The strange story of ! and ==(=)false
in [JavaScript]
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From: Evertjan. on 26 Jun 2010 07:56 Are ! and ==false equivalent? NO <script type='text/javascript'> alert('' == false); // true alert(!''); // true alert('000' == false); // true alert(!'000'); // false </script> ================================= Could it be that ! and ===false are equvalent? NO <script type='text/javascript'> alert('' === false); // false alert(!''); // true alert('000' === false); // false alert(!'000'); // false </script> ================================= So what the hell is the buggy logic of this? -- Evertjan. The Netherlands. (Please change the x'es to dots in my emailaddress)
From: VK on 26 Jun 2010 08:07 On Jun 26, 3:56 pm, "Evertjan." <exjxw.hannivo...(a)interxnl.net> wrote: > Are ! and ==false equivalent? NO > > <script type='text/javascript'> > > alert('' == false); // true > alert(!''); // true > > alert('000' == false); // true > alert(!'000'); // false > > </script> > > ================================= > > Could it be that ! and ===false are equvalent? NO > > <script type='text/javascript'> > > alert('' === false); // false > alert(!''); // true > > alert('000' === false); // false > alert(!'000'); // false > > </script> > > ================================= > > So what the hell is the buggy logic of this? What exactly illogical do you see? http://www.jibbering.com/faq/notes/type-conversion/#tcBool "Type conversion rules are even simpler for string to boolean conversion as all non-empty strings always become true and empty strings become false." '000' === false means "are '000' and false of the same type and do they contain the same value(s)?" What logic you want to apply to make the answer yes (true)?
From: Stefan Weiss on 26 Jun 2010 09:43 On 26/06/10 13:56, Evertjan. wrote: > Are ! and ==false equivalent? NO > > <script type='text/javascript'> > > alert('' == false); // true > alert(!''); // true > > alert('000' == false); // true > alert(!'000'); // false > > </script> > > ================================= > > Could it be that ! and ===false are equvalent? NO > > <script type='text/javascript'> > > alert('' === false); // false > alert(!''); // true > > alert('000' === false); // false > alert(!'000'); // false > > </script> > > ================================= > > So what the hell is the buggy logic of this? Short answer: none of these are equivalent, so you shouldn't expect them to return identical results. The rules for non-strict comparison can be very unintuitive. "000" evaluates to true in a boolean context, but in your first example, "000" == false. This can only happen if the string is converted to a number before the actual comparison. Sure enough, section 11.9.3 in E262-3 specifies that for a comparison x == y 1) If Type(x) is different from Type(y), go to step 14. ... 14-18) don't apply in our example ... 19) If Type(y) is Boolean, return the result of the comparison x == ToNumber(y). Starting again from the top for x == 0 1) If Type(x) is different from Type(y), go to step 14. ... 14-16) don't apply in our example ... 17) If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y. Third time's the charm, starting again for 0 == 0 1-6) don't apply ... 7) If x is the same number value as y, return true. And there you have it: "000" == false I don't think these rules are very logical or intuitive, but that's how the language was specified. Personally, I don't even bother to remember all of this. Don't use non-strict comparison with booleans and the problem disappears. -- stefan
From: Evertjan. on 26 Jun 2010 10:48 Stefan Weiss wrote on 26 jun 2010 in comp.lang.javascript: > I don't think these rules are very logical or intuitive, but that's how > the language was specified. Personally, I don't even bother to remember > all of this. Don't use non-strict comparison with booleans and the > problem disappears. I don't think that is enough for me, Stefan. (x ==false) returns a boolean based on x after conversion to a boolean. (! x) shoud do exactly the same. I showed it does not in the case of x = '000', so the Q is is this ment to be so by definition? or is this simply a definition mistake? Since both IE and Chrome have the same irregularity it would not be an oimplementation mistake. <FAQENTRY> Should there be a warning for this strange phenomenon? </FAQENTRY> -- Evertjan. The Netherlands. (Please change the x'es to dots in my emailaddress)
From: Stefan Weiss on 26 Jun 2010 11:22
On 26/06/10 16:48, Evertjan. wrote: > Stefan Weiss wrote on 26 jun 2010 in comp.lang.javascript: > >> I don't think these rules are very logical or intuitive, but that's how >> the language was specified. Personally, I don't even bother to remember >> all of this. Don't use non-strict comparison with booleans and the >> problem disappears. > > I don't think that is enough for me, Stefan. > > (x ==false) returns a boolean based on x after conversion to a boolean. Assuming that x is still a string, it will never evaluated as a boolean in this comparison. Its numerical value will be used instead. > (! x) shoud do exactly the same. The ! operator does evaluate its operand in a boolean context. Its behavior is specified as (equivalent to) ToBoolean(GetValue(x)). > so the Q is > > is this ment to be so by definition? > > or > > is this simply a definition mistake? It's been specified and implemented like this. Whether that's good or bad, or even a mistake, is not for me to decide. Anyway, it's too late to change it without breaking existing code, so we just have to live with it. I think most people would intuitively expect x to be evaluated as a boolean in the first example. That's why I recommended not to use the == operator with booleans; the ! or === operators are much better suited in such cases. It can certainly be confusing. There's too much DWIMmery going on with non-strict comparisons for my taste. -- stefan |