Why does parseInt() return an unexpected result when I pass a string with a leading zero?

Question

parseInt('1') = 1
parseInt('01') = 1
parseInt('5') = 5
parseInt('05') = 5
parseInt('8') = 8

But why:
parseInt('08') = 0
parseInt('09') = 0

Answer 1

Always use the overload that takes the radix as the second parameter:

parseInt('011',10) = 11

Answer 2

here is a hint:

parseInt('011') = 9

If it's starting with 0 it's parsed as an octal number.

Answer 3

Numbers start with 0(not 0x) are octal numbers. Therefore 8 and 9 are not a valid octal numbers.