Here is the explain to everyone about the
function plus(n) { return function (a) { return a + n } }

Let's say we need to do something like two(plus(three()))

I.The javascript is running from the inside 1st right?

then it will try to calculate three() meaning 3 , there is nothing hard here

II.now to the plus(three())

plus(three()) == plus(3)

if u trying to console.log(plus(three())), u will see it return

return function (a) { return a + n }

and nothing is calculated.
or i can say

two(plus(three))        ==        two(return function (a) { return a + 3 })

III. Remember two(something_in_here) function? it

return something_in_here(2)

or i can say

two(plus(three))    ==    return f(2)

To summarize all from 1-> 3, we have everything explained below

  two(plus(three))    ==    two(return function (a) { return a + n })
  return f(2)         ==    return function (a) { return a + 3 }) //we remove the sames of 2 sides of this equation
  f(2)                ==    function (a) { return a + 3 })

==> this mean 2 has been assigned to a!

So that' how.

let me show u why:
"This-i mean all of this-will be my string" => "hisTay-iay eanmay llaay foay histay-illway ebay ymay tringsay"