Stacks (LIFO)

A stack works like a pile of plates: you add to the top and take from the top. The last item in is the first out — LIFO. The two operations are push (add) and pop (remove the most recent).

class Stack {
  constructor() { this.items = [] }
  push(x) { this.items.push(x) }
  pop() { return this.items.pop() }
  peek() { return this.items[this.items.length - 1] }
  get size() { return this.items.length }
}

Edit and run this — it uses a stack to reverse a word: