Stack

Overview

A stack is a data structure that stores items according to the Last In First Out (LIFO) principle. The most recently added item is the first one to be removed.

Operations	Complexity
Push (insert end)	O(1)
Pop (delete last)	O(1)
Peek (view last)	O(1)

Implementation

Stacks can be implemented using different underlying structures. The most common versions are based on arrays or linked lists.

Array Based Stack

class Stack {
  constructor() {
    this.items = [];
  }

  push(element) {
    this.items.push(element);
  }

  pop() {
    if (this.items.length == 0) {
      throw new Error('stack is empty');
    }

    return this.items.pop();
  }

  peek() {
    return this.items.at(-1);
  }
}

Usage:

const myStack = new Stack();

myStack.push('One');
myStack.push('Two');
myStack.push('Three');

console.log(myStack.peek()); // Three

console.log(myStack.pop()); // Three
console.log(myStack.pop()); // Two
console.log(myStack.pop()); // One

console.log(myStack.peek()); // undefined

Linked List Based Stack

class StackNode {
  constructor(value) {
    this.value = value;
    this.next = null;
  }
}

class Stack {
  constructor() {
    this.top = null;
    this.size = 0;
  }

  push(value) {
    const newNode = new StackNode(value);
    newNode.next = this.top;
    this.top = newNode;
    this.size++;
  }

  pop() {
    if (this.top === null) {
      throw new Error('stack is empty');
    }

    const value = this.top.value;
    this.top = this.top.next;
    this.size--;

    return value;
  }

  peek() {
    if (this.top === null) {
      throw new Error('stack is empty');
    }

    return this.top.value;
  }
}