It returns 5 on first iteration if you pass 4 to it. In general it returns n + 1 for any n > 1.
Following the code path, it can be unrolled into
product = 1
n > product // true, loop
product *= n // so product is n
product++ // end of loop, product is n+1
n > product // false, exit loop
return product
If n === 1, the first check is false so it returns just 1.
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solved How; The most difficult question of your life [closed]