypeScript

A stack is an abstract data type that stores a collection of elements, with two principal operations:

• push: adds an element to the collection
• pop: removes the most recently added element that was not yet removed. The order in which elements are poped is Last In First Out aka. LIFO. In this lesson we discuss how to implement it using JavaScript / TypeScript.

A stack is a Last in First out (LIFO) with key operations having a time complexity of O(1). We can model this easily in TypeScript using a generic class for items of type T.

• It will have two key operations.
• The first one is push which adds an item into the stack in O(1).
• The other key operation pops an item from the stack in O(1). If there are no more items we can return an out of bound value like undefined. This fact can be modeled into the type system by using a union of T and undefined.

/**
 * Last In First Out (LIFO) with O(1) for key operations
 */
export class Stack<T>{
  /** Adds the item in O(1) */
  push(item: T): void {
  }

  /**
   * Pops the last inserted item in O(1)
   * If there are no more items it returns `undefined`
   */
  pop(): T | undefined {
  }
}

The objective is to implement these push and pop operations such that they operate in O(1) time. Fortunately in JavaScript implementations, array function that do not require any changes to the index of current items, have an average runtime of O(1).

• Therefore we can simply implement the operations of this data structure using an array.
• On push we push the item into the array
• On pop we simply return the same result as array.pop

/**
 * Last in First out (LIFO) with O(1) key operations
 */
export class Stack<T>{
  private data: T[] = [];

  /** Adds the item in O(1) */
  push(item: T): void {
    this.data.push(item);
  }

  /**
   * Pops the last inserted item in O(1)
   * If there are no more items it returns `undefined`
   */
  pop(): T | undefined {
    return this.data.pop();
  }
}

A common additional operation for collection data structures is the size as it allows you to safely find out if there are any elements present in the data structure. Fortunately JavaScript arrays implement this for us in the form of the length property:

  /**
   * Returns the number of elements in the stack in O(1)
   */
  size(): number {
    return this.data.length;
  }