BooleanQuiz.txt

Discussion about the "truthfulness" of Objects of the Boolean class.

Given some Javascript code of:

var bool_examples = [ 
  true == new Boolean(true), 
  true === new Boolean(true), 
  false == new Boolean(false), 
  false === new Boolean(false), 
  ! 0, 
  ! new Boolean(false), 
  ! "", 
  ! new Boolean(true), 
  ! new Boolean(false).valueOf(), 
  ! {}, 
  
];

and asked, "how many of these values are not equal to true? How many i where bool_examples[i] != true"


A newbie will think the first 6 are surely true, will assume that this one is false:
  ! new Boolean(true), 
but might not be so sure about these two:
  ! "",
  ! {},
.. especially if they are Java programmers and expect a Compiler Error.

Someone with a little experience who has used the || operator will think that the only false values are:
  ! new Boolean(true), 
  ! {},
.. as they say, a little experience is a dangerous thing.

Someone who has used the === operator a lot will know that it checks type as well as value, so these are also (surprisingly) false:
  true === new Boolean(true), 
  false === new Boolean(false), 

A programming logician or someone who has tried to use the Boolean class will recognize that
new Boolean(value) always returns an Object, so 
  ! new Boolean(false), 
is much like
  ! new Object(initialValue)
which is therefore the same as 
  ! {}
so therefore
  ! new Boolean(false), 
is false ("What!?" you say? Yes, try it: !new Boolean(false ), and will avoid using the Boolean class at all costs and use boolean primitives (true, false) instead, so _always_ downcast to a primitive using b.valueOf(). So the false values are:
  true === new Boolean(true), 
  false === new Boolean(false), 
  ! new Boolean(false), 
  ! new Boolean(true), 
  ! {},

Finally, a JavaScript Ninja will notice the extra Comma at the end of the Array:
  ! {}, 
  
];
and know that the compiler inserts an "undefined" value there, 
  ! {}, 
  undefined
];
and that undefined != true, so the number of values which are not true in bool_examples is 6!

Moral of the story: Avoid the Boolean class and use primitives (true,false) instead. 
If a Javascript Library you use is unfortunate enough to use Booleans, get a new Library or downcast you Booleans to primitives using the .valueOf() method call as soon as possible.

Bonus lesson: dangling commas will not throw an Exception (as Java does) but will insert an "undefined" value and increase the length of your array by 1.