JavaScript set.size

Question

Can you please explain me this weird thing?

let arr = [1,2,2,3,3,5];
let set = new Set(arr);

According to SoloLearn set.size is now 4, but I absolutly don't know why.

Answer 1

Sets don't allow duplicate values.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set

Answer 2

Set ignore duplicates (comparing value and type: === operator)