Q1

let b1 = [21,13,13,21]
let s1 = new Set()
for (let x of b1) s1.add(x)
b1.length == 4,  s1.size == 2
Repetitions are removed

Q2

URL

let k = t.split('\n').length

Q3

Q4

nw  Point (object)
se.x  number
se.toString  function
se.y.toString()  string

Q5

p2 is Object
p is Point, toString() redefined

Q6