Why does the radix for JavaScript's parseInt default to 8?

Question

Defaulting the radix to 8 (if the string starts with a 0) in JavaScript's parseInt function annoys me, only because I continue to forgot to pass the optional second argument as 10. I'm looking for an answer telling me why it makes sense to have it default to 8.

Answer 1

Now, a couple of years later, parseInt() seems to work fine with numbers starting with 0. Current browsers:

parseInt("019"); // 19 on Firefox 67
parseInt("019"); // 19 on Chrome 75
parseInt("019"); // 19 on Safari 12
parseInt("019"); // 19 on IE 11
parseInt("019"); // 19 on Edge 42

But still, this "fix" must break older scripts that rely on parseInt("019") returning 1 or 0 instead of 19...

Answer 2

It only "defaults" to 8 if the input string starts with 0. This is an unfortunate carryover from C and C++.

You can use Number('0123') instead, or, as you said in the question, parseInt('0123', 10).

How do I work around JavaScript's parseInt octal behavior?

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Can you tell me more about this carryover?

• Javascript eval function returning Octal value
• Octal number literals: When? Why? Ever?

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Note: ECMAScript strict mode removes octal syntax.

Answer 3

If a number starts with 0 and contains digits between (and inclusive) 0 to 7, it is interpreted as an octal number (with base 8 instead of 10).

In parseInt however, if a string starts with a 0 it's always interpeted as an octal, and stops searching when it encounters an invalid character (e.g. the digits 8 or 9 or a character like z).

parseInt("070");     //56
parseInt("70");      //70
parseInt("070", 10); //70
parseInt("78");      //78
parseInt("078");     //7, because it stops before 8

If you need to convert a string into a number, and you're sure that it contains no invalid characters or fractional parts, you can multiply it with 1 to make a number of it:

1 * "070";           //70

I personally prefer this approach, and believe it's faster than calling functions.