function add(a) {
const sum = b => add (a + b);
sum.valueOf = () => a;
total return;
}

• Function add() just returns function sum = () => (b) => add(a + b) with sum.valueOf = () => a; where a is the final sum, b is the new addition;
• Result(final sum) just return when sum.valueOf is run.

sum.valueOf will run when:
    + Do math
    + Compares with ==;

Example:
___ const addTwo = add (2) // sum = (b) => 2 + b; Returns sum function with sum.valueOf = () => 2;
|
|__ addTwo + 5 // 7 sum.valueOf ran when doing math and returned 2 so 2 + 5 = 7;
|
|__ addTwo + add(1) // 3 Both sum.valueOf of the two sum functions ran and returned 2 and 1 so 2 + 1 = 3

___ addTwo(3) // sum = () => 5 + b; Returns sum function with sum.valueOf = () => 5;
|
|__ addTwo(3) == 5 // true Because sum.valueOf ran when you compare with "==" and returned 5 so 5 == 5 is true;

Am I wrong?

Good job!