How does CoffeeScript's existential operator work? -

Coffeescript uses the survival operator to determine if a variable exists, and it shows that Some something will be compiled! == Undefined & amp; Amp; some! == Zero However I have noticed that my version of coffeescript only something! == Zero was compiling in , so I wrote a test to see how this would affect my code.

  Taco = Taco if undefined? Console.log "Fiesta!" Else console.log "No Taco!"   
 compiled  
  // generated by CoffeeScript 1.4.0 (function () {var taco; taco = zero =; (if Taco! =) Null) {console.log ("fiesta!");} Else {console.log ("no taco!"}}}}). Call (this);   
 And some unexpected  did not output to Taco!  So my question is two times why coffeescript does not check for  undefined  value and why is it strong?   
 
 
 What does the document say about ? :  
 
 The existence of the coffee script is the operator ?  is not correct unless a variable  null  or  undefined , which matches the value of the ruby ​​ zero? / Code>   
 Then it will definitely say "no taco":  
  Taco = Taco if undefined? Console.log "Fiesta!" Else console.log "No Taco!"   
 If your  taco  is clearly  undefined  then  taco?  is incorrect.  
 The coffee script announces the variable inherently, therefore, ? The javascript form depend on  for example, if you only say so:  
  if taco? Console.log "Fiesta!" Else console.log "No Taco!"   
 Will you see that  Taco?  becomes  typo taco! == "Undefined" & amp; Taco! == blank  You can still check "this is  zero " (in a tight form), but there is also a " var taco "; Note that  typo taco  exam also checks for  taco = undefined , so a strict ! ==  The test can be used to see if  taco  is  null .  
 You say this:  
 
 I have found that my version of coffeescript is only  something! == Faucet    
 But it's not what it's doing, it's actually  something! = Zero is compiled for ; Use the "sloppy" type changing the inequality (! == ) versus the inequality (! = ), which you claim ! =  and ! The difference between ==  is important here:  
 
 
 Tap and undefined type  ==  (but  === ) No    
 So if you know that the variable  v  has been declared (i.e.  var v  Is anywhere) then  v  is not  null , it is enough to see that  v! = Null  neither  undefined  However, if you do not know that  v  has been announced, then you have a comparison of an undeclared variable with the  null To avoid referencing, a  typeof  check is required . Consider this javascript:  
  if (tako! = Null) {console.log ("fiesta!"); } And {console.log ("no taco!"); }   
 This will put a reference error in your face because  taco  does not exist:  
  if (type of taco ! == "Undefined" & amp; amp; Taco! == faucet) console.log ("fiesta!"); } And {console.log ("no taco!"); }   
 On the other hand, check the guards for the investigation of  typeof  which is nothing which has not been declared. I do not think you can use Backlit to build the first in coffeehouse without embedding javascript.