In JavaScript, eval(010) returns 8 [duplicate]

Question

If I use the code:

string = '010';
write = eval(string);
document.write(write)

I get 8 written on the page. Why? This happens even if 010 isn't a string.

Answer 1

Because 010 is parsed as octal. Javascript treats a leading zero as indicating that the value is in base 8.

Similarly, 0x10 would give you 16, being parsed in hex.

If you want to parse a string using a specified base, use parseInt:

parseInt('010', 8); // returns 8.
parseInt('010',10); // returns 10.
parseInt('010',16); // returns 16.

Answer 2

Prefixing a number with an 0 means it's octal, i.e. base 8. Similar to prefixing with 0x for hexadecimal numbers (base 16).

Use the second argument of parseInt to force a base:

> parseInt('010')
8
> parseInt('010', 10)
10