So looking at the rest of the documentation, the parameter not being set causes the default radix to be 10 unless the number starts with 0 or 0x, in which case

So looking at the rest of the documentation, the parameter not being set causes the default radix to be 10 unless the number starts with 0 or 0x, in which case they radix is 8 or 16 respectively. Though newer versions of JS no longer support the octal syntax (it likely just caused bugs for people who had initial zeros in their strings sometimes) parseInt("0x10") == 16 parseInt("0x10", 10) == 0