poor explanation to the good task.

nope, the array returns the number of index the should choose the specific character in the hardcoded string. For instance, let's look at (~~(85/10))
85 / 10 = 8.5
~8.5 = -9 (rounding 8 to nine and giving minus ahead of it)
~(-9) = 8 (-9 + 1 and giving the opposite sign)
Finally we got 8, so 'DDDDDDCCBAS'[8] = 'B'

Hi!

Try please this: else if (isNaN(n)) and as the output: "Input number is Number.NaN" You have something like: "Input number is Number.Nan".

OP solved it, closing

Nothing can be enforced since this is a beginner-level kata

Because test suite calls your function without parameters, like this: let result = hello();

For that to work the value of s has to be saved to s for every loop

s += tmp is short for s = s + tmp. So, s + tmp is literally saved to s for every iteration of the loop. This expression is not "inside a function call". The result of the evaluated expression (s) is used as an argument to the function.

Any code of the form:

    someFunc( s += tmp );

can be rewritten as

    s += tmp;
    someFunc(s);