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

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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);

How come my code is working although I didn't set InitialValue for my reduce call, can anyone explain?

Too bad the best practice solution doesn't use neither slice or splice. If that is really best practice for this problem then the problem belongs in another lesson imo.