Problems with the var keyword

variables defined with the var keyword are function level scoped. meaning they are defined once, for the entirety of your function, which doesn't play well with loops and lamdas imagine creating buttons with something like this

for(var i = 0; i < people.length; i = i + 1)
{
  var person = people[i];
  var button = createButton(person);
  getList().addElement(button);
};

you might think that this would create a button for each person, one with each person's name. But since the var keyword scopes the button variable within the entire function... the button variable refers to one and only one button, and the person variable refers to one and only one person, both of which keep getting overidden, causing problems.

if you had declared the button variable with let instead of var You would see the intended result, since let has block level scoping. The person and button variables would be created anew each run of the loop.

var x = 1
let y = 2
{
   var x = 3
   let y = 4
   console.log(x); // 3
   console.log(y); // 4
}
console.log(x) // 3
console.log(y) // 2

In short, use let and const unless for some very special and very well understood reason, you need a global function variable, then and only then should you use var.