If there is a leading 0, it is converting it to octal (base 8) as long as its a valid number in base 8 (no numbers greater than 7).
For example:
017
in base 8 is 1 * 8 + 7 = 15
037
in base 8 is 3 * 8 + 7 = 31
018
is converted to 18
because 018
isn’t a valid number in base 8
Note that the behavior as to which base the number is converted to by default can be browser-specific, so its important to always specify the base/radix when using parseInt
:
parseInt("017",10) === 17
UPDATE based on comments:
parseInt
expects a string
as the first argument, so
parseInt("012",10) === 12