What is a Stack?

A stack is a container of objects that are inserted and removed according to the last-in, first-out (LIFO) principle.

Imagine you had a stack of big plates at the bottom, smaller plates on the bigger plates, and bowls on top.

If you wanted a bowl, it’d be pretty simple. You can just gram from the top of the stack. If you wanted a large plate however, it’d be a little more complicated. You’d have to remove the bowls, the small plates, and finally you could grab a large plate.

The Tower of Hanoi

A perfect example of a stack would be the Tower of Hanoi. You’re unable to grab the larger disks at the bottom without removing the smaller disks on top. This abides by the principles of LIFO.

Stacks in JavaScript

I’ve made a stack in JavaScript:

var App = (function () {

  function Stack () {
    this.top = null
  }

  Stack.prototype.pop = function() {
    this.top = this.top.next
  };

  Stack.prototype.push = function(el) {
    var next_node = this.top
    this.top = new Node(el, next_node)
  };

  Stack.prototype.peek = function() {
    if (this.top !== null) {
      console.log(this.top.value);
      return this.top.value
    };
    console.log("stack is empty");
    return null
  };

  function Node (value, next) {
    this.value = value
    this.next = next
  }

  return new Stack
})()

A stack is initialized by calling App and saving it to a variable. From there, you can #peek to see what’s on the stack. you can #push and #pop things from the stack.