It depend of the version of EcmaScript and the browser.
"If the input string begins with "0", radix is eight (octal). This feature is non-standard, and some implementations deliberately do not support it (instead using the radix 10)."
"The ECMAScript 5 specification of the function parseInt no longer allows implementations to treat Strings beginning with a 0 character as octal values."
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It depend of the version of EcmaScript and the browser.
"If the input string begins with "0", radix is eight (octal). This feature is non-standard, and some implementations deliberately do not support it (instead using the radix 10)."
"The ECMAScript 5 specification of the function parseInt no longer allows implementations to treat Strings beginning with a 0 character as octal values."
(Source : https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/parseInt )
So... For this reason, you should always specify a radix when using parseInt().
parseInt("03", 10);
Parent comment
And how would you check this "03" ?