JavaScript implementation of Stack Data Structure

stack.js

// Implement a stack using LinkedLists.
// Operations:
//  -> push
//  -> pop
//  -> peek


function StackNode(val) {
  this.val = val;
  this.next = null;
}

function Stack() {
  this.head = null;
}

Stack.prototype.push = function(val) {
  if (this.head == null) {
    this.head = new StackNode(val);
  } else {
    var head = this.head;
    this.head = new StackNode(val);
    this.head.next = head;
  }
};

Stack.prototype.pop = function() {
  var node = this.head;
  
  if (node !== null) {
    this.head = this.head.next;
    return node.val;
  }
  
  return null;
};

Stack.prototype.peek = function() {
  return (this.head) ? this.head.val : null;
}

var stack = new Stack();

stack.push(5);
stack.push(11);
stack.push(10);

console.log(stack.pop());
console.log(stack.pop());
console.log(stack.pop());
console.log(stack.peek());