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Re: falsy and truthy

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On 06-07-2013 00:01, tlbaxter99@yahoo.com wrote:
> Hi everyone,
>
> I thought I understood falsy and truthy until I ran this code:
>
> var falsyValues = [false, 0, null, "", undefined, NaN];
>
> for (var index in falsyValues)
>     if (!falsyValues[index])
>        console.log("FALSE");
>     else
>        console.log("TRUE");
>
> As I expected, "FALSE" was printed for all 6 values. No problem.

As described in ES 5.1, §9.2 (ToBoolean), there are seven falsy values: 
false, 0, -0, "", NaN, null, and undefined. All other values are truthy.

Logical NOT (!)	(!expr)	returns false if its single operand can be 
converted to true; otherwise, returns true.

>
> Now, when I modify the if() statement to this:
>     if (false == falsyValues[index])
>
> I see that null, undefined and NaN do NOT compare as being equal to false, yet when using the not operator(!) a shown earlier, they ARE considered false.
>
> Can someone explain this? Using the not operator (!) is apparently not the same as comparing directly against false.

In this particular case, we should use the strict Equals Operator (===)
than just the Equals Operator (==), because undefined == null, for
instance, produces true, whereas undefined === null yields false.

These are some interesting cases:

'' == '0' // false
0 == '' // true
0 == '0' // true
false == 'false' // false
false == '0' // true
false == undefined // false
false == null // false
null == undefined // true
' \t\r\n ' == 0 // true

Note: Douglas Crockford's advice is to never use [what he calls] the
evil twins (== and !=). Instead, we should always use === and !==,
although Crockford's advice is a tad exaggerated when dealing with the
typeof operator which always returns a string value.


--
Joao Rodrigues (J.R.)

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falsy and truthy tlbaxter99@yahoo.com - 2013-07-05 20:01 -0700
  Re: falsy and truthy Denis McMahon <denismfmcmahon@gmail.com> - 2013-07-06 07:20 +0000
  Re: falsy and truthy Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-07-06 10:24 +0200
    Re: falsy and truthy "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2013-07-06 11:22 +0200
  Re: falsy and truthy "Joao Rodrigues (J.R.)" <groups_jr-1@yahoo.com.br> - 2013-08-10 09:03 -0300

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