some-1 · HackerTrans

vor 14 Jahrendiscuss

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

vor 14 Jahrendiscuss

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

some-1

vor 14 Jahrendiscuss

-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();

Returns:

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

So there is no conversion going on in the test, it just returns the string "012". No wonder that it is fast. See my comment above for another jsperf where it is converted in each test.

some-1

vor 14 Jahrendiscuss

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();

And you get this result:

  "function anonymous() { return 12; }"

Notice that the expression has been converted to 12 in the function, so each call to it will just return 12 instead of converting it. That's why it is so much faster. The same is true for "<<", ">>" and "1 * '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.

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