some-1

No, the correct way is to use the global function isNaN(x);

See the following comment: http://news.ycombinator.com/item?id=4530557

The reason why some of them is so fast is because the constant expression is evaluated to a constant, so instead of doing a conversion in the test, the already converted number is returned.

Use this test instead: http://jsperf.com/parse-number-from-string

-0+'12' (and 0+'12') doesn't convert it to a number, it converts the zero to a string and then concatenate them to the string "012". And Firefox optimize it:

  Function('return -0+"12"').toString();

  "function anonymous() { return "012";}"

The reason why the wicked is so fast in Firefox is because of the literal string and the optimizations done by the compiler.

Try this (creates a function and then show the source):

  Function('return ~~(1 * "12");').toString();

  "function anonymous() { return 12; }"

I created another test where it actually do the conversion, by assigning the string to a variable and use that in each test. There parseInt is faster in Firefox, Opera, Chrome and Safari. Surprisingly it is the slowest in IE, where +str is the fastest.

You can find it here: http://jsperf.com/parse-number-from-string

Also notice:

* All the logical operators (~, <<, >>, |) operates internally on a 32 bit signed integer, while parseInt, Number and + operates on a 64 bit signed float. Try to convert 2147483648 or 2333444555 for a little surprise.

* At the other jsperf someone added "0+'12'". That returns a string, not a number.