See: https://docs.codewars.com/training/troubleshooting#print-input

array || [] evaluates to array when array is a truthy value and otherwise evaluates to an empty array.

So, array = array || [] will set array equal to an empty array if it does not already refer to an object.

I think you've got things backwards. Your code returns true for 47 but should return false.

If the goal of the function is to return a sorted array, not returning one in some cases seems counter-productive to me.

https://docs.codewars.com/training/troubleshooting#error-messages-and-printing-your-inputoutput

I'm not arguing with your solutions, it tricked me in the first glance, so notice for everyone else:

Such function only applies caching to a current call.
It means if the function takes 7 seconds to complete, calling it 3 times, in total, will take 21 seconds.

I posted an explanation of this solution to evarx78's comment. I hope it will help you too.

How many times do each of those numbers that 2 is a factor of occur?

what

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);

Can someone explain this code ?

Take a close look at your alphabet.